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Python Go
1
Print Hello World
Print a literal string on standard output 
print("Hello World")
Alternative implementation:
print('Hello World')
 
fmt.Println("Hello World")
2
Print Hello 10 times
Loop to execute some code a constant number of times 
for _ in range(10):
    print("Hello")
Alternative implementation:
print("Hello\n"*10)
Alternative implementation:
i = 0
while i < 10:
    print('Hello')
    i += 1
Alternative implementation:
def f(): print('Hello')
for x in range(10): f()
Alternative implementation:
f = lambda: print('Hello')
for x in range(10): f()
Alternative implementation:
for x in repeat('Hello', 10): print(x)
 
for i := 0; i < 10; i++ {
	fmt.Println("Hello")
}
Alternative implementation:
fmt.Println(strings.Repeat("Hello\n", 10))
Alternative implementation:
for range 10 {
	fmt.Println("Hello")
}
3
Create a procedure
Like a function which doesn't return any value, thus has only side effects (e.g. Print to standard output) 
def finish(name):
    print(f'My job here is done. Goodbye {name}')
Alternative implementation:
f = lambda: print('abc')
f()
 
func finish(name string) {
  fmt.Println("My job here is done. Good bye " + name)
}
Alternative implementation:
finish := func(name string) {
	fmt.Println("My job here is done. Good bye " + name)
}
4
Create a function
Create a function which returns the square of an integer 
def square(x):
    return x*x
Alternative implementation:
def square(x):
    return x**2
Alternative implementation:
square = lambda x: x * x
 
func square(x int) int {
  return x*x
}
5
Create a 2D Point data structure
Declare a container type for two floating-point numbers x and y 
@dataclass
class Point:
    x: float
    y: float
Alternative implementation:
Point = namedtuple("Point", "x y")
Alternative implementation:
point = {'x': 1.2, 'y': 3.4}
Alternative implementation:
class P:
    def __init__(self, x: float, y: float):
        self.x, self.y = x, y
Alternative implementation:
point = dict(x=1.2, y=3.4)
 
type Point struct {
    x, y float64
}
6
Iterate over list values
Do something with each item x of the list (or array) items, regardless indexes. 
for x in items:
        doSomething( x )
Alternative implementation:
[do_something(x) for x in items]
 
for _, x := range items {
    doSomething(x)
}
7
Iterate over list indexes and values
Print each index i with its value x from an array-like collection items 
for i, x in enumerate(items):
    print(i, x)
Alternative implementation:
for x in enumerate(items): print(x)
 
for i, x := range items {
    fmt.Printf("Item %d = %v \n", i, x)
}
8
Create a map (associative array)
Create a new map object x, and provide some (key, value) pairs as initial content. 
x = {"one" : 1, "two" : 2}
Alternative implementation:
x = dict(a=1, b=2, c=3)
 
x := map[string]int {"one": 1, "two": 2}
9
Create a Binary Tree data structure
The structure must be recursive because left child and right child are binary trees too. A node has access to children nodes, but not to its parent. 
class Node:
	def __init__(self, data):
		self.data = data
		self.left = None
		self.right = None
Alternative implementation:
class Node:
  def __init__(self, data, left_child, right_child):
    self.data = data
    self._left_child = left_child
    self._right_child = right_child
 
type BinTree struct {
	Label       valueType
	Left, Right *BinTree
}
Alternative implementation:
type BinTree[L any] struct {
	Label       L
	Left, Right *BinTree[L]
}
10
Shuffle a list
Generate a random permutation of the elements of list x 
shuffle(x)
Alternative implementation:
random.shuffle(x)
 
for i := range x {
	j := rand.Intn(i + 1)
	x[i], x[j] = x[j], x[i]
}
Alternative implementation:
y := make([]T, len(x))
perm := rand.Perm(len(x))
for i, v := range perm {
	y[v] = x[i]
}
Alternative implementation:
rand.Shuffle(len(x), func(i, j int) {
	x[i], x[j] = x[j], x[i]
})
Alternative implementation:
for i := len(x) - 1; i > 0; i-- {
	j := rand.Intn(i + 1)
	x[i], x[j] = x[j], x[i]
}
Alternative implementation:
func shuffle[T any](x []T) {
	rand.Shuffle(len(x), func(i, j int) {
		x[i], x[j] = x[j], x[i]
	})
}
11
Pick a random element from a list
The list x must be non-empty. 
random.choice(x)
 
x[rand.Intn(len(x))]
Alternative implementation:
func pickT(x []T) T {
	return x[rand.Intn(len(x))]
}
Alternative implementation:
func pick[T any](x []T) T {
	return x[rand.Intn(len(x))]
}
12
Check if list contains a value
Check if the list contains the value x.
list is an iterable finite container. 
x in list
 
func Contains(list []T, x T) bool {
	for _, item := range list {
		if item == x {
			return true
		}
	}
	return false
}
Alternative implementation:
slices.Contains(list, x)
13
Iterate over map keys and values
Access each key k with its value x from an associative array mymap, and print them. 
for k, v in mymap.items():
    print(k, v)
Alternative implementation:
for x in mymap.items(): print(x)
 
for k, x := range mymap {
  fmt.Println("Key =", k, ", Value =", x)
}
14
Pick uniformly a random floating point number in [a..b)
Pick a random number greater than or equals to a, strictly inferior to b. Precondition : a < b. 
random.uniform(a,b)
 
func pick(a, b  float64)  float64 {
	return a + (rand.Float64() * (b-a))
}
15
Pick uniformly a random integer in [a..b]
Pick a random integer greater than or equals to a, inferior or equals to b. Precondition : a < b. 
random.randint(a,b)
 
func pick(a,b int) int {
	return a + rand.Intn(b-a+1)
}
Alternative implementation:
func pick(a, b int) int {
	return a + rand.IntN(b-a+1)
}
16
Depth-first traversal of a binary tree
Call a function f on every node of binary tree bt, in depth-first infix order 
def dfs(bt):
	if bt is None:
		return
	dfs(bt.left)
	f(bt)
	dfs(bt.right)
 
func (bt *BinTree) Dfs(f func(*BinTree)) {
	if bt == nil {
		return
	}
	bt.Left.Dfs(f)
	f(bt)
	bt.Right.Dfs(f)
}
Alternative implementation:
func (bt *BinTree[L]) Dfs(f func(*BinTree[L])) {
	if bt == nil {
		return
	}
	bt.Left.Dfs(f)
	f(bt)
	bt.Right.Dfs(f)
}
17
Create a Tree data structure
The structure must be recursive. A node may have zero or more children. A node has access to its children nodes, but not to its parent. 
class Node:
    def __init__(self, value, *children):
        self.value = value
        self.children = list(children)
 
type Tree struct {
	Key keyType
	Deco valueType
	Children []*Tree
}
Alternative implementation:
type Tree[L any] struct {
	Label    L
	Children []*Tree[L]
}
18
Depth-first traversal of a tree
Call a function f on every node of a tree, in depth-first prefix order 
def DFS(f, root):
	f(root)
	for child in root:
		DFS(f, child)
 
func (t *Tree) Dfs(f func(*Tree)) {
	if t == nil {
		return
	}
	f(t)
	for _, child := range t.Children {
		child.Dfs(f)
	}
}
Alternative implementation:
func (t *Tree[L]) Dfs(f func(*Tree[L])) {
	if t == nil {
		return
	}
	f(t)
	for _, child := range t.Children {
		child.Dfs(f)
	}
}
19
Reverse a list
Reverse the order of the elements of the list x.
This may reverse "in-place" and destroy the original ordering. 
x = reversed(x)
Alternative implementation:
y = x[::-1]
Alternative implementation:
x.reverse()
 
for i, j := 0, len(x)-1; i < j; i, j = i+1, j-1 {
	x[i], x[j] = x[j], x[i]
}
Alternative implementation:
func reverse[T any](x []T) {
	for i, j := 0, len(x)-1; i < j; i, j = i+1, j-1 {
		x[i], x[j] = x[j], x[i]
	}
}
Alternative implementation:
slices.Reverse(x)
20
Return two values
Implement a function search which looks for item x in a 2D matrix m.
Return indices i, j of the matching cell.
Think of the most idiomatic way in the language to return the two values at the same time. 
def search(m, x):
    for idx, item in enumerate(m):
        if x in item:
            return idx, item.index(x)
Alternative implementation:
def search(x, m):
    for i, M in enumerate(m):
        for j, N in enumerate(M):
            if N == x: return (i, j)
 
func search(m [][]int, x int) (bool, int, int) {
	for i := range m {
		for j, v := range m[i] {
			if v == x {
				return true, i, j
			}
		}
	}
	return false, 0, 0
}
21
Swap values
Swap the values of the variables a and b
 
a, b = b, a
Alternative implementation:
a =int(input("enter a number"))
b =int(input("enter b number")) 
a, b = b, a
 
print("Value of a:", a)
print("Value of a", b)

a, b = b, a
22
Convert string to integer
Extract the integer value i from its string representation s (in radix 10) 
i = int(s)
 
i, err  := strconv.Atoi(s)
Alternative implementation:
i, err := strconv.ParseInt(s, 10, 0)
23
Convert real number to string with 2 decimal places
Given a real number x, create its string representation s with 2 decimal digits following the dot. 
s =  '{:.2f}'.format(x)
Alternative implementation:
s = f'{x:.2f}'
Alternative implementation:
s = '%.2f' % x
 
s := fmt.Sprintf("%.2f", x)
24
Assign to string the japanese word ネコ
Declare a new string s and initialize it with the literal value "ネコ" (which means "cat" in japanese) 
s = "ネコ"
 
s := "ネコ"
25
Send a value to another thread
Share the string value "Alan" with an existing running process which will then display "Hello, Alan" 
q = Queue()

def worker():
    while True:
        print(f"Hello, {q.get()}")
        q.task_done()

Thread(target=worker, daemon=True).start()

q.put("Alan")
q.join()
 
go func() {
	v := <-ch
	fmt.Printf("Hello, %v\n", v)
}()

ch <- "Alan"
26
Create a 2-dimensional array
Declare and initialize a matrix x having m rows and n columns, containing real numbers. 
x = [[0] * n for _ in range(m)]
 
const m, n = 3, 4
var x [m][n]float64
Alternative implementation:
func make2D(m, n int) [][]float64 {
	buf := make([]float64, m*n)

	x := make([][]float64, m)
	for i := range x {
		x[i] = buf[:n:n]
		buf = buf[n:]
	}
	return x
}
Alternative implementation:
func make2D[T any](m, n int) [][]T {
	buf := make([]T, m*n)

	x := make([][]T, m)
	for i := range x {
		x[i] = buf[:n:n]
		buf = buf[n:]
	}
	return x
}
27
Create a 3-dimensional array
Declare and initialize a 3D array x, having dimensions boundaries m, n, p, and containing real numbers. 
x = [[[0 for k in range(p)] for j in range(n)] for i in range(m)]
Alternative implementation:
x = numpy.zeros((m,n,p))
Alternative implementation:
x = []
for a in range(m):
    t = []
    for b in range(n):
        t.append([] * p)
    x.append(t)
 
const m, n, p = 2, 2, 3
var x [m][n][p]float64
Alternative implementation:
func make3D(m, n, p int) [][][]float64 {
	buf := make([]float64, m*n*p)

	x := make([][][]float64, m)
	for i := range x {
		x[i] = make([][]float64, n)
		for j := range x[i] {
			x[i][j] = buf[:p:p]
			buf = buf[p:]
		}
	}
	return x
}
Alternative implementation:
func make3D[T any](m, n, p int) [][][]T {
	buf := make([]T, m*n*p)

	x := make([][][]T, m)
	for i := range x {
		x[i] = make([][]T, n)
		for j := range x[i] {
			x[i][j] = buf[:p:p]
			buf = buf[p:]
		}
	}
	return x
}
28
Sort by a property
Sort the elements of the list (or array-like collection) items in ascending order of x.p, where p is a field of the type Item of the objects in items. 
items = sorted(items, key=lambda x: x.p)
Alternative implementation:
items = sorted(items, key=attrgetter('p'))
 
type ItemPSorter []Item
func (s ItemPSorter) Len() int{ return len(s) }
func (s ItemPSorter) Less(i,j int) bool{ return s[i].p<s[j].p }
func (s ItemPSorter) Swap(i,j int) { s[i],s[j] = s[j],s[i] }

func sortItems(items []Item){
	sorter := ItemPSorter(items)
	sort.Sort(sorter)
}
Alternative implementation:
less := func(i, j int) bool {
	return items[i].p < items[j].p
}
sort.Slice(items, less)
Alternative implementation:
compare := func(a, b Item) int {
	return cmp.Compare(a.p, b.p)
}
slices.SortFunc(items, compare)
29
Remove item from list, by its index
Remove i-th item from list items.
This will alter the original list or return a new list, depending on which is more idiomatic.
Note that in most languages, the smallest valid value for i is 0. 
del items[i]
Alternative implementation:
items.pop(i)
 
items = append(items[:i], items[i+1:]...)
Alternative implementation:
copy(items[i:], items[i+1:])
items[len(items)-1] = nil
items = items[:len(items)-1]
Alternative implementation:
items = slices.Delete(items, i, i+1)
30
Parallelize execution of 1000 independent tasks
Launch the concurrent execution of the procedure f with parameter i from 1 to 1000.
Tasks are independent and f(i) doesn't return any value.
Tasks need not run all at the same time, so you may use a pool. 
pool = Pool()
for i in range(1, 1001):
	pool.apply_async(f, [i])
 
for i := range 1_000 {
	go f(i)
}
31
Recursive factorial (simple)
Create the recursive function f which returns the factorial of the non-negative integer i, calculated from f(i-1) 
def f(i):
   if i == 0:
       return 1
   else:
       return i * f(i-1)
 
func f(i int) int {
  if i == 0 {
    return 1
  }
  return i * f(i-1)
}
32
Integer exponentiation by squaring
Create function exp which calculates (fast) the value x power n.
x and n are non-negative integers. 
def exp(x, n):
        return x**n

func exp(x, n int) int {
	switch {
	case n == 0:
		return 1
	case n == 1:
		return x
	case n%2 == 0:
		return exp(x*x, n/2)
	default:
		return x * exp(x*x, (n-1)/2)
	}
}
33
Atomically read and update variable
Assign to the variable x the new value f(x), making sure that no other thread may modify x between the read and the write. 
lock = threading.Lock()

lock.acquire()
try:
	x = f(x)
finally:
	lock.release()
Alternative implementation:
with threading.Lock():
    x = f(x)

var lock sync.Mutex

lock.Lock()
x = f(x)
lock.Unlock()
34
Create a set of objects
Declare and initialize a set x containing unique objects of type T. 
class T(object):
    pass

x = set(T())
Alternative implementation:
class T:
   ...

s = set(T() for _ in range(x))
 
x := make(map[T]bool)
Alternative implementation:
x := make(map[T]struct{})
35
First-class function : compose
Implement a function compose (A -> C) with parameters f (A -> B) and g (B -> C), which returns the composition function g ∘ f 
def compose(f, g):
    return lambda a: g(f(a))
 
func compose(f func(A) B, g func(B) C) func(A) C {
	return func(x A) C {
		return g(f(x))
	}
}
36
First-class function : generic composition
Implement a function compose which returns composition function g ∘ f for any functions f and g having exactly 1 parameter. 
def compose(f, g):
	return lambda x: g(f(x))
Alternative implementation:
compose = lambda f, g, x: \
    lambda x: g(f(x))
 
func composeIntFuncs(f func(int) int, g func(int) int) func(int) int {
	return func(x int) int {
		return g(f(x))
	}
}
Alternative implementation:
func compose[T, U, V any](f func(T) U, g func(U) V) func(T) V {
	return func(x T) V {
		return g(f(x))
	}
}
37
Currying
Transform a function that takes multiple arguments into a function for which some of the arguments are preset. 
def add(a, b):
	return a+b

add_to_two = partial(add, 2)
 
type PayFactory func(Company, *Employee, *Employee) Payroll

type CustomPayFactory func(*Employee) Payroll

func CurryPayFactory(pf PayFactory,company Company, boss *Employee) CustomPayFactory {
	return func(e *Employee) Payroll {
		return pf(company, boss, e)
	}
}
38
Extract a substring
Find substring t consisting in characters i (included) to j (excluded) of string s.
Character indices start at 0 unless specified otherwise.
Make sure that multibyte characters are properly handled. 
t = s[i:j]
Alternative implementation:
t = s[slice(i, j)]
 
t := string([]rune(s)[i:j])
39
Check if string contains a word
Set the boolean ok to true if the string word is contained in string s as a substring, or to false otherwise. 
ok = word in s
 
ok := strings.Contains(s, word)
40
Graph with adjacency lists
Declare a Graph data structure in which each Vertex has a collection of its neighbouring vertices. 
class Vertex(set): pass
class Graph(defaultdict):
  def __init__(self, *paths):
    self.default_factory = Vertex
    for path in paths:
      self.make_path(path)

  def make_path(self, labels):
    for l1, l2 in zip(labels, labels[1:]):
      self[l1].add(l2)
      self[l2].add(l1)

G = Graph((0, 1, 2, 3), (1, 4, 2))
 
type Vertex struct{
	Id int
	Label string
	Neighbours map[*Vertex]bool
}

type Graph []*Vertex
Alternative implementation:
type Graph[L any] []*Vertex[L]

type Vertex[L any] struct {
	Label      L
	Neighbours map[*Vertex[L]]bool
}
41
Reverse a string
Create the string t containing the same characters as the string s, in reverse order.
The original string s must remain unaltered. Each character must be handled correctly regardless its number of bytes in memory. 
t = s[::-1]
Alternative implementation:
t = ''.join(reversed(s))
 
runes := []rune(s)
for i, j := 0, len(runes)-1; i < j; i, j = i+1, j-1 {
   runes[i], runes[j] = runes[j], runes[i]
}
t := string(runes)
Alternative implementation:
func reverse(s string) string {
	if len(s) <= 1 {
		return s
	}
	var b strings.Builder
	b.Grow(len(s))
	for len(s) > 0 {
		r, l := utf8.DecodeLastRuneInString(s)
		s = s[:len(s)-l]
		b.WriteRune(r)
	}
	return b.String()
}
Alternative implementation:
runes := []rune(s)
slices.Reverse(runes)
t := string(runes)
42
Continue outer loop
Print each item v of list a which is not contained in list b.
For this, write an outer loop to iterate on a and an inner loop to iterate on b. 
for v in a:
    try:
        for u in b:
            if v == u:
                raise Exception()
        print(v)
    except Exception:
        continue
Alternative implementation:
for v in a:
  keep = True
  for w in b:
    if w == v:
      keep = False
      break
  if keep:
    print(v)
 
mainloop:
	for _, v := range a {
		for _, w := range b {
			if v == w {
				continue mainloop
			}
		}
		fmt.Println(v)
	}
43
Break outer loop
Look for a negative value v in 2D integer matrix m. Print it and stop searching. 
class BreakOuterLoop (Exception): pass

try:
    position = None
    for row in m:
        for column in m[row]:
            if m[row][column] == v:
                position = (row, column)
                raise BreakOuterLoop
except BreakOuterLoop:
    pass
Alternative implementation:
def loop_breaking(m, v): 
    for i, row in enumerate(m): 
        for j, value in enumerate(row): 
            if value == v: 
                return (i, j)
    return None

print(loop_breaking(([1,2,3],[4,5,6],[7,8,9]), 6))
Alternative implementation:
matrix = [[1,2,3],[4,-5,6],[7,8,9]]
try:
    print(next(i for i in chain.from_iterable(matrix) if i < 0))
except StopIteration:
    pass
Alternative implementation:
b = False
for r in m:
    for i in r:
        if i < 0:
            print(i)
            b = True
    if b: break
Alternative implementation:
b = False
for r in m:
    for i in r:
        if i < 0:
            print(i)
            b = True
    if b: break
 
mainloop:
	for i, line := range m {
		for _, v := range line {
			if v < 0 {
				fmt.Println(v)
				break mainloop
			}
		}
	}
44
Insert element in list
Insert the element x at position i in the list s. Further elements must be shifted to the right. 
s.insert(i, x)
 
s = append(s, 0)
copy(s[i+1:], s[i:])
s[i] = x
Alternative implementation:
s = slices.Insert(s, i, x)
45
Pause execution for 5 seconds
Sleep for 5 seconds in current thread, before proceeding with the next instructions. 
time.sleep(5)
 
time.Sleep(5 * time.Second)
46
Extract beginning of string (prefix)
Create the string t consisting of the 5 first characters of the string s.
Make sure that multibyte characters are properly handled. 
t = s[:5]
 
t := s
r := []rune(s)
if len(r) > 5 {
	t = string(r[:5])
}
Alternative implementation:
i := 0
count := 0
for i = range s {
	if count >= 5 {
		break
	}
	count++
}
t := s
if count >= 5 {
	t = s[:i]
}
47
Extract string suffix
Create string t consisting in the 5 last characters of string s.
Make sure that multibyte characters are properly handled. 
t = s[-5:]
 
t := s
r := []rune(s)
if len(r) > 5 {
	t = string(r[len(r)-5:])
}
Alternative implementation:
i := len(s)
for j := 0; i > 0 && j < 5; j++ {
	_, size := utf8.DecodeLastRuneInString(s[0:i])
	i -= size
}
t := s[i:]
48
Multi-line string literal
Assign to variable s a string literal consisting in several lines of text, including newlines. 
s = """Huey
Dewey
Louie"""
 
s := `Huey
Dewey
Louie`
49
Split a space-separated string
Build list chunks consisting in substrings of the string s, separated by one or more space characters. 
chunks = s.split()
Alternative implementation:
chunks = split(' +', s)
 
chunks := strings.Split(s, " ")
Alternative implementation:
chunks := strings.Fields(s)
50
Make an infinite loop
Write a loop that has no end clause. 
while True:
    pass
Alternative implementation:
while 1: ...
 
for {
	// Do something
}
51
Check if map contains key
Determine whether the map m contains an entry for the key k 
k in m
Alternative implementation:
m.get(k)
 
_, ok := m[k]
52
Check if map contains value
Determine whether the map m contains an entry with the value v, for some key. 
v in m.values()
 
func containsValue(m map[K]T, v T) bool {
	for _, x := range m {
		if x == v {
			return true
		}
	}
	return false
}
Alternative implementation:
func containsValue[M ~map[K]V, K, V comparable](m M, v V) bool {
	for _, x := range m {
		if x == v {
			return true
		}
	}
	return false
}
53
Join a list of strings
Concatenate elements of string list x joined by the separator ", " to create a single string y. 
y = ', '.join(x)
Alternative implementation:
y = ', '.join(map(str, x))
Alternative implementation:
f = lambda a, b: f'{a}, {b}'
y = reduce(f, x)
 
y := strings.Join(x, ", ")
54
Compute sum of integers
Calculate the sum s of the integer list or array x. 
s = sum(x)
Alternative implementation:
f = lambda a, b: a + b
s = reduce(f, x)
 
s := 0
for _, v := range x {
	s += v
}
55
Convert integer to string
Create the string representation s (in radix 10) of the integer value i. 
s = str(i)
 
s := strconv.Itoa(i)
Alternative implementation:
s := strconv.FormatInt(i, 10)
Alternative implementation:
s := fmt.Sprintf("%d", i)
56
Launch 1000 parallel tasks and wait for completion
Fork-join : launch the concurrent execution of procedure f with parameter i from 1 to 1000.
Tasks are independent and f(i) doesn't return any value.
Tasks need not run all at the same time, so you may use a pool.
Wait for the completion of the 1000 tasks and then print "Finished". 
def f(i):
	i * i

with Pool(1000) as p:
	p.map(func=f, iterable=range(1, 1001))

print('Finished')
 
var wg sync.WaitGroup
wg.Add(1_000)
for i := range 1_000 {
	go func() {
		f(i)
		wg.Done()
	}()
}
wg.Wait()
57
Filter list
Create the list y containing the items from the list x that satisfy the predicate p. Respect the original ordering. Don't modify x in-place. 
y = list(filter(p, x))
Alternative implementation:
y = [element for element in x if p(element)]
 
y := make([]T, 0, len(x))
for _, v := range x{
	if p(v){
		y = append(y, v)
	}
}
Alternative implementation:
n := 0
for _, v := range x {
	if p(v) {
		n++
	}
}
y := make([]T, 0, n)
for _, v := range x {
	if p(v) {
		y = append(y, v)
	}
}
Alternative implementation:
func filter[S ~[]T, T any](x S, p func(T) bool) S {
	var y S
	for _, v := range x {
		if p(v) {
			y = append(y, v)
		}
	}
	return y
}
Alternative implementation:
del := func(t *T) bool { return !p(t) }

y := slices.DeleteFunc(slices.Clone(x), del)
58
Extract file content to a string
Create the string lines from the content of the file with filename f. 
lines = open(f).read()
Alternative implementation:
with open(f) as fo:
    lines = fo.read()
 
b, err := os.ReadFile(f)
if err != nil {
	// Handle error...
}
lines := string(b)
59
Write to standard error stream
Print the message "x is negative" to standard error (stderr), with integer x value substitution (e.g. "-2 is negative"). 
print(x, "is negative", file=sys.stderr)
 
fmt.Fprintln(os.Stderr, x, "is negative")
60
Read command line argument
Assign to x the string value of the first command line parameter, after the program name. 
x = sys.argv[1]
 
x := os.Args[1]
61
Get current date
Assign to the variable d the current date/time value, in the most standard type. 
d = datetime.datetime.now()
 
d := time.Now()
62
Find substring position
Set i to the first position of string y inside string x, if exists.

Specify if i should be regarded as a character index or as a byte index.

Explain the behavior when y is not contained in x. 
i = x.find(y)
 
i := strings.Index(x, y)
63
Replace fragment of a string
Assign to x2 the value of string x with all occurrences of y replaced by z.
Assume occurrences of y are not overlapping. 
x2 = x.replace(y, z)
 
x2 := strings.Replace(x, y, z, -1)
Alternative implementation:
x2 := strings.ReplaceAll(x, y, z)
64
Big integer : value 3 power 247
Assign to x the value 3^247 
x = 3 ** 247
 
x := new(big.Int)
x.Exp(big.NewInt(3), big.NewInt(247), nil)
65
Format decimal number
From the real value x in [0,1], create its percentage string representation s with one digit after decimal point. E.g. 0.15625 -> "15.6%" 
s = '{:.1%}'.format(x)
Alternative implementation:
s = f"{x:.01%}"
 
s := fmt.Sprintf("%.1f%%", 100.0*x)
66
Big integer exponentiation
Calculate the result z of x power n, where x is a big integer and n is a positive integer. 
z = x**n
 
nb := big.NewInt(int64(n))
var z big.Int
z.Exp(x, nb, nil)
67
Binomial coefficient "n choose k"
Calculate binom(n, k) = n! / (k! * (n-k)!). Use an integer type able to handle huge numbers. 
def binom(n, k):
    return math.factorial(n) // math.factorial(k) // math.factorial(n - k)
Alternative implementation:
def binom(n, k):
    return math.comb(n, k)
 
z := new(big.Int)
z.Binomial(n, k)
68
Create a bitset
Create an object x to store n bits (n being potentially large). 
x = bytearray(int(math.ceil(n / 8.0)))
 
var x *big.Int = new(big.Int)
Alternative implementation:
x := make([]bool, n)
Alternative implementation:
x := make([]uint64, (n+63)/64)
69
Seed random generator
Use seed s to initialize a random generator.

If s is constant, the generator output will be the same each time the program runs. If s is based on the current value of the system clock, the generator output will be different each time. 
rand = random.Random(s)
 
r := rand.New(rand.NewSource(s))
Alternative implementation:
r := rand.New(rand.NewPCG(s, s))
70
Use clock as random generator seed
Get the current datetime and provide it as a seed to a random generator. The generator sequence will be different at each run. 
rand = random.Random()
 
r := rand.New(rand.NewSource(time.Now().UnixNano()))
71
Echo program implementation
Basic implementation of the Echo program: Print all arguments except the program name, separated by space, followed by newline.
The idiom demonstrates how to skip the first argument if necessary, concatenate arguments as strings, append newline and print it to stdout. 
print(' '.join(sys.argv[1:]))
 
func main() {
    fmt.Println(strings.Join(os.Args[1:], " "))
}
73
Create a factory
Create a factory named fact for any sub class of Parent and taking exactly one string str as constructor parameter. 
def fact(a_class, str_):
    if issubclass(a_class, Parent):
        return a_class(str_)
 
type ParentFactory func(string) Parent

var fact ParentFactory = func(str string) Parent {
	return Parent{
		name: str,
	}
}
74
Compute GCD
Compute the greatest common divisor x of big integers a and b. Use an integer type able to handle huge numbers. 
x = gcd(a, b)
Alternative implementation:
x = math.gcd(a, b)
 
x.GCD(nil, nil, a, b)
75
Compute LCM
Compute the least common multiple x of big integers a and b. Use an integer type able to handle huge numbers. 
x = (a*b)//gcd(a, b)
Alternative implementation:
x = math.lcm(a, b)
 
gcd.GCD(nil, nil, a, b)
x.Div(a, gcd).Mul(x, b)
76
Binary digits from an integer
Create the string s of integer x written in base 2.

E.g. 13 -> "1101" 
s = '{:b}'.format(x)
Alternative implementation:
s = format(x, 'b')
 
s := strconv.FormatInt(x, 2)
Alternative implementation:
s := fmt.Sprintf("%b", x)
77
Complex number
Declare a complex x and initialize it with value (3i - 2). Then multiply it by i. 
x = 3j-2
y = x * 1j
 
x := 3i - 2
x *= 1i
78
"do while" loop
Execute a block once, then execute it again as long as boolean condition c is true. 
while True:
    do_something()
    if not c:
        break
 
for{
   someThing()
   someOtherThing()
   if !c {
     break
   }
}
Alternative implementation:
for done := false; !done; {
	someThing()
	someOtherThing()
	done = !c()
}
79
Convert integer to floating point number
Declare the floating point number y and initialize it with the value of the integer x . 
y = float(x)
 
y := float64(x)
80
Truncate floating point number to integer
Declare integer y and initialize it with the value of floating point number x . Ignore non-integer digits of x .
Make sure to truncate towards zero: a negative x must yield the closest greater integer (not lesser). 
y = int(x)
 
y := int(x)
81
Round floating point number to integer
Declare the integer y and initialize it with the rounded value of the floating point number x .
Ties (when the fractional part of x is exactly .5) must be rounded up (to positive infinity). 
y = int(x + 0.5)
Alternative implementation:
c = Context(rounding=ROUND_HALF_UP)
y = round(Decimal(x, c))
 
y := int(math.Floor(x + 0.5))
82
Count substring occurrences
Find how many times string s contains substring t.
Specify if overlapping occurrences are counted. 
count = s.count(t)
 
x := strings.Count(s, t)
83
Regex with character repetition
Declare the regular expression r matching the strings "http", "htttp", "httttp", etc. 
r = re.compile(r"htt+p")
 
r := regexp.MustCompile("htt+p")
84
Count bits set in integer binary representation
Count number c of 1s in the integer i in base 2.

E.g. i=6 → c=2 
c = bin(i).count("1")
Alternative implementation:
c = i.bit_count()
 
func PopCountUInt64(i uint64) (c int) {
	i -= (i >> 1) & 0x5555555555555555
	i = (i>>2)&0x3333333333333333 + i&0x3333333333333333
	i += i >> 4
	i &= 0x0f0f0f0f0f0f0f0f
	i *= 0x0101010101010101
	return int(i >> 56)
}

func PopCountUInt32(i uint32) (n int) {
	i -= (i >> 1) & 0x55555555
	i = (i>>2)&0x33333333 + i&0x33333333
	i += i >> 4
	i &= 0x0f0f0f0f
	i *= 0x01010101
	return int(i >> 24)
}
Alternative implementation:
c := bits.OnesCount(i)
85
Check if integer addition will overflow
Write boolean function addingWillOverflow which takes two integers x, y and return true if (x+y) overflows.

An overflow may be above the max positive value, or below the min negative value. 
def adding_will_overflow(x,y):
    return False
 
func addingWillOverflow(x int, y int) bool {
	if x > 0 {
		return y > math.MaxInt-x
	}
	return y < math.MinInt-x
}
86
Check if integer multiplication will overflow
Write the boolean function multiplyWillOverflow which takes two integers x, y and returns true if (x*y) overflows.

An overflow may reach above the max positive value, or below the min negative value. 
def multiplyWillOverflow(x,y):
	return False
 
func multiplyWillOverflow(x, y uint64) bool {
   if x <= 1 || y <= 1 {
     return false
   }
   d := x * y
   return d/y != x
}
87
Stop program
Exit immediately.
If some extra cleanup work is executed by the program runtime (not by the OS itself), describe it. 
sys.exit(1)
 
os.Exit(0)
88
Allocate 1M bytes
Create a new bytes buffer buf of size 1,000,000. 
buf = bytearray(1000000)
 
buf := make([]byte, 1_000_000)
89
Handle invalid argument
You've detected that the integer value of argument x passed to the current function is invalid. Write the idiomatic way to abort the function execution and signal the problem. 
raise ValueError("x is invalid")
 
return nil, fmt.Errorf("invalid value for x: %v", x)
90
Read-only outside
Expose a read-only integer x to the outside world while being writable inside a structure or a class Foo. 
class Foo(object):
    def __init__(self):
        self._x = 0

    @property
    def x(self):
        """
        Doc for x
        """
        return self._x
 
type Foo struct {
	x int
}

func (f *Foo) X() int {
	return f.x
}
91
Load JSON file into object
Read from the file data.json and write its content into the object x.
Assume the JSON data is suitable for the type of x. 
with open("data.json", "r") as input:
    x = json.load(input)
 
buffer, err := os.ReadFile("data.json")
if err != nil {
	return err
}
err = json.Unmarshal(buffer, &x)
if err != nil {
	return err
}
Alternative implementation:
r, err := os.Open(filename)
if err != nil {
	return err
}
decoder := json.NewDecoder(r)
err = decoder.Decode(&x)
if err != nil {
	return err
}
92
Save object into JSON file
Write the contents of the object x into the file data.json. 
with open("data.json", "w") as output:
    json.dump(x, output)
 
buffer, err := json.MarshalIndent(x, "", "  ")
if err != nil {
	return err
}
err = os.WriteFile("data.json", buffer, 0644)
93
Pass a runnable procedure as parameter
Implement the procedure control which receives one parameter f, and runs f. 
def control(f):
    f()
Alternative implementation:
def control(f: Callable): f()
 
func control(f func()) {
	f()
}
94
Print the type of a variable
Print the name of the type of x. Explain if it is a static type or dynamic type.

This may not make sense in all languages. 
print(type(x))
Alternative implementation:
print(x.__class__)
 
fmt.Println(reflect.TypeOf(x))
Alternative implementation:
fmt.Printf("%T", x)
95
Get file size
Assign to variable x the length (number of bytes) of the local file at path. 
x = os.path.getsize(path)
 
info, err := os.Stat(path)
if err != nil {
	return err
}
x := info.Size()
96
Check string prefix
Set the boolean b to true if string s starts with prefix prefix, false otherwise. 
b = s.startswith(prefix)
 
b := strings.HasPrefix(s, prefix)
97
Check string suffix
Set boolean b to true if string s ends with string suffix, false otherwise. 
b = s.endswith(suffix)
 
b := strings.HasSuffix(s, suffix)
98
Epoch seconds to date object
Convert a timestamp ts (number of seconds in epoch-time) to a date with time d. E.g. 0 -> 1970-01-01 00:00:00 
d = datetime.date.fromtimestamp(ts)
 
d := time.Unix(ts, 0)
99
Format date YYYY-MM-DD
Assign to the string x the value of the fields (year, month, day) of the date d, in format YYYY-MM-DD. 
d = date(2016, 9, 28)
x = d.strftime('%Y-%m-%d')
Alternative implementation:
d = date.today()
x = d.isoformat()
 
x := d.Format("2006-01-02")
100
Sort by a comparator
Sort elements of array-like collection items, using a comparator c. 
items.sort(key=c)
Alternative implementation:
items.sort(key=functools.cmp_to_key(c))
 
type ItemCSorter []Item
func (s ItemCSorter) Len() int           { return len(s) }
func (s ItemCSorter) Less(i, j int) bool { return c(s[i], s[j]) }
func (s ItemCSorter) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }

func sortItems(items []Item) {
	sorter := ItemCSorter(items)
	sort.Sort(sorter)
}
Alternative implementation:
type ItemsSorter struct {
	items []Item
	c     func(x, y Item) bool
}

func (s ItemsSorter) Len() int           { return len(s.items) }
func (s ItemsSorter) Less(i, j int) bool { return s.c(s.items[i], s.items[j]) }
func (s ItemsSorter) Swap(i, j int)      { s.items[i], s.items[j] = s.items[j], s.items[i] }

func sortItems(items []Item, c func(x, y Item) bool) {
	sorter := ItemsSorter{
		items,
		c,
	}
	sort.Sort(sorter)
}
Alternative implementation:
sort.Slice(items, func(i, j int) bool {
	return c(items[i], items[j])
})
Alternative implementation:
slices.SortFunc(items, c)
101
Load from HTTP GET request into a string
Make an HTTP request with method GET to the URL u, then store the body of the response in the string s. 
with urllib.request.urlopen(u) as f:
    s = f.read()
Alternative implementation:
s = requests.get(u).content.decode()
 
res, err := http.Get(u)
if err != nil {
	return err
}
buffer, err := io.ReadAll(res.Body)
res.Body.Close()
if err != nil {
	return err
}
s := string(buffer)
102
Load from HTTP GET request into a file
Make an HTTP request with method GET to the URL u, then store the body of the response in the file result.txt. Try to save the data as it arrives if possible, without having all its content in memory at once. 
filename, headers = urllib.request.urlretrieve(u, 'result.txt')
Alternative implementation:
with open("results.txt", "wb") as fh:
	fh.write(requests.get(u).content)
 
out, err := os.Create("result.txt")
if err != nil {
	return err
}
defer out.Close()

resp, err := http.Get(u)
if err != nil {
	return err
}
defer func() {
	io.Copy(io.Discard, resp.Body)
	resp.Body.Close()
}()
if resp.StatusCode != 200 {
	return fmt.Errorf("Status: %v", resp.Status)
}

_, err = io.Copy(out, resp.Body)
if err != nil {
	return err
}
103
Load XML file into struct
Read from the file data.xml and write its contents into the object x.
Assume the XML data is suitable for the type of x. 
x = lxml.etree.parse('data.xml')
 
buffer, err := os.ReadFile("data.xml")
if err != nil {
	return err
}
err = xml.Unmarshal(buffer, &x)
if err != nil {
	return err
}
104
Save object into XML file
Write the contents of the object x into the file data.xml. 
buffer, err := xml.MarshalIndent(x, "", "  ")
if err != nil {
	return err
}
err = os.WriteFile("data.xml", buffer, 0644)
105
Current executable name
1 
s = sys.argv[0]
 
path := os.Args[0]
s = filepath.Base(path)
Alternative implementation:
path, err := os.Executable()
if err != nil {
  panic(err)
}
s = filepath.Base(path)
106
Get program working directory
Assign to string dir the path of the working directory.
(This is not necessarily the folder containing the executable itself) 
dir = os.getcwd()
 
dir, err := os.Getwd()
107
Get folder containing current program
Assign to string dir the path of the folder containing the currently running executable.
(This is not necessarily the working directory, though.) 
dir = os.path.dirname(os.path.abspath(__file__))
Alternative implementation:
dir = str(Path(__file__).parent)
 
programPath := os.Args[0]
absolutePath, err := filepath.Abs(programPath)
if err != nil {
	return err
}
dir := filepath.Dir(absolutePath)
108
Determine if variable name is defined
Print the value of variable x, but only if x has been declared in this program.
This makes sense in some languages, not all of them. (Null values are not the point, rather the very existence of the variable.) 
if 'x' in locals():
	print(x)
Alternative implementation:
try:
    x
except NameError:
    print("does not exist")
109
Number of bytes of a type
Set n to the number of bytes of a variable t (of type T). 
n = pympler.asizeof.asizeof(t)
 
var t T
tType := reflect.TypeOf(t)
n := tType.Size()
110
Check if string is blank
Set the boolean blank to true if the string s is empty, or null, or contains only whitespace ; false otherwise. 
blank = not s or s.isspace()
 
blank := strings.TrimSpace(s) == ""
111
Launch other program
From current process, run program x with command-line parameters "a", "b". 
subprocess.call(['x', 'a', 'b'])
 
err := exec.Command("x", "a", "b").Run()
112
Iterate over map entries, ordered by keys
Print each key k with its value x from an associative array mymap, in ascending order of k. 
for k in sorted(mymap):
    print(mymap[k])
Alternative implementation:
for e in sorted(m.items()):
    print(e)
 
keys := make([]string, 0, len(mymap))
for k := range mymap {
	keys = append(keys, k)
}
sort.Strings(keys)

for _, k := range keys {
	x := mymap[k]
	fmt.Println("Key =", k, ", Value =", x)
}
Alternative implementation:
keys := maps.Keys(mymap)
slices.Sort(keys)

for _, k := range keys {
	x := mymap[k]
	fmt.Println("Key =", k, ", Value =", x)
}
Alternative implementation:
keys := maps.Keys(mymap)
slices.SortFunc(keys, compare)

for _, k := range keys {
	x := mymap[k]
	fmt.Println("Key =", k, ", Value =", x)
}
113
Iterate over map entries, ordered by values
Print each key k with its value x from an associative array mymap, in ascending order of x.
Multiple entries may exist for the same value x. 
for x, k in sorted((x, k) for k,x in mymap.items()):
    print(k, x)
Alternative implementation:
for key, value in sorted(d.items(), key=operator.itemgetter(1)):
    print(key, value)
 
type entry struct {
	key   string
	value int
}

type entries []entry
func (list entries) Len() int { return len(list) }
func (list entries) Less(i, j int) bool { return list[i].value < list[j].value }
func (list entries) Swap(i, j int) { list[i], list[j] = list[j], list[i] }

entries := make(entries, 0, len(mymap))
for k, x := range mymap {
	entries = append(entries, entry{key: k, values: x})
}
sort.Sort(entries)

for _, e := range entries {
	fmt.Println("Key =", e.key, ", Value =", e.value)
}
Alternative implementation:
type entry struct {
	key   string
	value int
}

entries := make([]entry, 0, len(mymap))
for k, x := range mymap {
	entries = append(entries, entry{key: k, value: x})
}
sort.Slice(entries, func(i, j int) bool {
	return entries[i].value < entries[j].value
})

for _, e := range entries {
	fmt.Println("Key =", e.key, ", Value =", e.value)
}
114
Test deep equality
Set boolean b to true if objects x and y contain the same values, recursively comparing all referenced elements in x and y.
Tell if the code correctly handles recursive types. 
b = x == y
 
b := reflect.DeepEqual(x, y)
115
Compare dates
Set boolean b to true if date d1 is strictly before date d2 ; false otherwise. 
b = d1 < d2
 
b := d1.Before(d2)
116
Remove occurrences of word from string
Remove all occurrences of string w from string s1, and store the result in s2. 
s2 = s1.replace(w, '')
 
s2 := strings.Replace(s1, w, "", -1)
Alternative implementation:
s2 := strings.ReplaceAll(s1, w, "")
117
Get list size
Set n to the number of elements of the list x. 
n = len(x)
 
n := len(x)
118
List to set
Create the set y from the list x.
x may contain duplicates. y is unordered and has no repeated values. 
y = set(x)
Alternative implementation:
y = {*x}
 
y := make(map[T]struct{}, len(x))
for _, v := range x {
	y[v] = struct{}{}
}
Alternative implementation:
func sliceToSet[T comparable](x []T) map[T]struct{} {
	y := make(map[T]struct{}, len(x))
	for _, v := range x {
		y[v] = struct{}{}
	}
	return y
}
119
Deduplicate list
Remove duplicates from the list x.
Explain if the original order is preserved. 
x = list(set(x))
Alternative implementation:
x = list(OrderedDict(zip(x, x)))
Alternative implementation:
def dedup(x):
  y = []
  for i in x:
    if not i in y:
      y.append(i)
  return y
 
y := make(map[T]struct{}, len(x))
for _, v := range x {
	y[v] = struct{}{}
}
x2 := make([]T, 0, len(y))
for _, v := range x {
	if _, ok := y[v]; ok {
		x2 = append(x2, v)
		delete(y, v)
	}
}
x = x2
Alternative implementation:
seen := make(map[T]bool)
j := 0
for _, v := range x {
	if !seen[v] {
		x[j] = v
		j++
		seen[v] = true
	}
}
x = x[:j]
Alternative implementation:
seen := make(map[T]bool)
j := 0
for _, v := range x {
	if !seen[v] {
		x[j] = v
		j++
		seen[v] = true
	}
}
for i := j; i < len(x); i++ {
	x[i] = nil
}
x = x[:j]
Alternative implementation:
func deduplicate[S ~[]T, T comparable](x S) S {
	seen := make(map[T]bool)
	j := 0
	for _, v := range x {
		if !seen[v] {
			x[j] = v
			j++
			seen[v] = true
		}
	}
	var zero T
	for i := j; i < len(x); i++ {
		// Avoid memory leak
		x[i] = zero
	}
	return x[:j]
}
Alternative implementation:
slices.Sort(x)
x = slices.Compact(x)
120
Read integer from stdin
Read an integer value from the standard input into the variable n 
n = int(input("Input Prompting String: "))
 
_, err := fmt.Scan(&n)
Alternative implementation:
_, err := fmt.Scanf("%d", &n)
121
UDP listen and read
Listen UDP traffic on port p and read 1024 bytes into the buffer b. 
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.bind((UDP_IP, p))
while True:
    data, addr = sock.recvfrom(1024)
    print("received message:", data)
 
ServerAddr,err := net.ResolveUDPAddr("udp",p)
if err != nil {
	return err
}
ServerConn, err := net.ListenUDP("udp", ServerAddr)
if err != nil {
	return err
}
defer ServerConn.Close()
n,addr,err := ServerConn.ReadFromUDP(b[:1024])
if err != nil {
	return err
}
if n<1024 {
	return fmt.Errorf("Only %d bytes could be read.", n)
}
122
Declare an enumeration
Create an enumerated type Suit with 4 possible values SPADES, HEARTS, DIAMONDS, CLUBS. 
class Suit:
	SPADES, HEARTS, DIAMONDS, CLUBS = range(4)
Alternative implementation:
class Suit(Enum):
	SPADES = 1
	HEARTS = 2
	DIAMONDS = 3
	CLUBS = 4
 
type Suit int

const (
  Spades Suit = iota
  Hearts
  Diamonds
  Clubs
)
123
Assert condition
Verify that predicate isConsistent returns true, otherwise report assertion violation.
Explain if the assertion is executed even in production environment or not. 
assert isConsistent
 
if !isConsistent() {
	panic("State consistency violated")
}
124
Binary search for a value in sorted array
Write the function binarySearch which returns the index of an element having the value x in the sorted array a, or -1 if no such element exists. 
def binarySearch(a, x):
    i = bisect.bisect_left(a, x)
    return i if i != len(a) and a[i] == x else -1

func binarySearch(a []T, x T) int {
	imin, imax := 0, len(a)-1
	for imin <= imax {
		imid := imin + (imax-imin) / 2
		switch {
		case a[imid] == x:
			return imid
		case a[imid] < x:
			imin = imid + 1
		default:
			imax = imid - 1
		}
	}
	return -1
}
Alternative implementation:
func binarySearch(a []int, x int) int {
	i := sort.SearchInts(a, x)
	if i < len(a) && a[i] == x {
		return i
	}
	return -1
}
Alternative implementation:
func binarySearch(a []T, x T) int {
	f := func(i int) bool { return a[i] >= x }
	i := sort.Search(len(a), f)
	if i < len(a) && a[i] == x {
		return i
	}
	return -1
}
Alternative implementation:
func binarySearch(a []T, x T) int {
	if i, ok := slices.BinarySearch(a, x); ok {
		return i
	} else {
		return -1
	}
}
125
Measure function call duration
measure the duration t, in nanoseconds, of a call to the function foo. Print this duration. 
t1 = time.perf_counter_ns()
foo()
t2 = time.perf_counter_ns()
print('Nanoseconds:', t2 - t1)
 
t1 := time.Now()
foo()
t := time.Since(t1)
ns := int64(t / time.Nanosecond)
fmt.Printf("%dns\n", ns)
Alternative implementation:
t1 := time.Now()
foo()
t := time.Since(t1)
ns := t.Nanoseconds()
fmt.Printf("%dns\n", ns)
126
Multiple return values
Write a function foo that returns a string and a boolean value. 
def foo():
    return 'string', True
Alternative implementation:
foo = lambda: ('abc', True)
 
func foo() (string, bool) {
	return "Too good to be", true
}
127
Source code inclusion
Import the source code for the function foo body from a file "foobody.txt". 
foo = imp.load_module('foobody', 'foobody.txt').foo
 
//go:embed foobody.txt
var s string
128
Breadth-first traversing of a tree
Call a function f on every node of a tree, in breadth-first prefix order 
def BFS(f, root):
	Q = [root]
	while Q:
		n = Q.pop(0)
		f(n)
		for child in n:
			if not n.discovered:
				n.discovered = True
				Q.append(n)
 
func (root *Tree) Bfs(f func(*Tree)) {
	if root == nil {
		return
	}
	queue := []*Tree{root}
	for len(queue) > 0 {
		t := queue[0]
		queue = queue[1:]
		f(t)
		queue = append(queue, t.Children...)
	}
}
129
Breadth-first traversal in a graph
Call the function f on every vertex accessible from the vertex start, in breadth-first prefix order 
def breadth_first(start, f):
  seen = set()
  q = deque([start])
  while q:
    vertex = q.popleft()
    f(vertex)
    seen.add(vertex)
    q.extend(v for v in vertex.adjacent if v not in seen)
 
func (start *Vertex) Bfs(f func(*Vertex)) {
	queue := []*Vertex{start}
	seen := map[*Vertex]bool{start: true}
	for len(queue) > 0 {
		v := queue[0]
		queue = queue[1:]
		f(v)
		for next, isEdge := range v.Neighbours {
			if isEdge && !seen[next] {
				queue = append(queue, next)
				seen[next] = true
			}
		}
	}
}
130
Depth-first traversal in a graph
Call th function f on every vertex accessible from the vertex v, in depth-first prefix order 
def depth_first(start, f):
  seen = set()
  stack = [start]
  while stack:
    vertex = stack.pop()
    f(vertex)
    seen.add(vertex)
    stack.extend(
      v for v in vertex.adjacent if v not in seen
    )
 
func (v *Vertex) Dfs(f func(*Vertex), seen map[*Vertex]bool) {
	seen[v] = true
	f(v)
	for next, isEdge := range v.Neighbours {
		if isEdge && !seen[next] {
			next.Dfs(f, seen)
		}
	}
}
Alternative implementation:
func (v *Vertex[L]) Dfs(f func(*Vertex[L]), seen map[*Vertex[L]]bool) {
	seen[v] = true
	f(v)
	for next, isEdge := range v.Neighbours {
		if isEdge && !seen[next] {
			next.Dfs(f, seen)
		}
	}
}
131
Successive conditions
Execute f1 if condition c1 is true, or else f2 if condition c2 is true, or else f3 if condition c3 is true.
Don't evaluate a condition when a previous condition was true. 
f1() if c1 else f2() if c2 else f3() if c3 else None
Alternative implementation:
if c1:
    f1()
elif c2:
    f2()
elif c3:
    f3()
Alternative implementation:
if c1: f1()
elif c2: f2()
elif c3: f3()
 
switch {
case c1:
	f1()
case c2:
	f2()
case c3:
	f3()
}
132
Measure duration of procedure execution
Run the procedure f, and return the duration of the execution of f. 
duration = timeit.timeit("f()", setup="from __main__ import f")
Alternative implementation:
start = time.time()
f()
end = time.time()
return end - start
 
func clock(f func()) time.Duration {
	t := time.Now()
	f()
	return time.Since(t)
}
133
Case-insensitive string contains
Set boolean ok to true if string word is contained in string s as a substring, even if the case doesn't match, or to false otherwise. 
ok = word.lower() in s.lower()
Alternative implementation:
p = '(?i)' + escape(word)
ok = not not search(p, s)
 
lowerS, lowerWord := strings.ToLower(s), strings.ToLower(word)
ok := strings.Contains(lowerS, lowerWord)
134
Create a new list
Declare and initialize a new list items, containing 3 elements a, b, c. 
items = [a, b, c]
Alternative implementation:
items = list((a, b, c))
 
items := []T{a, b, c}
135
Remove item from list, by its value
Remove at most 1 item from list items, having the value x.
This will alter the original list or return a new list, depending on which is more idiomatic.
If there are several occurrences of x in items, remove only one of them. If x is absent, keep items unchanged. 
items.remove(x)
 
for i, y := range items {
	if y == x {
		items = append(items[:i], items[i+1:]...)
		break
	}
}
Alternative implementation:
for i, y := range items {
	if y == x {
		copy(items[i:], items[i+1:])
		items[len(items)-1] = nil
		items = items[:len(items)-1]
		break
	}
}
Alternative implementation:
func removeFirstByValue[S ~[]T, T comparable](items *S, x T) {
	for i, y := range *items {
		if y == x {
			*items = slices.Delete(*items, i, i+1)
			return
		}
	}
}
Alternative implementation:
func removeFirstByValue[S ~[]T, T comparable](items *S, x T) {
	if i := slices.Index(*items, x); i != -1 {
		*items = slices.Delete(*items, i, i+1)
	}
}
136
Remove all occurrences of a value from a list
Remove all occurrences of the value x from list items.
This will alter the original list or return a new list, depending on which is more idiomatic. 
newlist = [item for item in items if item != x]
 
items2 := make([]T, 0, len(items))
for _, v := range items {
	if v != x {
		items2 = append(items2, v)
	}
}
Alternative implementation:
j := 0
for i, v := range items {
	if v != x {
		items[j] = items[i]
		j++
	}
}
items = items[:j]
Alternative implementation:
j := 0
for i, v := range items {
	if v != x {
		items[j] = items[i]
		j++
	}
}
for k := j; k < len(items); k++ {
	items[k] = nil
}
items = items[:j]
Alternative implementation:
func removeAll[S ~[]T, T comparable](items *S, x T) {
	j := 0
	for i, v := range *items {
		if v != x {
			(*items)[j] = (*items)[i]
			j++
		}
	}
	var zero T
	for k := j; k < len(*items); k++ {
		(*items)[k] = zero
	}
	*items = (*items)[:j]
}
Alternative implementation:
items = slices.DeleteFunc(items, func(e T) bool {
	return e == x
})
137
Check if string contains only digits
Set the boolean b to true if the string s contains only characters in the range '0'..'9', false otherwise. 
b = s.isdigit()
Alternative implementation:
f = lambda x: x in digits
b = all(f(x) for x in s)
b = b and bool(len(s))
 
b := true
for _, c := range s {
	if c < '0' || c > '9' {
		b = false
		break
	}
}
Alternative implementation:
isNotDigit := func(c rune) bool { return c < '0' || c > '9' }
b := strings.ContainsFunc(s, isNotDigit)
138
Create temp file
Create a new temporary file on the filesystem. 
file = tempfile.TemporaryFile()
 
tmpfile, err := os.CreateTemp("", "")
139
Create temp directory
Create a new temporary folder on filesystem, for writing. 
td = tempfile.TemporaryDirectory()
 
dir, err := os.MkdirTemp("", "")
140
Delete map entry
Delete from map m the entry having key k.

Explain what happens if k is not an existing key in m. 
m.pop(k, None)
 
delete(m, k)
141
Iterate in sequence over two lists
Iterate in sequence over the elements of the list items1 then items2. For each iteration print the element. 
for x in items1 + items2:
    print(x)
Alternative implementation:
for x in chain(items1, items2): print(x)
 
for _, v := range items1 {
	fmt.Println(v)
}
for _, v := range items2 {
	fmt.Println(v)
}
142
Hexadecimal digits of an integer
Assign to string s the hexadecimal representation (base 16) of integer x.

E.g. 999 -> "3e7" 
s = hex(x)
Alternative implementation:
s = format(x, 'x')
Alternative implementation:
s = '%x' % x
 
s := strconv.FormatInt(x, 16)
Alternative implementation:
s := fmt.Sprintf("%x", x)
143
Iterate alternatively over two lists
Iterate alternatively over the elements of the lists items1 and items2. For each iteration, print the element.

Explain what happens if items1 and items2 have different size. 
for pair in zip(item1, item2): print(pair)
Alternative implementation:
a = zip_longest(items1, items2)
for x in a: print(x)
 
for i := 0; i < len(items1) || i < len(items2); i++ {
	if i < len(items1) {
		fmt.Println(items1[i])
	}
	if i < len(items2) {
		fmt.Println(items2[i])
	}
}
Alternative implementation:
for i := range min(len(items1), len(items2)) {
	fmt.Println(items1[i])
	fmt.Println(items2[i])

}
144
Check if file exists
Set boolean b to true if file at path fp exists on filesystem; false otherwise.

Beware that you should not do this and then in the next instruction assume the result is still valid, this is a race condition on any multitasking OS. 
b = os.path.exists(fp)
Alternative implementation:
b = Path(fp).exists()
 
_, err := os.Stat(fp)
b := !os.IsNotExist(err)
145
Print log line with datetime
Print message msg, prepended by current date and time.

Explain what behavior is idiomatic: to stdout or stderr, and what the date format is. 
logging.basicConfig(stream=sys.stdout, level=logging.DEBUG, format="%(asctime)-15s %(message)s")
logger = logging.getLogger('NAME OF LOGGER')

logger.info(msg)
 
log.Println(msg)
146
Convert string to floating point number
Extract floating point value f from its string representation s 
s = u'545,2222'
locale.setlocale(locale.LC_ALL, 'de')
f = locale.atof(s)
Alternative implementation:
f = float(s)
Alternative implementation:
f = float(s)
 
f, err := strconv.ParseFloat(s, 64)
147
Remove all non-ASCII characters
Create string t from string s, keeping only ASCII characters 
t = re.sub('[^\u0000-\u007f]', '',  s)
Alternative implementation:
t = s.encode("ascii", "ignore").decode()
Alternative implementation:
f = lambda x: ord(x) < 0x80
t = ''.join(filter(f, s))
 
re := regexp.MustCompile("[[:^ascii:]]")
t := re.ReplaceAllLiteralString(s, "")
Alternative implementation:
t := strings.Map(func(r rune) rune {
	if r > unicode.MaxASCII {
		return -1
	}
	return r
}, s)
148
Read list of integers from stdin
Read a list of integer numbers from the standard input, until EOF. 
list(map(int, input().split()))
Alternative implementation:
numbers = [int(x) for x in input().split()]
 
var ints []int
s := bufio.NewScanner(os.Stdin)
s.Split(bufio.ScanWords)
for s.Scan() {
	i, err := strconv.Atoi(s.Text())
	if err == nil {
		ints = append(ints, i)
	}
}
if err := s.Err(); err != nil {
	return err
}
149
Rescue the princess
As an exception, this content is not under license CC BY-SA 3.0 like the rest of this website.
150
Remove trailing slash
Remove the last character from the string p, if this character is a forward slash / 
p = p.rstrip("/")
 
p = strings.TrimSuffix(p, "/")
151
Remove string trailing path separator
Remove last character from string p, if this character is the file path separator of current platform.

Note that this also transforms unix root path "/" into the empty string! 
if p.endswith(os.sep):
    p = p[:-1]
 
sep := fmt.Sprintf("%c", os.PathSeparator)
p = strings.TrimSuffix(p, sep)
Alternative implementation:
sep := fmt.Sprintf("%c", filepath.Separator)
p = strings.TrimSuffix(p, sep)
152
Turn a character into a string
Create string s containing only the character c. 
s = c
 
s := fmt.Sprintf("%c", c)
153
Concatenate string with integer
Create the string t as the concatenation of the string s and the integer i. 
t = f"{s}{i}"
 
t := fmt.Sprintf("%s%d", s, i)
Alternative implementation:
t := s + strconv.Itoa(i)
154
Halfway between two hex color codes
Find color c, the average between colors c1, c2.

c, c1, c2 are strings of hex color codes: 7 chars, beginning with a number sign # .
Assume linear computations, ignore gamma corrections. 
r1, g1, b1 = [int(c1[p:p+2], 16) for p in range(1,6,2)]
r2, g2, b2 = [int(c2[p:p+2], 16) for p in range(1,6,2)]
c = '#{:02x}{:02x}{:02x}'.format((r1+r2) // 2, (g1+g2) //2, (b1+b2)// 2)
Alternative implementation:
class RGB(numpy.ndarray):
  @classmethod
  def from_str(cls, rgbstr):
    return numpy.array([
      int(rgbstr[i:i+2], 16)
      for i in range(1, len(rgbstr), 2)
    ]).view(cls)
 
  def __str__(self):
    self = self.astype(numpy.uint8)
    return '#' + ''.join(format(n, 'x') for n in self)
 
c1 = RGB.from_str('#a1b1c1')
print(c1)
c2 = RGB.from_str('#1A1B1C')
print(c2)

print((c1 + c2) / 2)
Alternative implementation:
s = slice(1, 7)
x = bytes.fromhex(c1[s])
y = bytes.fromhex(c2[s])
f = lambda x: sum(x) // 2
r, g, b = map(f, zip(x, y))
c = (r << 16) + (g << 8) + b
c = f'#{c:06x}'
 
r1, _ := strconv.ParseInt(c1[1:3], 16, 0)
r2, _ := strconv.ParseInt(c2[1:3], 16, 0)
r := (r1 + r2) / 2

g1, _ := strconv.ParseInt(c1[3:5], 16, 0)
g2, _ := strconv.ParseInt(c2[3:5], 16, 0)
g := (g1 + g2) / 2

b1, _ := strconv.ParseInt(c1[5:7], 16, 0)
b2, _ := strconv.ParseInt(c2[5:7], 16, 0)
b := (b1 + b2) / 2

c := fmt.Sprintf("#%02X%02X%02X", r, g, b)
Alternative implementation:
var buf [7]byte
buf[0] = '#'
for i := 0; i < 3; i++ {
	sub1 := c1[1+2*i : 3+2*i]
	sub2 := c2[1+2*i : 3+2*i]
	v1, _ := strconv.ParseInt(sub1, 16, 0)
	v2, _ := strconv.ParseInt(sub2, 16, 0)
	v := (v1 + v2) / 2
	sub := fmt.Sprintf("%02X", v)
	copy(buf[1+2*i:3+2*i], sub)
}
c := string(buf[:])
155
Delete file
Delete from filesystem the file having path filepath. 
path = pathlib.Path(_filepath)
path.unlink()
Alternative implementation:
os.remove(filepath)
 
err := os.Remove(filepath)
156
Format integer with zero-padding
Assign to the string s the value of the integer i in 3 decimal digits. Pad with zeros if i < 100. Keep all digits if i ≥ 1000. 
s = format(i, '03d')
Alternative implementation:
s = '%03d' % i
 
s := fmt.Sprintf("%03d", i)
157
Declare constant string
Initialize a constant planet with string value "Earth". 
PLANET = 'Earth'
 
const planet = "Earth"
158
Random sublist
Create a new list y from randomly picking exactly k elements from list x.

It is assumed that x has at least k elements.
Each element must have same probability to be picked.
Each element from x must be picked at most once.
Explain if the original ordering is preserved or not. 
y = random.sample(x, k)
 
y := make([]T, k)
perm := rand.Perm(len(x))
for i, v := range perm[:k] {
	y[i] = x[v]
}
159
Trie
Define a Trie data structure, where entries have an associated value.
(Not all nodes are entries) 
class Trie:
   def __init__(self, prefix, value=None):
       self.prefix = prefix
       self.children = []
       self.value = value
 
type Trie struct {
	c        rune
	children map[rune]*Trie
	isEntry  bool
	value    V
}
160
Detect if 32-bit or 64-bit architecture
Execute f32() if platform is 32-bit, or f64() if platform is 64-bit.
This can be either a compile-time condition (depending on target) or a runtime detection. 
if sys.maxsize > 2**32:
    f64()
else:
    f32()


if strconv.IntSize==32 {
	f32()
}
if strconv.IntSize==64 {
	f64()
}
161
Multiply all the elements of a list
Multiply all the elements of the list elements by a constant c 
elements = [c * x for x in elements]
Alternative implementation:
f = lambda x: x * c
elements = list(map(f, elements))
 
for i := range elements {
	elements[i] *= c
}
162
Execute procedures depending on options
execute bat if b is a program option and fox if f is a program option. 
if 'b' in sys.argv[1:]: bat()
if 'f' in sys.argv[1:]: fox()
Alternative implementation:
options = {
	'b': bat
	'f': fox
}

for option, function in options:
	if option in sys.argv[1:]:
		function()
 
var b = flag.Bool("b", false, "Do bat")
var f = flag.Bool("f", false, "Do fox")

func main() {
	flag.Parse()
	if *b {
		bar()
	}
	if *f {
		fox()
	}
}
163
Print list elements by group of 2
Print all the list elements, two by two, assuming list length is even. 
for x in zip(list[::2], list[1::2]):
    print(x)
Alternative implementation:
def pairwise(iterable):
    "s -> (s0,s1), (s1,s2), (s2, s3), ..."
    a, b = tee(iterable)
    next(b, None)
    return zip(a, b)

for a, b in pairwise(list):
    print(a, b)
Alternative implementation:
for x in batched(a, 2): print(x)
Alternative implementation:
i = iter(a)
for x in zip_longest(i, i):
    print(x)
 
for i := 0; i+1 < len(list); i += 2 {
	fmt.Println(list[i], list[i+1])
}
164
Open URL in the default browser
Open the URL s in the default browser.
Set the boolean b to indicate whether the operation was successful. 
webbrowser.open(s)
 
b := open.Start(s) == nil
Alternative implementation:
var err error
switch runtime.GOOS {
case "linux":
	err = exec.Command("xdg-open", s).Start()
case "windows":
	err = exec.Command("rundll32", "url.dll,FileProtocolHandler", s).Start()
case "darwin":
	err = exec.Command("open", s).Start()
default:
	err = fmt.Errorf("unsupported platform")
}
b := err == nil
165
Last element of list
Assign to the variable x the last element of the list items. 
x = items[-1]
 
x := items[len(items)-1]
166
Concatenate two lists
Create the list ab containing all the elements of the list a, followed by all the elements of the list b. 
ab = a + b
 
ab := append(a, b...)
Alternative implementation:
var ab []T
ab = append(append(ab, a...), b...)
Alternative implementation:
ab := make([]T, len(a)+len(b))
copy(ab, a)
copy(ab[len(a):], b)
167
Trim prefix
Create the string t consisting of the string s with its prefix p removed (if s starts with p). 
t = s[s.startswith(p) and len(p):]
Alternative implementation:
t = s.removeprefix(p)
 
t := strings.TrimPrefix(s, p)
168
Trim suffix
Create string t consisting of string s with its suffix w removed (if s ends with w). 
t = s.removesuffix(w)
 
t := strings.TrimSuffix(s, w)
169
String length
Assign to the integer n the number of characters of the string s.
Make sure that multibyte characters are properly handled.
n can be different from the number of bytes of s. 
n = len(s)
 
n := utf8.RuneCountInString(s)
170
Get map size
Set n to the number of elements stored in mymap.

This is not always equal to the map capacity. 
n = len(mymap)
 
n := len(mymap)
171
Add an element at the end of a list
Append the element x to the list s. 
s.append(x)
 
s = append(s, x)
172
Insert an entry in a map
Insert value v for key k in map m. 
m[k] = v
 
m[k] = v
173
Format a number with grouped thousands
Number will be formatted with a comma separator between every group of thousands. 
f'{1000:,}'
Alternative implementation:
format(1000, ',')
Alternative implementation:
'{:,}'.format(1000)
 
p := message.NewPrinter(language.English)
s := p.Sprintf("%d\n", 1000)
174
Make HTTP POST request
Make a HTTP request with method POST to the URL u 
data = parse.urlencode(<your data dict>).encode()
req =  request.Request(u, data=data, method="POST")
resp = request.urlopen(req)
 
response, err := http.Post(u, contentType, body)
Alternative implementation:
response, err := http.PostForm(u, formValues)
175
Bytes to hex string
From the array a of n bytes, build the equivalent hex string s of 2n digits.
Each byte (256 possible values) is encoded as two hexadecimal characters (16 possible values per digit). 
s = a.hex()
 
s := hex.EncodeToString(a)
Alternative implementation:
s := fmt.Sprintf("%x", a)
176
Hex string to byte array
From hex string s of 2n digits, build the equivalent array a of n bytes.
Each pair of hexadecimal characters (16 possible values per digit) is decoded into one byte (256 possible values). 
a = bytearray.fromhex(s)
 
a, err := hex.DecodeString(s)
if err != nil {
	log.Fatal(err)
}
177
Find files for a list of filename extensions
Construct a list L that contains all filenames that have the extension ".jpg" , ".jpeg" or ".png" in directory D and all its subdirectories. 
extensions = [".jpg", ".jpeg", ".png"]
L = [f for f in os.listdir(D) if os.path.splitext(f)[1] in extensions]
Alternative implementation:
filtered_files = ["{}/{}".format(dirpath, filename) for dirpath, _, filenames in os.walk(D) for filename in filenames if re.match(r'^.*\.(?:jpg|jpeg|png)$', filename)]
Alternative implementation:
list(itertools.chain(*(glob.glob("*/**.%s" % ext) for ext in ["jpg", "jpeg", "png"])))
Alternative implementation:
extensions = [".jpg", ".jpeg", ".png"]
L = [f for f in glob.glob(os.path.join(D, "**/*"), recursive=True) if os.path.splitext(f)[1] in extensions]
 
L := []string{}
err := filepath.Walk(D, func(path string, info os.FileInfo, err error) error {
	if err != nil {
		fmt.Printf("failure accessing a path %q: %v\n", path, err)
		return err
	}
	for _, ext := range []string{".jpg", ".jpeg", ".png"} {
		if strings.HasSuffix(path, ext) {
			L = append(L, path)
			break
		}
	}
	return nil
})
178
Check if point is inside rectangle
Set boolean b to true if if the point with coordinates (x,y) is inside the rectangle with coordinates (x1,y1,x2,y2) , or to false otherwise.
Describe if the edges are considered to be inside the rectangle. 
b = (x1 < x < x2) and (y1 < y < y2)
Alternative implementation:
class Rectangle:
    def __init__(self, x, y, w, h):
        self.x, self.y = x, y
        self.w, self.h = w, h
    def contains(self, x, y):
        a, b = self.x, self.y
        w, h = self.w, self.h
        return a <= x <= (a + w) and \
               b <= y <= (b + h)
w, h = x2 - x1, y2 - y1
r = Rectangle(x1, y1, w, h)
b = r.contains(x, y)
 
p := image.Pt(x, y)
r := image.Rect(x1, y1, x2, y2)
b := p.In(r)
179
Get center of a rectangle
Return the center c of the rectangle with coördinates(x1,y1,x2,y2) 
center = ((x1+x2)/2, (y1+y2)/2)
Alternative implementation:
Point = namedtuple('Point', 'x y')
center = Point((x1+x2)/2, (y1+y2)/2)
Alternative implementation:
class R:
    def __init__(self, x, y, w, h):
        self.x, self.y = x, y
        self.w, self.h = w, h
    def center(self):
        return {
            'x': (self.x + self.w) / 2,
            'y': (self.y + self.h) / 2
        }
w, h = x2 - x1, y2 - y1
c = R(x1, y1, w, h).center()
 
c := image.Pt((x1+x2)/2, (y1+y2)/2)
180
List files in directory
Create the list x containing the contents of the directory d.

x may contain files and subfolders.
No recursive subfolder listing. 
x = os.listdir(d)
 
x, err := os.ReadDir(d)
182
Quine program
Output the source of the current program. A quine is a computer program that takes no input and produces a copy of its own source code as its only output.

Reading the source file from disk is cheating. 
s = 's = %r\nprint(s%%s)'
print(s%s)
 
package main

import "fmt"

func main() {
	fmt.Printf("%s%c%s%c\n", s, 0x60, s, 0x60)
}

var s = `package main

import "fmt"

func main() {
	fmt.Printf("%s%c%s%c\n", s, 0x60, s, 0x60)
}

var s = `
183
Make HTTP PUT request
Make a HTTP request with method PUT to the URL u 
content_type = 'text/plain'
headers = {'Content-Type': content_type}
data = {}

r = requests.put(u, headers=headers, data=data)
status_code, content = r.status_code, r.content

req, err := http.NewRequest("PUT", u, body)
if err != nil {
	return err
}
req.Header.Set("Content-Type", contentType)
req.ContentLength = contentLength
response, err := http.DefaultClient.Do(req)
184
Tomorrow
Assign to variable t a string representing the day, month and year of the day after the current date. 
t = str(date.today() + timedelta(days=1))
 
t := time.Now().Add(24 * time.Hour).Format("2006-01-02")
185
Execute function in 30 seconds
Schedule the execution of f(42) in 30 seconds. 
timer = threading.Timer(30.0, f, args=(42,) ) 
timer.start() 

timer := time.AfterFunc(
	30*time.Second,
	func() {
		f(42)
	})
Alternative implementation:
go func() {
	time.Sleep(30 * time.Second)
	f(42)
}()
186
Exit program cleanly
Exit a program cleanly indicating no error to OS 
sys.exit(0)
 
os.Exit(0)
Alternative implementation:
defer os.Exit(0)
187
Disjoint Set
Disjoint Sets hold elements that are partitioned into a number of disjoint (non-overlapping) sets. 
class UnionFind:
    def __init__(self, size):
        self.rank = [0] * size
        self.p = [i for i in range(size)]

    def find_set(self, i):
        if self.p[i] == i:
            return i
        else:
            self.p[i] = self.find_set(self.p[i])
            return self.p[i]

    def is_same_set(self, i, j):
        return self.find_set(i) == self.find_set(j)

    def union_set(self, i, j):
        if not self.is_same_set(i, j):
            x, y = self.find_set(i), self.find_set(j)
188
Matrix multiplication
Perform matrix multiplication of a real matrix a with nx rows and ny columns, a real matrix b with ny rows and nz columns and assign the value to a real matrix c with nx rows and nz columns. 
c = a @ b
Alternative implementation:
c = np.matmul(a, b)
 
c := new(mat.Dense)
c.Mul(a, b)
189
Filter and transform list
Produce a new list y containing the result of the function T applied to all elements e of the list x that match the predicate P. 
y = [T(e) for e in x if P(e)]
Alternative implementation:
y = list(map(T, filter(P, x)))
 
var y []Result
for _, e := range x {
	if P(e) {
		y = append(y, T(e))
	}
}
190
Call an external C function
Declare an external C function with the prototype

void foo(double *a, int n);

and call it, passing an array (or a list) of size 10 to a and 10 to n.

Use only standard features of your language. 
a = (c_int * 10)(*range(10))
n = 10
libc = cdll.LoadLibrary('/path/to/c_library')
libc.foo(c_void_p(a), c_int(n))

// void foo(double *a, int n);
// double a[] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
import "C"

C.foo(C.a, 10)
191
Check if any value in a list is larger than a limit
Given a one-dimensional array a, check if any value is larger than x, and execute the procedure f if that is the case 
if any(v > x for v in a):
    f()
 
for _, v := range a {
	if v > x {
		f()
		break
	}
}
Alternative implementation:
if slices.ContainsFunc(a, func(i int) bool {
	return i > x
}) {
	f()
}
192
Declare a real variable with at least 20 digits
Declare a real variable a with at least 20 digits; if the type does not exist, issue an error at compile time. 
a = decimal.Decimal('1234567890.123456789012345')
 
a, _, err := big.ParseFloat("123456789.123456789123465789", 10, 200, big.ToZero)
193
Transpose a two-dimensional matrix
Declare two two-dimensional arrays a and b of dimension n*m and m*n, respectively. Assign to b the transpose of a (i.e. the value with index interchange). 
a = np.array([[1,2], [3,4], [5,6]])
b = a.T
Alternative implementation:
a = [[1,2], [3,4], [5,6]]
b = list(map(list, zip(*a)))
Alternative implementation:
b = list(zip(*a))
194
Circular shift of a two-dimensional array
Given an array a, set b to an array which has the values of a along its second dimension shifted by n. Elements shifted out should come back at the other end. 
b = np.roll(a, m, axis=1)
195
Pass a two-dimensional array
Pass an array a of real numbers to the procedure (resp. function) foo. Output the size of the array, and the sum of all its elements when each element is multiplied with the array indices i and j (assuming they start from one). 
def foo(a):
    print(len(a))
    print(sum(
        x*(i+1) + y*(i+1)*2 for i, (x, y) in enumerate(a)
    ))
Alternative implementation:
def foo(a: list[list[float]]):
    print(len(a))
    z = 0
    for i, x in enumerate(a):
        for j, y in enumerate(x):
            z = z + (y * (i + 1))
            z = z + (y * (j + 1))
    print(z)
Alternative implementation:
def foo(a):
    z = 0
    for i, m in enumerate(a, 1):
        for j, n in enumerate(m, 1):
            z = z + (n * i * j)
    print(len(a), z)
 
func foo(a [][]int) {
	fmt.Println("array is ", len(a)[0], " x ", len(a))
	x := 0
	for i, v1 := range a {
		for j, v2 := range v1 {
			x += v2*(i+1)*(j+1)
		}
	}
	fmt.Println("result: ", x)
}
196
Pass a sub-array
Given an integer array a of size n, pass the first, third, fifth and seventh, ... up to the m th element to a routine foo which sets all these elements to 42. 
def foo(data, r):
    for i in r: 
        data[i] = 42

foo(a, range(0, m+1, 2))
Alternative implementation:
def foo(s: slice):
    global a
    m = (s.stop - 1) - s.start
    a[s] = [42] * ((m // s.step) + 1)
foo(slice(0, m + 1, 2))
197
Get a list of lines from a file
Retrieve the contents of file at path into a list of strings lines, in which each element is a line of the file. 
with open(path) as f:
    lines = f.readlines()
 
func readLines(path string) ([]string, error) {
	b, err := os.ReadFile(path)
	if err != nil {
		return nil, err
	}
	lines := strings.Split(string(b), "\n")
	return lines, nil
}
Alternative implementation:
func readLines(path string) ([][]byte, error) {
	b, err := os.ReadFile(path)
	if err != nil {
		return nil, err
	}
	lines := bytes.Split(b, []byte{'\n'})
	return lines, nil
}
198
Abort program execution with error condition
Abort program execution with error condition x (where x is an integer value) 
sys.exit(x)
 
os.Exit(x)
199
Truncate a file at the current file position
Truncate a file F at the given file position. 
F.truncate(F.tell())
 
err := os.Truncate(F, position)
200
Return hypotenuse
Compute the hypotenuse h of the triangle where the sides adjacent to the square angle have lengths x and y. 
h = math.hypot(x, y)
 
h := math.Hypot(x, y)
201
Euclidean norm
Calculate n, the Euclidean norm of data (an array or list of floating point values). 
np.linalg.norm(adata2[:, 0:3] - adata1[ipc1, 0:3], axis=1)
Alternative implementation:
n = np.linalg.norm(data)
 
func Euclidean(data []float64) float64 {
	n := 0.0
	for _, val := range data {
		n += val * val
	}
	return math.Sqrt(n)
202
Sum of squares
Calculate the sum of squares s of data, an array of floating point values. 
s = sum(i**2 for i in data)
Alternative implementation:
f = lambda x: x * x
s = sum(map(f, data))
 
var s float64
for _, d := range data {
	s += math.Pow(d, 2)
}
203
Calculate mean and standard deviation
Calculate the mean m and the standard deviation s of the list of floating point values data. 
m = statistics.mean(data)
sd = statistics.stdev(data)

m, s := stat.MeanStdDev(data, nil)
204
Return fraction and exponent of a real number
Given a real number a, print the fractional part and the exponent of the internal representation of that number. For 3.14, this should print (approximately)

0.785 2
 
print(math.frexp(a))
 
fmt.Println(math.Frexp(a))
205
Get an environment variable
Read an environment variable with the name "FOO" and assign it to the string variable foo. If it does not exist or if the system does not support environment variables, assign a value of "none". 
try:
    foo = os.environ['FOO']
except KeyError:
    foo = "none"
Alternative implementation:
foo = getenv('FOO', 'none')
Alternative implementation:
foo = os.environ.get('FOO', 'none')
 
foo, ok := os.LookupEnv("FOO")
if !ok {
	foo = "none"
}
Alternative implementation:
foo := os.Getenv("FOO")
if foo == "" {
	foo = "none"
}
206
Switch statement with strings
Execute different procedures foo, bar, baz and barfl if the string str contains the name of the respective procedure. Do it in a way natural to the language. 
switch = {'foo': foo, 
	'bar': bar, 
	'baz': baz, 
	'barfl': barfl
	}

switch_funct = switch.get(string)
if switch_funct : switch_funct()
Alternative implementation:
match str:
    case 'foo': foo()
    case 'bar': bar()
    case 'baz': baz()
    case 'barfl': barfl()
 
switch str {
case "foo":
	foo()
case "bar":
	bar()
case "baz":
	baz()
case "barfl":
	barfl()
}
207
Allocate a list that is automatically deallocated
Allocate a list a containing n elements (n assumed to be too large for a stack) that is automatically deallocated when the program exits the scope it is declared in. 
def func():
    a = [0] * n
    # local variable automatically deallocated at end of function
    return
 
a := make([]T, n)
208
Formula with arrays
Given the arrays a,b,c,d of equal length and the scalar e, calculate a = e*(a+b*c+cos(d)).
Store the results in a. 
for i in xrange(len(a)):
	a[i] = e*(a[i] + b[i] + c[i] + math.cos(a[i]))
Alternative implementation:
a = [e*(a[i] + b[i] + c[i] + math.cos(d[i])) for i in range(len(a))]
Alternative implementation:
def f(a, b, c, d):
    return e * (a + b * c + cos(d))
a = sum(map(f, a, b, c, d))
 
func applyFormula(a, b, c, d []float64, e float64) {
	for i, v := range a {
		a[i] = e * (v + b[i] + c[i] + math.Cos(d[i]))
	}
}
209
Type with automatic deep deallocation
Declare a type t which contains a string s and an integer array n with variable size, and allocate a variable v of type t. Allocate v.s and v.n and set them to the values "Hello, world!" for s and [1,4,9,16,25], respectively. Deallocate v, automatically deallocating v.s and v.n (no memory leaks). 
class T:
    def __init__(self, s, n):
        self.s = s
        self.n = n
        return

v = T('hello world', [1, 4,  9, 16, 25])
del v

type t struct {
	s string
	n []int
}

v := t{
	s: "Hello, world!",
	n: []int{1, 4, 9, 16, 25},
}
210
Compiler version and options
Assign, at runtime, the compiler version and the options the program was compiled with to variables version and options, respectively, and print them. For interpreted languages, substitute the version of the interpreter.

Example output:

GCC version 10.0.0 20190914 (experimental)
-mtune=generic -march=x86-64
 
version = sys.version
options = sys.flags
 
version := runtime.Version()
211
Create folder
Create the folder at path on the filesystem 
os.mkdir(path)
 
err := os.Mkdir(path, os.ModeDir)
Alternative implementation:
err := os.MkdirAll(path, os.ModeDir)
212
Check if folder exists
Set the boolean b to true if path exists on the filesystem and is a directory; false otherwise. 
b = os.path.isdir(path)
 
info, err := os.Stat(path)
b := !os.IsNotExist(err) && info.IsDir()
213
Case-insensitive string compare
Compare four strings in pair-wise variations. The string comparison can be implemented with an equality test or a containment test, must be case-insensitive and must apply Unicode casefolding. 
strings = ['ᾲ στο διάολο', 
           'ὰι στο διάολο', 
           'Ὰͅ ΣΤΟ ΔΙΆΟΛΟ', 
           'ᾺΙ ΣΤΟ ΔΙΆΟΛΟ']

for a, b in itertools.combinations(strings, 2):
    print(a, b, a.casefold() == b.casefold())
214
Pad string on the right
Append extra character c at the end of string s to make sure its length is at least m.
The length is the number of characters, not the number of bytes. 
s = s.ljust(m, c)
Alternative implementation:
s = '{:{}<{}}'.format(s, c, m)
 
if n := utf8.RuneCountInString(s); n < m {
	s += strings.Repeat(c, m-n)
}
215
Pad string on the left
Prepend extra character c at the beginning of string s to make sure its length is at least m.
The length is the number of characters, not the number of bytes. 
s = s.rjust(m, c)
Alternative implementation:
s = '{:{}>{}}'.format(s, c, m)
 
if n := utf8.RuneCountInString(s); n < m {
	s = strings.Repeat(c, m-n) + s
}
216
Pad a string on both sides
Add the extra character c at the beginning and ending of string s to make sure its length is at least m.
After the padding the original content of s should be at the center of the result.
The length is the number of characters, not the number of bytes.

E.g. with s="abcd", m=10 and c="X" the result should be "XXXabcdXXX". 
s = s.center(m, c)
Alternative implementation:
s = '{:{}^{}}'.format(s, c, m)
 
n := utf8.RuneCountInString(s)
if n < m {
	nleft := (m - n) / 2
	nright := (m - n) - nleft
	left, right := strings.Repeat(string(c), nleft), strings.Repeat(string(c), nright)
	s = left + s + right
}
217
Create a Zip archive
Create a zip-file with filename name and add the files listed in list to that zip-file. 
with zipfile.ZipFile(name, 'w', zipfile.ZIP_DEFLATED) as zip:
    for file in list_:
        zip.write(file)
 
buf := new(bytes.Buffer)
w := zip.NewWriter(buf)
for _, filename := range list {
	input, err := os.Open(filename)
	if err != nil {
		return err
	}
	output, err := w.Create(filename)
	if err != nil {
		return err
	}
	_, err = io.Copy(output, input)
	if err != nil {
		return err
	}
}

err := w.Close()
if err != nil {
	return err
}

err = os.WriteFile(name, buf.Bytes(), 0777)
if err != nil {
	return err
}
218
List intersection
Create the list c containing all unique elements that are contained in both lists a and b.
c should not contain any duplicates, even if a and b do.
The order of c doesn't matter. 
c = list(set(a) & set(b))
Alternative implementation:
c = list(set(a).intersection(b))
 
seta := make(map[T]bool, len(a))
for _, x := range a {
	seta[x] = true
}
setb := make(map[T]bool, len(a))
for _, y := range b {
	setb[y] = true
}

var c []T
for x := range seta {
	if setb[x] {
		c = append(c, x)
	}
}
Alternative implementation:
func intersection[S ~[]T, T comparable](a, b S) S {
	seta := make(map[T]bool, len(a))
	for _, x := range a {
		seta[x] = true
	}
	setb := make(map[T]bool, len(b))
	for _, y := range b {
		setb[y] = true
	}

	var c S
	for x := range seta {
		if setb[x] {
			c = append(c, x)
		}
	}
	return c
}
Alternative implementation:
func intersection[S ~[]T, T comparable](a, b S) S {
	s, l := a, b
	if len(b) < len(a) {
		s, l = b, a
	}

	set := make(map[T]struct{}, len(s))
	for _, x := range s {
		set[x] = struct{}{}
	}

	c := make(S, 0, len(s))
	for _, x := range l {
		if _, found := set[x]; found {
			c = append(c, x)
			delete(set, x)
		}
	}
	return c
}
219
Replace multiple spaces with single space
Create the string t from the value of string s with each sequence of spaces replaced by a single space.

Explain if only the space characters will be replaced, or the other whitespaces as well: tabs, newlines. 
t = re.sub(' +', ' ', s)
Alternative implementation:
t: str = " ".join(s.split())
Alternative implementation:
t = ' '.join(split(' {2,}', s))
Alternative implementation:
t = ' '.join(split(r'\s{2,}', s))
 
whitespaces := regexp.MustCompile(`\s+`)
t := whitespaces.ReplaceAllString(s, " ")
220
Create a tuple value
Create t consisting of 3 values having different types.

Explain if the elements of t are strongly typed or not. 
t = (2.5, "hello", -1)
Alternative implementation:
t = tuple('abc', 123, true)
 
t := []any{
	2.5,
	"hello",
	make(chan int),
}
Alternative implementation:
a, b, c := 2.5, "hello", make(chan int)
221
Remove all non-digits characters
Create string t from string s, keeping only digit characters 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. 
t = re.sub(r"\D", "", s)
Alternative implementation:
f = lambda x: x in digits
t = ''.join(filter(f, s))
 
re := regexp.MustCompile("[^\\d]")
t := re.ReplaceAllLiteralString(s, "")
222
Find the first index of an element in list
Set i to the first index in list items at which the element x can be found, or -1 if items does not contain x. 
i = items.index(x) if x in items else -1
 
i := -1
for j, e := range items {
	if e == x {
		i = j
		break
	}
}
Alternative implementation:
i := slices.Index(items, x)
223
for else loop
Loop through list items checking a condition. Do something else if no matches are found.

A typical use case is looping through a series of containers looking for one that matches a condition. If found, an item is inserted; otherwise, a new container is created.

These are mostly used as an inner nested loop, and in a location where refactoring inner logic into a separate function reduces clarity. 
for item in items:
    if item == 'baz':
        print('found it')
        break
else:
    print('never found it')
 
for _, item := range items {
    if item == "baz" {
        fmt.Println("found it")
        goto exit
    }
}
{
    fmt.Println("not found")
}
exit:
224
Add element to the beginning of the list
Insert the element x at the beginning of the list items. 
items = [x] + items
Alternative implementation:
items.insert(0, x)
 
items = append([]T{x}, items...)
Alternative implementation:
items = append(items, x)
copy(items[1:], items)
items[0] = x
Alternative implementation:
func prepend[S ~[]T, T any](items *S, x ...T) {
	*items = append(x, *items...)
}
Alternative implementation:
func prepend[S ~[]T, T any](items *S, x ...T) {
	*items = append(*items, x...)
	copy((*items)[len(x):], *items)
	copy(*items, x)
}
225
Declare and use an optional argument
Declare an optional integer argument x to procedure f, printing out "Present" and its value if it is present, "Not present" otherwise 
def f(x=None):
    if x is None:
        print("Not present")
    else:
        print("Present", x)
 
func f(x ...int) {
	if len(x) > 0 {
		println("Present", x[0])
	} else {
		println("Not present")
	}
}
226
Delete last element from list
Remove the last element from the list items. 
items.pop()
 
items = items[:len(items)-1]
Alternative implementation:
items = slices.Delete(items, len(items)-1, len(items))
227
Copy a list
Create the new list y containing the same elements as the list x.

Subsequent modifications of y must not affect x (except for the contents referenced by the elements themselves if they contain pointers). 
y = x[:]
Alternative implementation:
y = x.copy()
 
y := make([]T, len(x))
copy(y, x)
Alternative implementation:
y := slices.Clone(x)
228
Copy a file
Copy the file at path src to dst. 
shutil.copy(src, dst)
 
func copy(dst, src string) error {
	data, err := os.ReadFile(src)
	if err != nil {
		return err
	}
	stat, err := os.Stat(src)
	if err != nil {
		return err
	}
	return os.WriteFile(dst, data, stat.Mode())
}
Alternative implementation:
func copy(dst, src string) error {
	data, err := os.ReadFile(src)
	if err != nil {
		return err
	}
	stat, err := os.Stat(src)
	if err != nil {
		return err
	}
	err = os.WriteFile(dst, data, stat.Mode())
	if err != nil {
		return err
	}
	return os.Chmod(dst, stat.Mode())
}
Alternative implementation:
func copy(dst, src string) error {
	f, err := os.Open(src)
	if err != nil {
		return err
	}
	defer f.Close()
	stat, err := f.Stat()
	if err != nil {
		return err
	}
	g, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, stat.Mode())
	if err != nil {
		return err
	}
	defer g.Close()
	_, err = io.Copy(g, f)
	if err != nil {
		return err
	}
	return os.Chmod(dst, stat.Mode())
}
229
Cancel an operation
Interrupt an ongoing processing p. 
ctx, cancel := context.WithCancel(context.Background())
go p(ctx)

somethingElse()

cancel()
230
Timeout
Cancel an ongoing processing p if it has not finished after 5s. 
ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
defer cancel()
p(ctx)
231
Test if bytes are a valid UTF-8 string
Set b to true if the byte sequence s consists entirely of valid UTF-8 character code points, false otherwise. 
try:
    s.decode('utf8')
    b = True
except UnicodeError:
    b = False


b := utf8.Valid(s)
232
Read a command line boolean flag
Print "verbose is true" if the flag -v was passed to the program command line, "verbose is false" otherwise. 
parser = argparse.ArgumentParser()
parser.add_argument('-v', action='store_true', dest='verbose')
args = parser.parse_args()
print('verbose is', args.verbose)

var verbose = flag.Bool("v", false, "verbose")
flag.Parse()
fmt.Println("verbose is", *verbose)
233
Read a command line string flag
Print the value of the flag -country passed to the program command line, or the default value "Canada" if no such flag was passed. 
parser = argparse.ArgumentParser()
parser.add_argument('-country', default='Canada', dest='country')
args = parser.parse_args()
print('country is', args.country)

var country = flag.String("country", "Canada", "user home country")
flag.Parse()
fmt.Println("country is", *country)
234
Encode bytes to base64
Assign to the string s the standard base64 encoding of the byte array data, as specified by RFC 4648. 
b = base64.b64encode(data)
s = b.decode()
 
s := base64.StdEncoding.EncodeToString(data)
235
Decode base64
Assign to byte array data the bytes represented by the base64 string s, as specified by RFC 4648. 
data = base64.decode(s)
 
data, err := base64.StdEncoding.DecodeString(s)
236
Large quotient
Initialize a quotient q = a/b of arbitrary precision. a and b are large integers. 
q = fractions.Fraction(a, b)
 
q := new(big.Rat)
q.SetString(str)
Alternative implementation:
q := new(big.Rat)
q.SetFrac(a, b)
Alternative implementation:
q := big.NewRat(a, b)
237
Xor integers
Assign to c the result of (a xor b) 
c = a ^ b
 
c := a ^ b
Alternative implementation:
c := new(big.Int)
c.Xor(a, b)
238
Xor byte arrays
Write in a new byte array c the xor result of byte arrays a and b.

a and b have the same size. 
c = bytes([aa ^ bb for aa, bb in zip(a, b)])
Alternative implementation:
c = list(map(xor, a, b))
 
c := make([]byte, len(a))
for i := range a {
	c[i] = a[i] ^ b[i]
}
Alternative implementation:
var c T
for i := range a {
	c[i] = a[i] ^ b[i]
}
239
Find first regular expression match
Assign to string x the first word of string s consisting of exactly 3 digits, or the empty string if no such match exists.

A word containing more digits, or 3 digits as a substring fragment, must not match. 
m = re.search(r'\b\d\d\d\b', s)
x = m.group(0) if m else ''
 
re := regexp.MustCompile(`\b\d\d\d\b`)
x := re.FindString(s)
240
Sort 2 lists together
Lists a and b have the same length. Apply the same permutation to a and b to have them sorted based on the values of a. 
temp = list(zip(a, b))
temp.sort(key=operator.itemgetter(0))
a, b = zip(*work)
 
type sorter struct {
	k []K
	t []T
}

func (s *sorter) Len() int {
	return len(s.k)
}

func (s *sorter) Swap(i, j int) {
	s.k[i], s.k[j] = s.k[j], s.k[i]
	s.t[i], s.t[j] = s.t[j], s.t[i]
}

func (s *sorter) Less(i, j int) bool {
	return s.k[i] < s.k[j]
}

sort.Sort(&sorter{
	k: a,
	t: b,
})
241
Yield priority to other threads
Explicitly decrease the priority of the current process, so that other execution threads have a better chance to execute now. Then resume normal execution and call the function busywork. 
runtime.Gosched()
busywork()
242
Iterate over a set
Call a function f on each element e of a set x. 
for e in x:
    f(e)
Alternative implementation:
list(map(lambda e: f(e), x))
Alternative implementation:
for e in x: f(e)
 
for e := range x {
	f(e)
}
243
Print list
Print the contents of the list or array a on the standard output. 
print(a)
 
fmt.Println(a)
244
Print a map
Print the contents of the map m to the standard output: keys and values. 
print(m)
 
fmt.Println(m)
Alternative implementation:
fmt.Printf("%q", m)
245
Print value of custom type
Print the value of object x having custom type T, for log or debug. 
print(x)
 
fmt.Println(x)
246
Count distinct elements
Set c to the number of distinct elements in the list items. 
c = len(set(items))
 
distinct := make(map[T]bool)
for _, v := range items {
	distinct[v] = true
}
c := len(distinct)
Alternative implementation:
func count[T comparable](items []T) int {
	distinct := make(map[T]bool)
	for _, v := range items {
		distinct[v] = true
	}
	return len(distinct)
}
247
Filter list in-place
Remove all the elements from list x that don't satisfy the predicate p, without allocating a new list.
Keep all the elements that do satisfy p.

For languages that don't have mutable lists, refer to idiom #57 instead. 
del_count = 0
for i in range(len(x)):
    if not p(x[i - del_count]):
        del x[i - del_count]
        del_count += 1
Alternative implementation:
x = list(filter(p, x))
 
j := 0
for i, v := range x {
	if p(v) {
		x[j] = x[i]
		j++
	}
}
x = x[:j]
Alternative implementation:
j := 0
for i, v := range x {
	if p(v) {
		x[j] = x[i]
		j++
	}
}
for k := j; k < len(x); k++ {
	x[k] = nil
}
x = x[:j]
Alternative implementation:
func Filter[S ~[]T, T any](x *S, p func(T) bool) {
	j := 0
	for i, v := range *x {
		if p(v) {
			(*x)[j] = (*x)[i]
			j++
		}
	}
	var zero T
	for k := j; k < len(*x); k++ {
		(*x)[k] = zero
	}
	*x = (*x)[:j]
}
Alternative implementation:
del := func(t *T) bool { return !p(t) }

x = slices.DeleteFunc(x, del)
248
Construct a 64-bit floating-point value
Construct the "double precision" (64-bit) floating point number d from the mantissa m, the exponent e and the sign flag s (true means the sign is negative). 
sign = -1 if s else 1
d = math.ldexp(sign*m,e)
 
if s {
	m = -m
}
d := math.Ldexp(m, e)
249
Declare and assign multiple variables
Define variables a, b and c in a concise way.
Explain if they need to have the same type. 
a, b, c = 42, 'hello', 5.0
Alternative implementation:
a, b, c = 'xyz'
Alternative implementation:
a, b, *c = '1111011'
Alternative implementation:
*a, b, c = '1111011'
Alternative implementation:
a, b, c = Decimal(1.23).as_tuple()
 
a, b, c := 42, "hello", 5.0
250
Pick a random value from a map
Choose a value x from map m.
m must not be empty. Ignore the keys. 
x = random.choice(list(m.values()))
 
func pick(m map[K]V) V {
	k := rand.Intn(len(m))
	i := 0
	for _, x := range m {
		if i == k {
			return x
		}
		i++
	}
	panic("unreachable")
}
Alternative implementation:
func pick(m map[K]V) V {
	k := rand.Intn(len(m))
	for _, x := range m {
		if k == 0 {
			return x
		}
		k--
	}
	panic("unreachable")
}
Alternative implementation:
func pick[K comparable, V any](m map[K]V) V {
	k := rand.Intn(len(m))
	i := 0
	for _, x := range m {
		if i == k {
			return x
		}
		i++
	}
	panic("unreachable")
}
Alternative implementation:
import "sync"

var mu sync.RWMutex

func pick(m map[int]any) any {
	mu.RLock()
	defer mu.RUnlock()
	for _, v := range m {
		return v
	}
	return nil
}
251
Parse binary digits
Extract integer value i from its binary string representation s (in radix 2)
E.g. "1101" -> 13 
i = int(s, 2)
 
i, err := strconv.ParseInt(s, 2, 0)
252
Conditional assignment
Assign to the variable x the string value "a" if calling the function condition returns true, or the value "b" otherwise. 
x = "a" if condition() else "b"
 
if condition() {
	x = "a"
} else {
	x = "b"
}
253
Print stack trace
Print the stack frames of the current execution thread of the program. 
for frame in inspect.stack():
    print(frame)

debug.PrintStack()
254
Replace value in list
Replace all exact occurrences of "foo" with "bar" in the string list x 
for i, v in enumerate(x):
  if v == "foo":
    x[i] = "bar"
Alternative implementation:
x = ["bar" if v=="foo" else v for v in x]
Alternative implementation:
f = lambda x: 'bar' if x == 'foo' else x
x = list(map(f, x))
 
for i, v := range x {
	if v == "foo" {
		x[i] = "bar"
	}
}
Alternative implementation:
func replaceAll[T comparable](s []T, old, new T) {
	for i, v := range s {
		if v == old {
			s[i] = new
		}
	}
}

replaceAll(x, "foo", "bar")
255
Print a set
Print the values of the set x to the standard output.
The order of the elements is irrelevant and is not required to remain the same next time. 
print(x)
 
for _, v := range x {
	fmt.Println(v)
}
256
Count backwards
Print the numbers 5, 4, ..., 0 (included), one line per number. 
for i in range(5, -1, -1):
    print(i)
Alternative implementation:
a = reversed(range(0, 5 + 1))
for x in a: print(x)
 
for i := 5; i >= 0; i-- {
	fmt.Println(i)
}
257
Traverse list backwards
Print each index i and value x from the list items, from the last down to the first. 
for i in range(len(items)-1, -1, -1):
    print(i, items[i])
Alternative implementation:
for i, x in enumerate(reversed(items)):
  print(f'{i} {x}')
Alternative implementation:
a = list(enumerate(items))
for e in reversed(a): print(e)
 
for i := len(items) - 1; i >= 0; i-- {
	x := items[i]
	fmt.Printf("Item %d = %v \n", i, x)
}
258
Convert list of strings to list of integers
Convert the string values from list a into a list of integers b. 
b = [int(elem) for elem in a]
Alternative implementation:
b = list(map(int, a))
 
b := make([]int, len(a))
var err error
for i, s := range a {
	b[i], err = strconv.Atoi(s)
	if err != nil {
		return err
	}
}
259
Split on several separators
Build the list parts consisting of substrings of the input string s, separated by any of the characters ',' (comma), '-' (dash), '_' (underscore). 
parts = re.split('[,_\-]', s)
Alternative implementation:
z = escape(',-_')
parts = split(f'[{z}]', s)
 
re := regexp.MustCompile("[,\\-_]")
parts := re.Split(s, -1)
260
Create an empty list of strings
Declare a new list items of string elements, containing zero elements 
items = []
 
var items []string
261
Format time hours-minutes-seconds
Assign to the string x the value of fields (hours, minutes, seconds) of the date d, in format HH:MM:SS. 
d = datetime.datetime.now()
x = d.strftime('%H:%M:%S')
 
x := d.Format("15:04:05")
262
Count trailing zero bits
Assign to t the number of trailing 0 bits in the binary representation of the integer n.

E.g. for n=112, n is 1110000 in base 2 ⇒ t=4 
t = bin(n)[::-1].find('1')
Alternative implementation:
b = len(s := format(n, 'b'))
t = b - len(s.rstrip('0'))
 
t := bits.TrailingZeros(n)
263
Integer logarithm in base 2
Write two functions log2d and log2u, which calculate the binary logarithm of their argument n rounded down and up, respectively. n is assumed to be positive. Print the result of these functions for numbers from 1 to 12. 
def log2d(n):
    return math.floor(math.log2(n))

def log2u(n):
    return math.ceil(math.log2(n))

for n in range(1, 13):
    print(n, log2d(n), log2u(n))
Alternative implementation:
log2d = lambda x: floor(log2(x))
log2u = lambda x: ceil(log2(x))
for i in range(1, 13):
    print(i, log2d(i), log2u(i))
264
Automated passing of array bounds
Pass a two-dimensional integer array a to a procedure foo and print the size of the array in each dimension. Do not pass the bounds manually. Call the procedure with a two-dimensional array. 
def foo(a):
    print(len(a), len(a[0]))
    return


a = [[1,2,3], [4,5,6]]
foo(a)
Alternative implementation:
foo = lambda x: \
    [print(len(y)) for y in x]
foo(a)
265
Even parity bit
Calculate the parity p of the integer variable i : 0 if it contains an even number of bits set, 1 if it contains an odd number of bits set. 
p = bin(i).count('1') % 2
266
Repeated string
Assign to the string s the value of the string v repeated n times, and write it out.

E.g. v="abc", n=5 ⇒ s="abcabcabcabcabc" 
s = v * n
Alternative implementation:
s = ''.join(repeat(v, n))
 
s := strings.Repeat(v, n)
fmt.Println(s)
267
Pass string to argument that can be of any type
Declare an argument x to a procedure foo that can be of any type. If the type of the argument is a string, print it, otherwise print "Nothing."

Test by passing "Hello, world!" and 42 to the procedure. 
def foo(x):
    if isinstance(x, str):
        print(x)
    else:
        print('Nothing.')
    return

foo('Hello, world!')
foo(42)

func foo(x any) {
	if s, ok := x.(string); ok {
		fmt.Println(s)
	} else {
		fmt.Println("Nothing.")
	}
}

func main() {
	foo("Hello, world!")
	foo(42)
}
268
User-defined operator
Define a type vector containing three floating point numbers x, y, and z. Write a user-defined operator x that calculates the cross product of two vectors a and b. 
class Vector:
    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z
        return

    def __mul__(self, other):
        return Vector(self.y * other.z - self.z * other.y,
                      self.z * other.x - self.x * other.z,
                      self.x * other.y - self.y * other.x)

result = a * b
Alternative implementation:
class Vector:
    def __init__(self, x, y, z):
        self.a = x, y, z
    def __getitem__(self, item):
        return self.a[item]
    def __mul__(self, b):
        a = self.a
        return {
            'x': a[1] * b[2] - a[2] * b[1],
            'y': a[2] * b[0] - a[0] * b[2],
            'z': a[0] * b[1] - a[1] * b[0]
        }
a = Vector(.1, .2, .3)
b = Vector(.4, .5, .6)
x = a * b
269
Enum to String
Given the enumerated type t with 3 possible values: bike, car, horse.
Set the enum value e to one of the allowed values of t.
Set the string s to hold the string representation of e (so, not the ordinal value).
Print s. 
e = T.horse
s = e.name
print(s)
270
Test for quiet or signaling NaN
Given a floating point number r1 classify it as follows:
If it is a signaling NaN, print "This is a signaling NaN."
If it is a quiet NaN, print "This s a quiet NaN."
If it is not a NaN, print "This is a number."
 
if math.isnan(r1):
    print('This is a quiet NaN.')
else:
    print('This is a number.')
Alternative implementation:
d = Decimal(r1)
if d.is_snan(): print('This is a signaling NaN.')
if d.is_qnan(): print('This s a quiet NaN.')
if not d.is_nan(): print('This is a number.')
271
Test for type extension
If a variable x passed to procedure tst is of type foo, print "Same type." If it is of a type that extends foo, print "Extends type." If it is neither, print "Not related." 
def tst(x):
    if type(x) == foo:
        print("Same type.")
    elif isinstance(x, foo):
        print("Extends type.")
    else:
        print("Not related.")
272
Play FizzBuzz
Fizz buzz is a children's counting game, and a trivial programming task used to affirm that a programmer knows the basics of a language: loops, conditions and I/O.

The typical fizz buzz game is to count from 1 to 100, saying each number in turn. When the number is divisible by 3, instead say "Fizz". When the number is divisible by 5, instead say "Buzz". When the number is divisible by both 3 and 5, say "FizzBuzz" 
for i in range(1,101):
    if i % 15 == 0:
        print("FizzBuzz")
    elif i % 3 == 0:
        print("Fizz")
    elif i % 5 == 0:
        print("Buzz")
    else:
        print(i)
Alternative implementation:
n=1
while(n<=100):
    out=""
    if(n%3==0):
        out=out+"Fizz"
    if(n%5==0):
        out=out+"Buzz"
    if(out==""):
        out=out+str(n)
    print(out)
    n=n+1
Alternative implementation:
for i in range(100, 1):
    if i % 5 == 0 and not i % 3 == 0:
        print(i, "Buzz");
    if i % 3 == 0 and not i % 5 == 0:
        print(i, "Fizz");
    if i % 3 == 0 and i % 5 == 0:
        print(i, "FizzBuzz");
Alternative implementation:
for i in range(1, 100+1):
    out = ""
    if i % 3 == 0:
        out += "Fizz"
    if i % 5 == 0:
        out += "Buzz"
    print(out or i)
 
for n:=1; n<=100; n++ {
	    out:=""
	    if n%3==0 {
		    out=out+"Fizz"
	    }
	    if n%5==0 {
		    out=out+"Buzz"
	    }
	    if out=="" {
		    out=out+strconv.Itoa(n)
	    }
	    fmt.Println(out)
    }
273
Check if folder is empty
Set the boolean b to true if the directory at filepath p is empty (i.e. doesn't contain any other files and directories) 
b = os.listdir(p) == []
 
dir, err := os.Open(p)
if err != nil {
	panic(err)
}
defer dir.Close()
_, err = dir.Readdirnames(1)
b := err == io.EOF
274
Remove all white space characters
Create the string t from the string s, removing all the spaces, newlines, tabulations, etc. 
t = re.sub('\\s', '', s)
Alternative implementation:
t = ''.join(s.split())
Alternative implementation:
f = lambda x: x not in whitespace
t = ''.join(filter(f, s))
 
t := strings.Map(func(r rune) rune {
	if unicode.IsSpace(r) {
		return -1
	}
	return r
}, s)
275
Binary digits to byte array
From the string s consisting of 8n binary digit characters ('0' or '1'), build the equivalent array a of n bytes.
Each chunk of 8 binary digits (2 possible values per digit) is decoded into one byte (256 possible values). 
n = (len(s) - 1) // 8 + 1
a = bytearray(n)
for i in range(n):
    b = int(s[i * 8:(i + 1) * 8], 2)
    a[i] = b
Alternative implementation:
f = lambda x: int(s[x:x + 8], 2)
a = range(0, len(s), 8)
a = list(map(f, a))
Alternative implementation:
f = lambda x: int(''.join(x), 2)
a = list(map(f, batched(s, 8)))
Alternative implementation:
p = re.findall('.{8}', s)
f = lambda x: int(x, 2)
a = list(map(f, p))
 
n := len(s) / 8
a := make([]byte, n)
for i := range a {
	b, err := strconv.ParseInt(s[i*8:i*8+8], 2, 0)
	if err != nil {
		log.Fatal(err)
	}
	a[i] = byte(b)
}
276
Insert an element in a set
Insert an element e into the set x. 
x.add(e)
 
x[e] = struct{}{}
Alternative implementation:
x[e] = true
277
Remove an element from a set
Remove the element e from the set x.

Explains what happens if e was already absent from x. 
x.remove(e)
 
delete(x, e)
Alternative implementation:
delete(x, e)
278
Read one line from the standard input
Read one line into the string line.

Explain what happens if EOF is reached. 
line = sys.stdin.readline()
 
s := bufio.NewScanner(os.Stdin)
if ok := s.Scan(); !ok {
	log.Fatal(s.Err())
}
line := s.Text()
279
Read list of strings from the standard input
Read all the lines (until EOF) into the list of strings lines. 
lines = sys.stdin.readlines()
 
var lines []string
s := bufio.NewScanner(os.Stdin)
for s.Scan() {
	line := s.Text()
	lines = append(lines, line)
}
if err := s.Err(); err != nil {
	log.Fatal(err)
}
280
Filter map
Remove all the elements from the map m that don't satisfy the predicate p.
Keep all the elements that do satisfy p.

Explain if the filtering happens in-place, i.e. if m is reused or if a new map is created. 
m = {k:v for k, v in m.items() if p(v)}
Alternative implementation:
for k in list(m):
    if p(m[k]): m.pop(k)
Alternative implementation:
m = dict(filter(p, m.items()))
 
for k, v := range m {
	if !p(v) {
		delete(m, k)
	}
}
Alternative implementation:
maps.DeleteFunc(m, func(k K, v V) bool {
	return !p(v)
})
281
Use a Point as a map key
You have a Point with integer coordinates x and y. Create a map m with key type Point (or equivalent) and value type string. Insert "Hello" at position (42, 5). 
m = dict()
p = Point(x=42, y=5)
m[p] = 'Hello'
Alternative implementation:
Point = namedtuple('Point', 'x y')

p = Point(42, 5)

m = {p: "Hello"}
Alternative implementation:
m = {Point(42, 5): 'Hello'}
 
m := map[Point]string{}
p := Point{x: 42, y: 5}
m[p] = "Hello"
282
Use a custom type as map key
Declare a type Foo, and create a new map with Foo as key type.

Mention the conditions on Foo required to make it a possible map key type. 
class Foo:
    def __init__(self, x, y):
        self.x = x
        self.y = y
    def __hash__(self):
        return hash((Foo, self.x, self.y))
    def __eq__(self, other):
        return (self.x, self.y) == (other.x, other.y)

foo = Foo(1, 2)
m = {foo: 'hello'}
 
type Foo struct {
	name string
	x, y int
}

m := make(map[Foo]string)
283
Split with a custom string separator
Build the list parts consisting of substrings of input string s, separated by the string sep. 
parts = s.split(sep)
 
parts := strings.Split(s, sep)
284
Create a zeroed list of integers
Create a new list a (or array, or slice) of size n, where all elements are integers initialized with the value 0. 
a = [0] * n
Alternative implementation:
a = list(repeat(0, n))
 
a := make([]int, n)
285
Set variable to NaN
Given two floating point variables a and b, set a to a to a quiet NaN and b to a signalling NaN. Use standard features of the language only, without invoking undefined behavior. 
a = float('nan')
286
Iterate over characters of a string
Print a line "Char i is c" for each character c of the string s, where i is the character index of c in s (not the byte index).

Make sure that multi-byte characters are properly handled, and count for a single character. 
for i, c in enumerate(s):
    print(f'Char {i} is {c}')
Alternative implementation:
f = lambda i, c: f'Char {i} is {c}'
for e in enumerate(s): print(f(*e))
 
i := 0
for _, c := range s {
	fmt.Printf("Char %d is %c\n", i, c)
	i++
}
287
Number of bytes of a string
Assign to n the number of bytes in the string s.

This can be different from the number of characters. If n includes more bytes than the characters per se (trailing zero, length field, etc.) then explain it. One byte is 8 bits. 
n = len(s.encode('utf8'))
 
n := len(s)
288
Check if set contains a value
Set the boolean b to true if the set x contains the element e, false otherwise. 
b = e in x
 
b := x[e]
Alternative implementation:
_, b := x[e]
289
Concatenate two strings
Create the string s by concatenating the strings a and b. 
s = a + b
Alternative implementation:
s = f'{a}{b}'
 
s := a + b
290
Sort sublist
Sort the part of the list items from index i (included) to index j (excluded), in place, using the comparator c.

Elements before i and after j must remain unchanged. 
items[i:j] = sorted(items[i:j], key=functools.cmp_to_key(c))
Alternative implementation:
items[i:j] = sorted(items[i:j], key=c)
 
sub := items[i:j]
sort.Slice(sub, func(a, b int) bool {
	return c(sub[a], sub[b])
})
Alternative implementation:
slices.SortFunc(items[i:j], c)
291
Remove sublist
Delete all the elements from index i (included) to index j (excluded) from the list items. 
items[i:j] = []
Alternative implementation:
del items[i:j]
 
copy(items[i:], items[j:])
for k, n := len(items)-j+i, len(items); k < n; k++ {
	items[k] = nil
}
items = items[:len(items)-j+i]
Alternative implementation:
items = append(items[:i], items[j:]...)
Alternative implementation:
items = slices.Delete(items, i, j)
292
Write "Ni Hao" in Chinese to standard output in UTF-8
Write "Hello World and 你好" to standard output in UTF-8.
 
print('Hello World and \u4f60\u597d')
Alternative implementation:
print('Hello World and 你好')

fmt.Println("Hello World and 你好")
293
Create a stack
Create a new stack s, push an element x, then pop the element into the variable y. 
s = []
s.append(x)
y = s.pop()
 
type Stack[T any] struct {
	items []T
}

func (s *Stack[T]) Push(t T) {
	s.items = append(s.items, t)
}

func (s *Stack[T]) Pop() T {
	n := len(s.items)
	t := s.items[n-1]
	var zero T
	s.items[n-1] = zero
	s.items = s.items[:n-1]
	return t
}

var s = new(Stack[string])
s.Push(x)
y := s.Pop()
294
Print a comma-separated list of integers
Given an array a containing the three values 1, 12, 42, print out
"1, 12, 42" with a comma and a space after each integer except the last one.
 
a = [1, 12, 42]
print(*a, sep=', ')
Alternative implementation:
print(a[0], end='')
for i in range(1, len(a)):
    print(',', a[i], end='')
Alternative implementation:
print(', '.join(map(str, a)))
Alternative implementation:
f = lambda x, y: f'{x}, {y}'
print(reduce(f, a))
 
a := []int{1, 12, 42}

for i, j := range a {
	if i > 0 {
		fmt.Print(", ")
	}
	fmt.Print(j)
}
295
String to Enum
Given the enumerated type T, create a function TryStrToEnum that takes a string s as input and converts it into an enum value of type T.

Explain whether the conversion is case sensitive or not.
Explain what happens if the conversion fails.
 
t = T[s]
296
Replace last occurrence of substring
Assign to x2 the value of string x with the last occurrence of y replaced by z.
If y is not contained in x, then x2 has the same value as x. 
x2 = z.join(x.rsplit(y, 1))

func replaceLast(x, y, z string) (x2 string) {
	i := strings.LastIndex(x, y)
	if i == -1 {
		return x
	}
	return x[:i] + z + x[i+len(y):]
}
297
Sort a list of strings, case-insensitively
Sort the string list data in a case-insensitive manner.

The sorting must not destroy the original casing of the strings. 
data.sort(key=str.lower)

func lessCaseInsensitive(s, t string) bool {
	for {
		if len(t) == 0 {
			return false
		}
		if len(s) == 0 {
			return true
		}
		c, sizec := utf8.DecodeRuneInString(s)
		d, sized := utf8.DecodeRuneInString(t)

		lowerc := unicode.ToLower(c)
		lowerd := unicode.ToLower(d)

		if lowerc < lowerd {
			return true
		}
		if lowerc > lowerd {
			return false
		}

		s = s[sizec:]
		t = t[sized:]
	}
}

sort.Slice(data, func(i, j int) bool { return lessCaseInsensitive(data[i], data[j]) })
Alternative implementation:
sort.Slice(data, func(i, j int) bool {
	return strings.ToLower(data[i]) < strings.ToLower(data[j])
})
Alternative implementation:
slices.SortFunc(data, func(a, b string) int {
	return cmp.Compare(strings.ToLower(a), strings.ToLower(b))
})
298
Copy a map
Create the map y by cloning the map x.

y is a shallow copy, not a deep copy. 
y = x.copy()
 
y := make(map[K]V, len(x))
for k, v := range x {
	y[k] = v
}
Alternative implementation:
y := maps.Clone(x)
299
Comment out a single line
Write a line of comments.

This line will not be compiled or executed. 
# This is a comment
 
// This is a comment
301
Recursive Fibonacci sequence
Compute the Fibonacci sequence of n numbers using recursion.

Note that naive recursion is extremely inefficient for this task. 
def fib(n):
    if n < 0:
        raise ValueError
    if n < 2:
        return n
    return fib(n - 2) + fib(n - 1)
        
func fibonacci(n int) int {
	if n <= 1 {
		return n
	}
	return fibonacci(n-1) + fibonacci(n-2)
}
302
String interpolation
Given the integer x = 8, assign to the string s the value "Our sun has 8 planets", where the number 8 was evaluated from x. 
s = f'Our sun has {x} planets'
Alternative implementation:
s = 'Our sun has {} planets'.format(x)
Alternative implementation:
s = 'Our sun has %s planets' % x
 
s := fmt.Sprintf("Our sun has %d planets", x)
303
Array with non-default lower bound
Declare an array a of integers with six elements, where the first index is 42 and consecutive elements have the indices 43, 44, 45, 46, 47. 
a = array.array("i", range(42,48))
304
Encode string into UTF-8 bytes
Create the array of bytes data by encoding the string s in UTF-8. 
data = s.encode('utf8')
 
data := []byte(s)
305
Calculate exponentiation of real numbers
Compute and print a^b, and a^n, where a and b are floating point numbers and n is an integer.
 
print(a ** b, a ** n)
306
Ensure list capacity
Preallocate memory in the list x for a minimum total capacity of 200 elements.

This is not possible in all languages. It is only meant as a performance optimization, should not change the length of x, and should not have any effect on correctness. 
if cap(x) < 200 {
	y := make([]T, len(x), 200)
	copy(y, x)
	x = y
}
Alternative implementation:
x = slices.Grow(x, 200)
307
XOR encrypt/decrypt string
Create a function that XOR encrypts/decrypts a string 
def xor(data, key):
	return ''.join(chr(ord(x)^ord(y)) for x,y in zip(data, cycle(key)))
308
Integer to string in base b
Create the string representation s of the integer value n in base b.

18 in base 3 -> "200"
26 in base 5 -> "101"
121 in base 12 -> "a1"

 
def int_to_base_str(n, b):
    digits = '0123456789abcdefghijklmnopqrstuvwxyz'
    s = ''
    if n == 0: return '0'
    while n:
        n, remainder = divmod(n, b)
        s = digits[remainder] + s
    return s
Alternative implementation:
s = numpy.base_repr(n, b)
 
s := strconv.FormatInt(int64(n), b)
309
Clone a 2D array
Create the new 2-dimensional array y containing a copy of the elements of the 2-dimensional array x.

x and y must not share memory. Subsequent modifications of y must not affect x. 
y = copy.deepcopy(x)
 
buf := make([]T, m*n)
y = make([][]T, m)
for i := range y {
	y[i] = buf[:n:n]
	buf = buf[n:]
	copy(y[i], x[i])
}
Alternative implementation:
func clone2D[M ~[][]T, T any](in M) (out M) {
	if len(in) == 0 {
		return nil
	}

	m, n := len(in), len(in[0])

	buf := make([]T, m*n)

	out = make(M, m)
	for i := range out {
		out[i] = buf[:n:n]
		buf = buf[n:]
		copy(out[i], in[i])
	}
	return out
}
310
Fill array with random bytes
Fill the byte array a with randomly generated bytes. 
a[:] = random.randbytes(len(a))
Alternative implementation:
a = random.randbytes(N)
 
_, err := rand.Read(a)
311
Deep copy an object
Create the new object y by cloning the all the contents of x, recursively. 
y = copy.deepcopy(x)
312
List equality
Set b to true if the lists p and q have the same size and the same elements, false otherwise. 
b = p == q
 
b := slices.Equal(p, q)
313
Map equality
Set b to true if the maps m and n have the same key/value entries, false otherwise. 
b = m == n
 
b := maps.Equal(m, n)
314
Fill array with value
Set all the elements in the array x to the same value v 
x[:] = [v] * len(x)
 
for i := range x {
	x[i] = v
}
Alternative implementation:
func fill[T any](x []T, v T) {
	for i := range x {
		x[i] = v
	}
}
315
Memoization
Given any function f, create an object or function m that stores the results of f, and calls f only on inputs for which the result is not stored yet. 
@functools.lru_cache
def m(*args):
    return f(*args)


func memoize[T comparable, U any](f func(T) U) func(T) U {
	memory := make(map[T]U)

	return func(t T) U {
		if u, seen := memory[t]; seen {
			return u
		}
		u := f(t)
		memory[t] = u
		return u
	}
}
316
Count occurrences in a list
Determine the number c of elements in the list x that satisfy the predicate p. 
c = sum(p(v) for v in x)
 
c := 0
for _, v := range x {
	if p(v) {
		c++
	}
}
Alternative implementation:
func count[T any](x []T, p func(T) bool) int {
	c := 0
	for _, v := range x {
		if p(v) {
			c++
		}
	}
	return c
}
317
Random string
Create a string s of n characters having uniform random values out of the 62 alphanumeric values A-Z, a-z, 0-9 
alphanum = string.ascii_letters + string.digits
s = ''.join(random.choices(alphanum, k=n))
Alternative implementation:
a = list(s.ascii_letters + s.digits)
r.shuffle(a)
s = ''.join(a[:n])
 
const alphanum = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"

func randomString(n int) string {
	a := make([]byte, n)
	for i := range a {
		a[i] = alphanum[rand.Intn(len(alphanum))]
	}
	return string(a)
}
Alternative implementation:
const alphanum = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"

func randomString(n int, rng *rand.Rand) string {
	a := make([]byte, n)
	for i := range a {
		a[i] = alphanum[rng.Intn(len(alphanum))]
	}
	return string(a)
}
Alternative implementation:
var alphanum = []rune("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789")

func randomString(n int, rng *rand.Rand) string {
	a := make([]rune, n)
	for i := range a {
		a[i] = alphanum[rng.Intn(len(alphanum))]
	}
	return string(a)
}
318
Cryptographically secure random number
Assign to the integer x a random number between 0 and 17 (inclusive), from a crypto secure random number generator. 
x = secrets.choice(range(0, 18))
Alternative implementation:
x = secrets.randbelow(18)
 
bi, err := rand.Int(rand.Reader, big.NewInt(18))
x := int(bi.Int64())
319
Generator function
Write a function g that behaves like an iterator.
Explain if it can be used directly in a for loop. 
def g():
    for i in range(6):
        yield i
 
func generator() chan int {
	ch := make(chan int, 16)
	go func() {
		defer close(ch)
		timeout := time.After(2 * time.Minute)
		
		for i := 0; i < 1024; i++ {
			select {
			case ch <- i:
			case <-timeout:
				return
			}
		}
	}()
	return ch
}
320
Test if string is empty
Set b to true if the string s is empty, false otherwise 
b = s == ''
Alternative implementation:
b = not s
 
b := s == ""
321
Access character in string, by index
Assign to c the value of the i-th character of the string s.

Make sure to properly handle multi-byte characters. i is the character index, which may not be equal to the byte index. 
c = s[i]
 
c := []rune(s)[i]
322
replace value of variable with new one and return old value 
old, x = x, new
 
old, x = x, new
323
Set HTTP request header
Make an HTTP request with method GET to the URL u, with the request header "accept-encoding: gzip", then store the body of the response in the buffer data. 
response = requests.get(u, headers={'accept-encoding': 'gzip'})
data = response.content[:]
 
req, err := http.NewRequest("GET", u, nil)
if err != nil {
	return err
}
req.Header.Set("accept-encoding", "gzip")
res, err := http.DefaultClient.Do(req)
if err != nil {
	return err
}
data, err := io.ReadAll(res.Body)
res.Body.Close()
if err != nil {
	return err
}
324
Read HTTP response header
Set the string c to the (first) value of the header "cache-control" of the HTTP response res. 
c = res.headers['cache-control']
 
c := res.Header.Get("cache-control")
325
Create a queue
Create a new queue q, then enqueue two elements x and y, then dequeue an element into the variable z. 
q = queue.Queue()
q.put(x)
q.put(y)
z = q.get()
 
type Queue[T any] struct {
	items []T
}

func (q *Queue[T]) Enqueue(t T) {
	q.items = append(q.items, t)
}

func (q *Queue[T]) Dequeue() T {
	t := q.items[0]
	var zero T
	q.items[0] = zero
	q.items = q.items[1:]
	return t
}

q := new(Queue[string])
q.Enqueue(x)
q.Enqueue(y)
z := q.Dequeue()
326
Milliseconds since Unix epoch
Assign to t the number of milliseconds elapsed since 00:00:00 UTC on 1 January 1970. 
t = time.time() * 1000
 
t := time.Now().UnixMilli()
327
Convert string to lower case
Assign to t the value of the string s, with all letters mapped to their lower case. 
t = s.lower()
 
t := strings.ToLower(s)
328
Convert string to upper case
Assign to t the value of the string s, with all letters mapped to their upper case. 
t = s.upper()
 
t := strings.ToUpper(s)
329
Read value in a map
Assign to v the value stored in the map m for the key k.

Explain what happens if there is no entry for k in m. 
v = m[k]
Alternative implementation:
v = m.get(k, "default value")
Alternative implementation:
v = m.get(k)
 
v := m[k]
Alternative implementation:
v, ok := m[k]
330
Map to list
Create the list a containing all the values of the map m.

Ignore the keys of m. The order of a doesn't matter. a may contain duplicate values. 
a = list(m.values())
 
a := make([]V, 0, len(m))
for _, v := range m {
	a = append(a, v)
}
Alternative implementation:
a := maps.Values(m)
331
Clear map
Remove all entries from the map m.

Explain if other references to the same map now see an empty map as well. 
m.clear()
 
clear(m)
332
List of the keys of a map
Create the list k containing all the keys of the map m 
k = m.keys()
Alternative implementation:
k = [*m.keys()]
Alternative implementation:
k = list(m.keys())
 
k := maps.Keys(m)
Alternative implementation:
k := make([]K, 0, len(m))
for key := range m {
	k = append(k, key)
}
333
Pretty-print object in JSON
Print the object x in human-friendly JSON format, with newlines and indentation. 
print(json.dumps(x, indent=4))
 
buffer, err := json.MarshalIndent(x, "", "  ")
if err != nil {
	log.Fatal(err)
}
fmt.Println(string(buffer))
334
Combine 2 maps
Create the new map c containing all of the (key, value) entries of the two maps a and b.

Explain what happens for keys existing in both a and b. 
c = {**a, **b}
Alternative implementation:
c = a | b
 
c := make(M, len(a)+len(b))
for k, v := range a {
	c[k] = v
}
for k, v := range b {
	c[k] = v
}
335
List to map
Create the map m containing all the elements e of the list a, using as key the field e.id. 
m = {e.id:e for e in a}
Alternative implementation:
f = lambda x: (x.id, x)
m = dict(map(f, a))
 
m := make(map[K]V, len(a))
for _, e := range a {
	m[e.id] = e
}
336
Exponent
Compute x = b ⁿ

b raised to the power of n is equal to the product of n terms b × b × ... × b 
x = b ** n
337
Convert string to integer, explicit base
Extract the integer value i from its string representation s, in radix b 
i = int(s, b)
 
i, err := strconv.ParseInt(s, b, 0)
338
Create a bidirectional map
Create a new bidirectional map bm and insert the pair (42, "forty-two") 
bm = bidict.bidict({42: 'forty-two'})
339
Clear a byte array
Set all the elements of the byte array a to zero 
a = numpy.ones((n,), numpy.uint8)
a.fill(0)
 
clear(a[:])
Alternative implementation:
clear(a)
340
Last character of string
Assign to c the value of the last character of the string s.

Explain the type of c, and what happens if s is empty.

Make sure to properly handle multi-bytes characters. 
c = s[-1]
 
r := []rune(s)
c := r[len(r)-1]
Alternative implementation:
c, _ := utf8.DecodeLastRuneInString(s)
341
Find substring last position
Set i to the position of the last occurrence of the string y inside the string x, if exists.

Specify if i should be regarded as a character index or as a byte index.

Explain the behavior when y is not contained in x. 
i = x.rfind(y)
 
i := strings.LastIndex(x, y)
342
Leap year?
Determine if the current year is a leap year. 
if calendar.isleap(datetime.date.today().year):
    print('This year is a leap year.')
Alternative implementation:
y = date.today().year
b = not y % 4 and bool(y % 100 or not y % 400)
343
Rename file
Rename the file at path1 into path2 
os.rename(path1, path2)
 
err := os.Rename(path1, path2)
344
Extract filename extension
Assign to ext the fragment of the string f after the last dot character, or the empty string if f does not contain a dot.

E.g. "photo.jpg" -> "jpg"

ext must not contain the dot character. 
ext = os.path.splitext(f)[1][1:]
 
ext := strings.TrimPrefix(filepath.Ext(f), ".")
345
Convert string to big integer
Create the integer value i initialized from its string representation s (in radix 10)

Use an integer type that can hold huge values. Explain what happens if s cannot be parsed. 
i = int(s)
 
i := new(big.Int)
_, ok := i.SetString(s, 10)
346
Repeated list
Create the list y from the list x repeated n times 
94102
 
94102
347
Copy folder
Copy the directory path1 to path2, with all its contents 
shutil.copytree(path1, path2)
 
err := os.CopyFS(path2, os.DirFS(path1))
348
Convert a decimal value into a fraction
Parse a number, a, into a mathematical fraction, f.

For example, 0.5 is `1/2`, and 3.125 is `3 1/8`.

https://en.wikipedia.org/wiki/Fraction 
s = str(a).split('.')
i, n = map(int, s)
d = 10 ** len(str(n))
v = gcd(n, d) or 1
n, d = n // v, d // v
if not n: f = str(i)
elif i: f = f'{i} {n}/{d}'
else: f = f'{n}/{d}'
Alternative implementation:
f = Fraction(a)
349
Convert a fraction into a decimal value
Parse a value, f, into a decimal number, a.

For example, `1/2` is 0.5, and `3 1/8` is 3.125.

https://en.wikipedia.org/wiki/Fraction 
s = split('[ /]', f)
*i, n, d = map(int, s)
a = sum(i, n / d)
Alternative implementation:
a = map(Fraction, f.split())
a = sum(map(float, a))
350
Create an enumerated collection
Generate a collection of "enumerated" values.

For example, Python's `enumerate` built-in function.

https://docs.python.org/3/library/functions.html#enumerate 
e = enumerate(a)
351
Create a zipped collection
Generate a "zipped" list z of pairs of elements from the lists a, b having the same length n.

The result z will contain n pairs. 
z = zip(a, b)
352
Parallelize 3 computations
Execute the functions f1, f2, f3 concurrently, and assign their respective return values to a1, a2, a3. 
var wg sync.WaitGroup
wg.Add(3)
go func() {
	a1 = f1()
	wg.Done()
}()
go func() {
	a2 = f2()
	wg.Done()
}()
go func() {
	a3 = f3()
	wg.Done()
}()
wg.Wait()
353
Test if list is sorted
Set the boolean b to true if the list items has all its elements in increasing order, false otherwise.

The order is not strict: the ordered list may contain duplicate elements. 
b = sorted(items) == items
 
b := slices.IsSorted(items)
354
Increment map entry
Set the value for the key k in the map m to 1 if the entry doesn't exist yet in m, or increment the entry value if it already exists. 
m.update(k=m.get('k', 0) + 1)
 
m[k]++
355
Absolute value
Assign to y the absolute value of the number n 
y = abs(n)
Alternative implementation:
y = fabs(n)
 
y := math.Abs(x)
Alternative implementation:
y := new(big.Float)
y.Abs(x)
Alternative implementation:
y := new(big.Int)
y.Abs(x)
Alternative implementation:
y := x
if y < 0 {
	y = -x
}
356
Parse list of integers
Create the list of integers items for the string s containing integers separated by one or more whitespace characters (space, tab, newline). 
a = map(int, s.split())
items = list(a)
 
parts := strings.Fields(s)
items := make([]int, len(parts))
for i, part := range parts {
	v, err := strconv.Atoi(part)
	if err != nil {
		return err
	}
	items[i] = v
}