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C++ Rust
1
Print Hello World
Print a literal string on standard output 
std::cout << "Hello World" << std::endl;
Alternative implementation:
using namespace std;
cout << "Hello World";
Alternative implementation:
printf("Hello World");
Alternative implementation:
fprintf(stdout, "Hello World");
 
println!("Hello World");
2
Print Hello 10 times
Loop to execute some code a constant number of times 
for (int i = 0; i < 10; ++i)
  cout << "Hello\n";
Alternative implementation:
std::string a {"Hello"};
char const* p {a.data()};
for (int i {}; i != 10; ++i)
    printf("%s\n", p);
Alternative implementation:
char* a {"Hello"};
for (int i {}; i != 10; ++i)
    printf("%s\n", a);
 
for _ in 0..10 { println!("Hello"); }
Alternative implementation:
print!("{}", "Hello\n".repeat(10));
3
Create a procedure
Like a function which doesn't return any value, thus has only side effects (e.g. Print to standard output) 
void finish(char* name){
    cout << "My job here is done. Goodbye " << name << "\n";
}
Alternative implementation:
void f() { std::cout << "abc"; }
Alternative implementation:
auto f = [] { printf("abc"); };
 
fn finish(name: &str) {
    println!("My job here is done. Goodbye {}", name);
}
4
Create a function
Create a function which returns the square of an integer 
int square(int x){
  return x*x;
}
Alternative implementation:
int square(int& x) { return x * x; }
Alternative implementation:
auto square = [] (int x) {
    return x * x;
};
 
fn square(x : u32) -> u32 { x * x }
5
Create a 2D Point data structure
Declare a container type for two floating-point numbers x and y 
struct Point{
  double x;
  double y;
};
Alternative implementation:
struct point { double x, y; };
point p { .x = 1.2, .y = 3.4 };
Alternative implementation:
class point {
    public:
        double x, y;
        point(double x, double y) {
            this->x = x;
            this->y = y;
        }
};
Alternative implementation:
namespace point {
    double x, y;
}
 
struct Point {
    x: f64,
    y: f64,
}
Alternative implementation:
struct Point(f64, f64);
6
Iterate over list values
Do something with each item x of the list (or array) items, regardless indexes. 
for(const auto &x : items) {
	doSomething(x);
}
 
for x in items {
	do_something(x);
}
Alternative implementation:
items.into_iter().for_each(|x| do_something(x));
7
Iterate over list indexes and values
Print each index i with its value x from an array-like collection items 
std::size_t i = 0;
for(const auto & x: items) {
  std::cout << "Item " << i++ << " = " << x << std::endl;
}
Alternative implementation:
for (std::size_t ix = 0; ix < items.size(); ++ix) {
  std::cout << "Item " << ix << " = " << items[ix] << std::endl;
}
Alternative implementation:
for (auto const [i, x] : std::views::enumerate(items)) {
  std::cout << i << ": " << x << '\n';
}
Alternative implementation:
for (int i {}; auto& x : items)
    printf("%d, %s\n", i++, x);
 
for (i, x) in items.iter().enumerate() {
    println!("Item {} = {}", i, x);
}
Alternative implementation:
items.iter().enumerate().for_each(|(i, x)| {
    println!("Item {} = {}", i, x);
})
8
Create a map (associative array)
Create a new map object x, and provide some (key, value) pairs as initial content. 
std::map<const char*, int> x;
x["one"] = 1;
x["two"] = 2;
Alternative implementation:
std::unordered_map<std::string, double> mymap = {
    {"mom", 5.4},
    {"dad", 6.1},
    {"bro", 5.9}
};
Alternative implementation:
using namespace std;
map<char, int> m {
    { 'x', 1 },
    { 'y', 2 }
};
Alternative implementation:
using namespace std;
pair a { 'x', 1 },
     b { 'y', 2 };
map m { a, b };
 
let mut x = BTreeMap::new();
x.insert("one", 1);
x.insert("two", 2);
Alternative implementation:
let x: HashMap<&str, i32> = [
    ("one", 1),
    ("two", 2),
].into_iter().collect();
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. 
struct binary_tree
{
 int data;
 binary_tree *left = nullptr, *right = nullptr;
};
 
struct BinTree<T> {
    value: T,
    left: Option<Box<BinTree<T>>>,
    right: Option<Box<BinTree<T>>>,
}
10
Shuffle a list
Generate a random permutation of the elements of list x 
std::random_shuffle(x.begin(), x.end());
 
let mut rng = StdRng::new().unwrap();
rng.shuffle(&mut x);
Alternative implementation:
let mut rng = thread_rng();
x.shuffle(&mut rng);
Alternative implementation:
pub fn shuffle<T>(vec: &mut [T]) {
    let n = vec.len();
    for i in 0..(n - 1) {
        let j = rand() % (n - i) + i;
        vec.swap(i, j);
    }
}

pub fn rand() -> usize {
    RandomState::new().build_hasher().finish() as usize
}
11
Pick a random element from a list
The list x must be non-empty. 
std::mt19937 gen;
std::uniform_int_distribution<size_t> uid (0, x.size () - 1);
x[uid (gen)];
Alternative implementation:
std::ranges::sample(x, &result, 1, std::mt19937{std::random_device{}()});
 
x[rand::thread_rng().gen_range(0..x.len())]
Alternative implementation:
let mut rng = rand::thread_rng();
let choice = x.choose(&mut rng).unwrap();
12
Check if list contains a value
Check if the list contains the value x.
list is an iterable finite container. 
bool Contains(const std::vector<int> &list, int x)
{
	return std::find(list.begin(), list.end(), x) != list.end();
}
Alternative implementation:
auto contains(auto list, auto x) -> bool {
  return std::ranges::find(list, x) != std::ranges::end(list);
}
 
list.contains(&x);
Alternative implementation:
list.iter().any(|v| v == &x)
Alternative implementation:
(&list).into_iter().any(|v| v == &x)
Alternative implementation:
list.binary_search(&x).is_ok()
13
Iterate over map keys and values
Access each key k with its value x from an associative array mymap, and print them. 
for (const auto& kx: mymap) {
	std::cout << "Key: " << kx.first << " Value: " << kx.second << std::endl;
}
Alternative implementation:
for (const auto& [k, x]: mymap) {
	std::cout << "Key: " << k << " Value: " << x << '\n';
}
Alternative implementation:
for (auto entry : mymap) {
  auto k = entry.first;
  auto x = entry.second;
  std::cout << k << ": " << x << "\n";
}
Alternative implementation:
for (auto& [k, x] : mymap)
    printf("%s %s\n", k, x);
Alternative implementation:
K k;
X x;
auto i {mymap.begin()}, n {mymap.end()};
while (i not_eq n) {
    k = i->first;
    x = i->second;
    printf("%s %s\n", k, x);
    ++i;
}
 
for (k, x) in &mymap {
    println!("Key={key}, Value={val}", key=k, val=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. 
float pick(float a, float b)
{
	std::default_random_engine generator;
	std::uniform_real_distribution distribution(a, b);
	return distribution(generator);
}
 
thread_rng().gen_range(a..b);
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. 
std::mt19937 gen;
std::uniform_int_distribution<size_t> uid (a, b);
uid (gen);

fn pick(a: i32, b: i32) -> i32 {
    let between = Range::new(a, b);
    let mut rng = rand::thread_rng();
    between.ind_sample(&mut rng)
}
Alternative implementation:
Uniform::new_inclusive(a, b).sample(&mut rand::thread_rng())
16
Depth-first traversal of a binary tree
Call a function f on every node of binary tree bt, in depth-first infix order 
fn depthFirstTraverse<T>(bt: &mut BiTree<T>, f: fn(&mut BiTree<T>)) {
    if let Some(left) = &mut bt.left {
        depthFirstTraverse(left, f);
    }
    
    f(bt);
    
    if let Some(right) = &mut bt.right {
        depthFirstTraverse(right, 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. 
template<typename T>
struct Node{
  T value;
  std::vector<Node<T>> children;
};
 
struct Node<T> {
  value: T,
  children: Vec<Node<T>>,
}
18
Depth-first traversal of a tree
Call a function f on every node of a tree, in depth-first prefix order 
void dfs(const auto &tree, const auto &root)
{
	f(root);

	for (auto child : tree)
		dfs(tree, child);
}

pub struct Tree<V> {
    children: Vec<Tree<V>>,
    value: V
}

impl<V> Tree<V> {
    pub fn dfs<F: Fn(&V)>(&self, f: F) {
        self.dfs_helper(&f);
    }
    fn dfs_helper<F: Fn(&V)>(&self, f: &F) {
        (f)(&self.value);
        for child in &self.children {
            child.dfs_helper(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. 
std::reverse(begin(x), end(x));
 
let y: Vec<_> = x.into_iter().rev().collect();
Alternative implementation:
x.reverse();
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. 
template<typename T, size_t len_x, size_t len_y>
std::pair<size_t, size_t> search (const T (&m)[len_x][len_y], const T &x) {
    for(size_t pos_x = 0; pos_x < len_x; ++pos_x) {
        for(size_t pos_y = 0; pos_y < len_y; ++pos_y) {
            if(m[pos_x][pos_y] == x) {
                return std::pair<size_t, size_t>(pos_x, pos_y);
            }
        }
    }

    // return an invalid value if not found
    return std::pair<size_t, size_t>(len_x, len_y);
}
Alternative implementation:
bool _search(const Matrix& _m, const float _x, size_t* const out_i, size_t* const out_j) {
  for (size_t j = 0; j < _m.rows; j++) {
    for (size_t i = 0; i < _m.cols; i++) {
      if (_m.at(i, j) == _x) {
         *out_i = i;
         *out_j = j;
         return true;
      }
    }
  }
  return false;
}
 
fn search<T: Eq>(m: &Vec<Vec<T>>, x: &T) -> Option<(usize, usize)> {
    for (i, row) in m.iter().enumerate() {
        for (j, column) in row.iter().enumerate() {
            if *column == *x {
                return Some((i, j));
            }
        }
    }

    None
}
21
Swap values
Swap the values of the variables a and b
 
swap(a, b);
 
std::mem::swap(&mut a, &mut b);
Alternative implementation:
let (a, b) = (b, a);
22
Convert string to integer
Extract the integer value i from its string representation s (in radix 10) 
int i = std::atoi(s);
Alternative implementation:
int i = std::stoi(s);
Alternative implementation:
std::stringstream ss(str);
int i;
ss >> i;
Alternative implementation:
std::string s("123");
int i;
std::from_chars(s.data(), s.data() + s.size(), i, 10);
 
let i = s.parse::<i32>().unwrap();
Alternative implementation:
let i: i32 = s.parse().unwrap_or(0);
Alternative implementation:
let i = match s.parse::<i32>() {
  Ok(i) => i,
  Err(_e) => -1,
};
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. 
std::stringstream ss;
ss << std::fixed << std::setprecision(2) << x;
s = ss.str();
Alternative implementation:
std::array<char, 23> buffer;
std::string s{};
if (auto [ptr, ec] = std::to_chars(buffer.data(), buffer.data() + buffer.size(), x, std::chars_format::fixed, 2); ec == std::errc{}) {
  s = std::string(buffer.data(), ptr);
} else {
  s = std::make_error_code(ec).message();
}
Alternative implementation:
auto s = std::format("{.2f}", x);
Alternative implementation:
std::string s;
sprintf(s.data(), "%.2f", x);
 
let s = format!("{:.2}", 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) 
std::string s = u8"ネコ";
 
let 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" 
//declaration
auto mutex = std::mutex{};
auto cv = std::condition_variable{};
auto variable = std::optional<std::string>{};
auto future = std::async([&]() {
	auto lock = std::unique_lock{mutex, std::defer_lock};
	cv.wait(lock, [&variable](){ return variable.has_value(); });
	std::cout << "Hello, " << *variable << "!" << std::endl;
});

//passing value in main thread (or it can be done in others as well)
{
	auto lock = std::unique_lock{mutex};
	variable = "Alan";
	cv.notify_all();
}
Alternative implementation:
using namespace std;
string s;
atomic a {&s};
mutex x;
condition_variable c;
thread t {[&] {
    unique_lock u {x};
    c.wait(u, [&a] {
        return !a.load()->empty();
    });
    printf("Hello, %s", a.load()->data());
    c.notify_all();
}};
x.lock();
*a.load() = "Alan";
c.notify_all();
x.unlock();
t.join();
 
let (send, recv) = channel();

thread::spawn(move || {
    loop {
        let msg = recv.recv().unwrap();
        println!("Hello, {:?}", msg);
    }  
});

send.send("Alan").unwrap();
26
Create a 2-dimensional array
Declare and initialize a matrix x having m rows and n columns, containing real numbers. 
std::vector<std::vector<double>> x (m, std::vector<double>(n));
Alternative implementation:
::std::array<::std::array<int, n>, m> x;
 
let mut x = vec![vec![0.0f64; N]; M];
Alternative implementation:
let mut x = [[0.0; N] ; M];
27
Create a 3-dimensional array
Declare and initialize a 3D array x, having dimensions boundaries m, n, p, and containing real numbers. 
std::vector<std::vector<std::vector<double>>> x (m, std::vector<std::vector<double>> (n, std::vector<double> (p)));

let x = vec![vec![vec![0.0f64; p]; n]; m];
Alternative implementation:
let x = [[[0.0f64; P]; N]; M];
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. 
std::sort(begin(items), end(items), [](const auto& a, const auto& b) { return a.p < b.p; });
Alternative implementation:
struct {
    bool operator()(const Item &lhs, const Item &rhs) const { return lhs.p < rhs.p; }
} custom_sort;

std::sort(begin(items), end(items), custom_sort);
Alternative implementation:
std::ranges::sort(items, {}, &Item::p);

items.sort_by(|a,b| a.p.cmp(&b.p));
Alternative implementation:
items.sort_by_key(|x| x.p);
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. 
items.erase (items.begin () + i);

items.remove(i)
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. 
auto futures = std::vector<std::future<void>>{};
for (auto i = 1; i <= 1000; ++i)
{
	futures.emplace_back(std::async(f, i));
}
 
let threads: Vec<_> = (0..1000).map(|i| {
	thread::spawn(move || f(i))
}).collect();

for thread in threads {
	thread.join();
}
Alternative implementation:
(0..1000).into_par_iter().for_each(f);
31
Recursive factorial (simple)
Create the recursive function f which returns the factorial of the non-negative integer i, calculated from f(i-1) 
unsigned int f( unsigned int i ) {
	if ( i == 0 ) return 1;
	
	return i * f( i - 1 );
}
Alternative implementation:
using namespace std;
function<unsigned(unsigned)> f;
f = [&f] (unsigned i) {
    return not i ? 1 : i * f(i - 1);
};
Alternative implementation:
unsigned f(unsigned i) {
    return not i ? 1 : i * f(i - 1);
}
 
fn f(n: u32) -> u32 {
    if n < 2 {
        1
    } else {
        n * f(n - 1)
    }
}
Alternative implementation:
fn factorial(num: u64) -> u64 {
    match num {
        0 | 1 => 1,
        _ => factorial(num - 1) * num,
    }
}
32
Integer exponentiation by squaring
Create function exp which calculates (fast) the value x power n.
x and n are non-negative integers. 
int pow(int x, int p)
{
  if (p == 0) return 1;
  if (p == 1) return x;

  int tmp = pow(x, p/2);
  if (p%2 == 0) return tmp * tmp;
  else return x * tmp * tmp;
}
Alternative implementation:
auto exp = [] (unsigned x, unsigned n) {
    while (--n) x *= x;
    return x;
};
 
fn exp(x: u64, n: u64) -> u64 {
    match n {
        0 => 1,
        1 => x,
        i if i % 2 == 0 => exp(x * x, n / 2),
        _ => x * exp(x * x, (n - 1) / 2),
    }     
}
Alternative implementation:
fn exp(x: u64, n: u32) -> u64 {
    x.pow(n)
}
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. 
std::atomic<int> x{};

auto local_x = x.load();
while(!x.compare_exchange_strong(local_x, f(local_x))) {
	local_x = x.load();
}
Alternative implementation:
//declaration
auto mutex = std::mutex{};
auto x = someValue();

//use
{
	auto lock = std::unique_lock{mutex};
	x = f(x);
}
Alternative implementation:
using namespace std;
template <typename T>
struct object {
    T x;
    atomic<T*> p {&x};
    mutex m;
    object(T x) { set(x); }
    void set(T x) {
        lock_guard g {m};
        *p.load() = x;
    }
    T get() {
        lock_guard g {m};
        return *p.load();
    }
};
 
let mut x = x.lock().unwrap();
*x = f(x);
34
Create a set of objects
Declare and initialize a set x containing unique objects of type T. 
std::unordered_set<T> x;
Alternative implementation:
std::unordered_set<T, hasher, eq> x;
 
let x: HashSet<T> = HashSet::new();
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 
std::function<C (A)> compose(B (&f)(A), C (&g)(B))
{
    return [&f, &g](A a){return g(f(a));};
}

auto fn = compose(f, g);
Alternative implementation:
template <typename A, typename B>
auto compose(A a, B b) {
    return [=] (auto x) {
        return b(a(x));
    };
}
 
fn compose<'a, A, B, C, G, F>(f: F, g: G) -> Box<Fn(A) -> C + 'a>
	where F: 'a + Fn(A) -> B, G: 'a + Fn(B) -> C
{
	Box::new(move |x| g(f(x)))
}
Alternative implementation:
fn compose<A, B, C>(f: impl Fn(A) -> B, g: impl Fn(B) -> C) -> impl Fn(A) -> C {
    move |x| 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. 
template<typename Pred1, typename Pred2>
auto compose(Pred1 f, Pred2 g){
  return [f,g](auto x){ return g(f(x)); };	
}
 
fn compose<'a, A, B, C, G, F>(f: F, g: G) -> Box<Fn(A) -> C + 'a>
	where F: 'a + Fn(A) -> B, G: 'a + Fn(B) -> C
{
	Box::new(move |x| g(f(x)))
}
Alternative implementation:
fn compose<A, B, C>(f: impl Fn(A) -> B, g: impl Fn(B) -> C) -> impl Fn(A) -> C {
    move |x| g(f(x))
}
37
Currying
Transform a function that takes multiple arguments into a function for which some of the arguments are preset. 
// function taking many parameters
int add(int a, int b)
{
    return a + b;
}

// define a new function preseting the first parameter
std::function<int (int)> add_def(int a)
{
    return [a](int b){return add(a, b);};
}

int result = add_def(4)(6);
Alternative implementation:
//function
auto add(int a, int b) -> int {
	return a + b;
}

//curry with std::bind
using namespace std::placeholders;
auto add5 = std::bind(add, _1, 5);

//curry with lambda
auto add5 = [](int x) { return add(x, 5); };

//use
auto result = add5(1);
assert(result == 6);
 
fn add(a: u32, b: u32) -> u32 {
    a + b
}

let add5 = move |x| add(5, x);
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. 
auto t = s.substr(i, j-i);
 
let t = s.graphemes(true).skip(i).take(j - i).collect::<String>();
Alternative implementation:
let t = s.substring(i, j);
Alternative implementation:
let mut iter = s.grapheme_indices(true);
let i_idx = iter.nth(i).map(|x|x.0).unwrap_or(0);
let j_idx = iter.nth(j-i).map(|x|x.0).unwrap_or(0);
let t = s[i_idx..j_idx];
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. 
bool ok = s.find(word) != std::string::npos;
 
let ok = s.contains(word);
40
Graph with adjacency lists
Declare a Graph data structure in which each Vertex has a collection of its neighbouring vertices. 
struct Vertex {
    neighbours: Vec<Weak<Vertex>>
}

struct Graph {
    vertices: Vec<Rc<Vertex>>
}
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. 
auto t = ::std::ranges::reverse(s);
 
let t = s.chars().rev().collect::<String>();
Alternative implementation:
let t: String = s.chars().rev().collect();
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. 
auto a = {1,2,3,4,5};
auto b = {3,5};

for (auto va: a){
    for (auto vb: b){
        if (va==vb) goto OUTER;
    }
    std::cout << va << '\n';
    OUTER: continue;
}
Alternative implementation:
int x {}, y, v,
    m {a.size()}, n {b.size()};
while (x not_eq m) {
    v = a[x];
    for (y = 0; y not_eq n; ++y)
        if (b[y] == v) goto a;
    printf("%d\n", v);
    a: ++x;
}
 
'outer: for va in &a {
    for vb in &b {
        if va == vb {
            continue 'outer;
        }
    }
    println!("{}", va);
}
43
Break outer loop
Look for a negative value v in 2D integer matrix m. Print it and stop searching. 
auto indices = findNegativeValue (m, 10, 20);
std::cout << m[indices.first][indices.second] << '\n';

std::pair<int, int> findNegativeValue (int **m, int rows, int columns) {
  for (int i = 0; i < rows; ++i) {
    for (int j = 0; j < columns; ++j) {
      if (m[i][j] < 0) return make_pair (i, j);
    }
  }
  throw "No negative value!";
}
 
'outer: for v in m {
    'inner: for i in v {
        if i < 0 {
            println!("Found {}", i);
            break 'outer;
        }
    }
}
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 (s.begin () + i, x);

s.insert(i, x);
45
Pause execution for 5 seconds
Sleep for 5 seconds in current thread, before proceeding with the next instructions. 
using namespace std::chrono_literals;
std::this_thread::sleep_for(5s);
 
thread::sleep(time::Duration::from_secs(5));
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. 
auto t = s.substr(0, 5);
 
let t = s.char_indices().nth(5).map_or(s, |(i, _)| &s[..i]);
Alternative implementation:
let t = s.chars().take(5).collect::<String>();
47
Extract string suffix
Create string t consisting in the 5 last characters of string s.
Make sure that multibyte characters are properly handled. 
std::string t = s.substr(s.length() - 5);
 
let last5ch = s.chars().count() - 5;
let t: String = s.chars().skip(last5ch).collect();
Alternative implementation:
let s = "a̐éö̲\r\n";
let t = s.grapheme_indices(true).rev().nth(5).map_or(s, |(i,_)|&s[i..]);
48
Multi-line string literal
Assign to variable s a string literal consisting in several lines of text, including newlines. 
std::string s = R"( Earth is a planet.
So is the Jupiter)";
 
let s = "line 1
line 2
line 3";
Alternative implementation:
let s = r#"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. 
std::istringstream x(s);
std::list<std::string> chunks;
std::copy(std::istream_iterator<std::string>(x), std::istream_iterator<std::string>(), std::back_inserter(chunks));
Alternative implementation:
vector<string> chunks;
istringstream x {s};
string y;
while (x >> y)
    chunks.push_back(y);
 
let chunks: Vec<_> = s.split_whitespace().collect();
Alternative implementation:
let chunks: Vec<_> = s.split_ascii_whitespace().collect();
Alternative implementation:
let chunks: Vec<_> = s.split(' ').collect();
50
Make an infinite loop
Write a loop that has no end clause. 
for (;;) {
	/// Do something
}
Alternative implementation:
while (true) {
	// Do something
}
 
loop {
	// Do something
}
51
Check if map contains key
Determine whether the map m contains an entry for the key k 
bool key_exists = m.find(k) != m.end();
Alternative implementation:
bool key_exists = m.count(k) != 0;
Alternative implementation:
bool key_exists = m.contains(k);
 
m.contains_key(&k)
52
Check if map contains value
Determine whether the map m contains an entry with the value v, for some key. 
std::find_if(m.begin(), m.end(), [v](const auto& mo) {return mo.second == v; }) != m.end();
 
let does_contain = m.values().any(|&val| *val == v);
53
Join a list of strings
Concatenate elements of string list x joined by the separator ", " to create a single string y. 
std::vector<std::string> x;
std::string y;
const char* const delim = ", ";

switch (x.size())
{
	case 0: y = "";   break;
	case 1: y = x[0]; break;
	default:
		std::ostringstream os;
		std::copy(x.begin(), x.end() - 1,
			std::ostream_iterator<std::string>(os, delim));
		os << *x.rbegin();
		y = os.str();
}
Alternative implementation:
y = x | views::join_with(',') | to<std::string>();
Alternative implementation:
string d {", "};
auto f = [&d] (auto& a, auto& b) {
    return a + d + b;
};
string y {
    x[0] +
    reduce(begin(x) + 1, end(x), ""s, f)
};
 
let y = x.join(", ");
54
Compute sum of integers
Calculate the sum s of the integer list or array x. 
int s = std::accumulate(x.begin(), x.end(), 0, std::plus<int>());
Alternative implementation:
auto s = std::ranges::fold_left(x, 0, std::plus<int>{});

x.iter().sum()
Alternative implementation:
let s = x.iter().sum::<i32>();
55
Convert integer to string
Create the string representation s (in radix 10) of the integer value i. 
auto s = std::to_string(i);
 
let s = i.to_string();
Alternative implementation:
let s = format!("{}", 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". 
size_t taskCount = 1000;
auto remainingTasks = std::latch(static_cast<std::ptrdiff_t>(taskCount));
auto f = [&remainingTasks](int i) {
  // do work
  remainingTasks.count_down();
};

std::vector<std::jthread> threads{ 10 };
for (auto i = 0; i < taskCount; i++) {
  auto& workerThread = threads[i % 10];
  if (workerThread.joinable()) {
    workerThread.join();
  }
  workerThread = std::jthread(f, i);
}

remainingTasks.wait();
std::cout << "Finished";

let threads: Vec<_> = (0..1000).map(|i| thread::spawn(move || f(i))).collect();

for t in threads {
	t.join();
}
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. 
std::copy_if (x.begin (), x.end (), std::back_inserter(y), p);
Alternative implementation:
auto y = x | std::views::filter(p);
 
let y: Vec<_> = x.iter().filter(p).collect();
58
Extract file content to a string
Create the string lines from the content of the file with filename f. 
std::string fromFile(std::string _f)
{
    std::ifstream t(_f);
    t.seekg(0, std::ios::end);
    size_t size = t.tellg();
    std::string buffer(size, ' ');
    t.seekg(0);
    t.read(&buffer[0], size); 
}
 
let mut file = File::open(f)?;
let mut lines = String::new();
file.read_to_string(&mut lines)?;
Alternative implementation:
let lines = fs::read_to_string(f).expect("Can't read file.");
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"). 
std::cerr << x << " is negative\n";
Alternative implementation:
int main(){
	int x = -2;
	std::cerr << x <<" is negative\n";
}
 
eprintln!("{} is negative", x);
60
Read command line argument
Assign to x the string value of the first command line parameter, after the program name. 
int main(int argc, char *argv[])
{
 vector<string> args(1 + argv, argc + argv);

 string x = args.at(0);
}
 
let first_arg = env::args().skip(1).next();

let fallback = "".to_owned();
let x = first_arg.unwrap_or(fallback);
Alternative implementation:
let x = env::args().nth(1).unwrap_or("".to_string());
61
Get current date
Assign to the variable d the current date/time value, in the most standard type. 
std::time_t d = std::time(nullptr);
 
let d = time::now();
Alternative implementation:
let d = SystemTime::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. 
int i = x.find(y);
 
let i = x.find(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. 
std::stringstream s;
std::regex_replace(std::ostreambuf_iterator<char>(s), x.begin(), x.end(), y, z);
auto x2 = s.str()
Alternative implementation:
constexpr std::string ReplaceAllCopy(const std::string& x, const std::string& y, const std::string& z) {
	auto x2 = x;
	size_t i = 0;

	while (true) {
		i = x2.find(y, i);
		if (i == std::string::npos) break;
		x2.replace(i, y.length(), z);
	}
	return x2;
}
 
let x2 = x.replace(&y, &z);
64
Big integer : value 3 power 247
Assign to x the value 3^247 
let a = 3.to_bigint().unwrap();
let x = num::pow(a, 247);
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%" 
auto s = std::format("{.1f}%", x * 100);
 
let s = format!("{:.1}%", 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. 
let z = num::pow(x, n);
67
Binomial coefficient "n choose k"
Calculate binom(n, k) = n! / (k! * (n-k)!). Use an integer type able to handle huge numbers. 
fn binom(n: u64, k: u64) -> BigInt {
    let mut res = BigInt::one();
    for i in 0..k {
        res = (res * (n - i).to_bigint().unwrap()) /
              (i + 1).to_bigint().unwrap();
    }
    res
}
68
Create a bitset
Create an object x to store n bits (n being potentially large). 
std::vector<bool> x(n, 0);
Alternative implementation:
std::bitset<n> x;
 
let mut x = vec![false; n];
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. 
srand(s);
 
let s = 32;
let mut rng = StdRng::seed_from_u64(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. 
let d = SystemTime::now()
    .duration_since(SystemTime::UNIX_EPOCH)
    .expect("Duration since UNIX_EPOCH failed");
let mut rng = StdRng::seed_from_u64(d.as_secs());
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. 
int main(int argc, char * argv[])
{
	for(int i = 1; i < argc; ++i)
	{
		std::cout <<(1==i ? "" : " ") << argv[i];
	}
        std::cout <<std::endl;
}
 
println!("{}", env::args().skip(1).collect::<Vec<_>>().join(" "));
Alternative implementation:
println!("{}", std::env::args().skip(1).format(" "));
73
Create a factory
Create a factory named fact for any sub class of Parent and taking exactly one string str as constructor parameter. 
template <typename T,
          std::enable_if_t<
            std::is_base_of_v<Parent, T>, bool> = true>
T fact(const std::string& str) {
  return T(str);
}
 
fn fact<Parent: std::str::FromStr>(str: String, _: Parent) -> Parent where <Parent as FromStr>::Err: Debug 
{
    return str.parse::<Parent>().unwrap();
}
74
Compute GCD
Compute the greatest common divisor x of big integers a and b. Use an integer type able to handle huge numbers. 
unsigned long long int GCD(unsigned long long int a, unsigned long long int b)
{
    unsigned long long int c=a%b;
    if(c==0)
        return b;
    return GCD(b, c);
}
Alternative implementation:
auto x = std::gcd(a, b);
Alternative implementation:
using namespace std;
using u64 = uint64_t;
function<u64(u64, u64)> gcd;
gcd = [&gcd] (u64 a, u64 b) {
    return !b ? a : gcd(b, a % b);
};
auto x {gcd(a, b)};
 
let x = a.gcd(&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. 
auto x = std::lcm(a, b);
 
let x = a.lcm(&b);
76
Binary digits from an integer
Create the string s of integer x written in base 2.

E.g. 13 -> "1101" 
std::bitset<sizeof(x)*8> y(x);
auto s = y.to_string();
Alternative implementation:
std::string ToBinary(int x) {
  std::array<char, 64> buf;
  auto[ptr, ec] = std::to_chars(buf.data(), buf.data() + buf.size(), x, 2);
  auto s = std::string(buf.data(), ptr);
  return s;
}
Alternative implementation:
bitset<(sizeof x) * 8> b (x);
string s {b.to_string()};
 
let s = format!("{:b}", x);
Alternative implementation:
let s = format!("{x:b}");
77
Complex number
Declare a complex x and initialize it with value (3i - 2). Then multiply it by i. 
using namespace std::complex_literals;
	
auto x = 3i - 2.;
x *= 1i;

let mut x = Complex::new(-2, 3);
x *= Complex::i();
78
"do while" loop
Execute a block once, then execute it again as long as boolean condition c is true. 
do {
  someThing();
  someOtherThing();
} while(c);
 
loop {
    doStuff();
    if !c { break; }
}
Alternative implementation:
while {
   doStuff();
   c
} { /* EMPTY */ }
79
Convert integer to floating point number
Declare the floating point number y and initialize it with the value of the integer x . 
float y = static_cast<float>(x);
Alternative implementation:
float y (x);
 
let y = x as f64;
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). 
int y = static_cast<int>(x);
Alternative implementation:
int y (x);
 
let y = x as i32;
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). 
int y = static_cast<int>(std::floor(x + 0.5f));
 
let y = x.round() as i64;
82
Count substring occurrences
Find how many times string s contains substring t.
Specify if overlapping occurrences are counted. 
let c = s.matches(t).count();
83
Regex with character repetition
Declare the regular expression r matching the strings "http", "htttp", "httttp", etc. 
 auto r = regex("htt+p");
Alternative implementation:
std::regex r {"ht{2,}p"};
 
let r = Regex::new(r"htt+p").unwrap();
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 
uint32_t c = 0;
for (; i; i &= i - 1, ++c);
 
let c = i.count_ones();
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. 
bool addingWillOverflow(int x, int y) {
  return ((x > 0) && (y > std::numeric_limits<int>::max() - x)) ||
         ((x < 0) && (y < std::numeric_limits<int>::min() - x));
}
 
fn adding_will_overflow(x: usize, y: usize) -> bool {
    x.checked_add(y).is_none()
}
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. 
fn multiply_will_overflow(x: i64, y: i64) -> bool {
    x.checked_mul(y).is_none()
}
87
Stop program
Exit immediately.
If some extra cleanup work is executed by the program runtime (not by the OS itself), describe it. 
exit(1);
 
std::process::exit(0);
88
Allocate 1M bytes
Create a new bytes buffer buf of size 1,000,000. 
vector<byte> _buf(1'000'000);
 
let buf: Vec<u8> = Vec::with_capacity(1000000);
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. 
throw domain_error("oops!");
 
enum CustomError { InvalidAnswer }

fn do_stuff(x: i32) -> Result<i32, CustomError> {
    if x != 42 {
        Err(CustomError::InvalidAnswer)
    } else {
        Ok(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 final {
  int mX = 0;

public:
  const auto& X() const { return mX; }
};
 
struct Foo {
    x: usize
}

impl Foo {
    pub fn new(x: usize) -> Self {
        Foo { x }
    }

    pub fn x<'a>(&'a self) -> &'a usize {
        &self.x
    }

    pub fn bar(&mut self) {
        self.x += 1;
    }
}
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. 
let x = ::serde_json::from_reader(File::open("data.json")?)?;
92
Save object into JSON file
Write the contents of the object x into the file data.json. 
::serde_json::to_writer(&File::create("data.json")?, &x)?
93
Pass a runnable procedure as parameter
Implement the procedure control which receives one parameter f, and runs f. 
void control(invocable auto&& f)
{
 f();
}
Alternative implementation:
template <typename F>
void control(F& f) {
    f();
}
Alternative implementation:
void control(void(* f)()) {
    f();
}
 
fn control(f: impl Fn()) {
    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. 
std::cout<<typeid(x).name();
 
fn type_of<T>(_: &T) -> &'static str {
    std::intrinsics::type_name::<T>()
}

println!("{}", type_of(&x));
95
Get file size
Assign to variable x the length (number of bytes) of the local file at path. 
std::ifstream f(path, std::ios::in | std::ios::binary);
f.seekg(0, std::ios::end);
size_t x = f.tellg();

let x = fs::metadata(path)?.len();
Alternative implementation:
let x = path.metadata()?.len();
Alternative implementation:
let x = fs::metadata(path)?.st_size();
96
Check string prefix
Set the boolean b to true if string s starts with prefix prefix, false otherwise. 
bool b = s.starts_with(prefix);
Alternative implementation:
std::string prefix = "something";
bool b = s.compare(0, prefix.size(), prefix) == 0;
 
let b = s.starts_with(prefix);
97
Check string suffix
Set boolean b to true if string s ends with string suffix, false otherwise. 
return suffix.size() == s.size() - s.rfind(suffix);
Alternative implementation:
bool b = s.ends_with(suffix);
 
let b = s.ends_with(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 
std::time_t d = std::chrono::system_clock::to_time_t(ts);
 
let d = NaiveDateTime::from_timestamp(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. 
int main()
{
	char x[32]{};
	time_t a = time(nullptr);
	struct tm d;
	if (localtime_s(&d, &a) == 0) {
		strftime(x, sizeof(x), "%F", &d);
		std::cout << x << std::endl;
	}

	return 0;

}
 
Utc::today().format("%Y-%m-%d")
Alternative implementation:
let format = format_description!("[year]-[month]-[day]");
let x = d.format(&format).expect("Failed to format the date");
100
Sort by a comparator
Sort elements of array-like collection items, using a comparator c. 
struct is_less {
    bool operator () (int a, int b){
        return a < b;
    }
};

int main(){
    std::vector<int> items = {1337, 666, -666, 0, 0, 666, -666};
    std::sort(items.begin(), items.end(), is_less());

    std::vector<int> expected = {-666, -666, 0, 0, 666, 666, 1337};
    assert(items.size() == expected.size());
    for (size_t i = 0; i < items.size(); i++){
        assert(items[i] == expected[i]);
    }
    return 0;
}
Alternative implementation:
std::ranges::sort(items, c);
 
items.sort_by(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. 
let client = Client::new();
let s = client.get(u).send().and_then(|res| res.text())?;
Alternative implementation:
let s = ureq::get(u).call().into_string()?;
Alternative implementation:
let mut response = reqwest::blocking::get(u)?;
let mut s = String::new();
response.read_to_string(&mut s)?;
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. 
let client = Client::new();
match client.get(&u).send() {
    Ok(res) => {
        let file = File::create("result.txt")?;
        ::std::io::copy(res, file)?;
    },
    Err(e) => eprintln!("failed to send request: {}", e),
};
105
Current executable name
Assign to the string s the name of the currently executing program (but not its full path). 

int main(int argc, char* argv[])
{
    std::cout << std::filesystem::path(argv[0]).filename() << '\n';
}
 
fn get_exec_name() -> Option<String> {
    std::env::current_exe()
        .ok()
        .and_then(|pb| pb.file_name().map(|s| s.to_os_string()))
        .and_then(|s| s.into_string().ok())
}

fn main() -> () {
    let s = get_exec_name().unwrap();
    println!("{}", s);
}
Alternative implementation:
let s = std::env::current_exe()
    .expect("Can't get the exec path")
    .file_name()
    .expect("Can't get the exec name")
    .to_string_lossy()
    .into_owned();
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 = std::filesystem::current_path();
 
let dir = env::current_dir().unwrap();
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.) 
let dir = std::env::current_exe()?
    .canonicalize()
    .expect("the current exe should exist")
    .parent()
    .expect("the current exe should be a file")
    .to_string_lossy()
    .to_owned();
109
Number of bytes of a type
Set n to the number of bytes of a variable t (of type T). 
std::size_t n = sizeof(t);
Alternative implementation:
template <typename T>
auto f(T x) { return sizeof x; }
 
let n = std::mem::size_of::<T>();
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. 
bool blank = false;
if (s.empty() || std::all_of(s.begin(), s.end(), [](char c){return std::isspace(c);})) {
      blank = true;
}
 
let blank = s.trim().is_empty();
Alternative implementation:
let blank = s.chars().all(|c| c.is_whitespace());
111
Launch other program
From current process, run program x with command-line parameters "a", "b". 
let output = Command::new("x")
    .args(&["a", "b"])
    .spawn()
    .expect("failed to execute process");
Alternative implementation:
let output = Command::new("x")
        .args(&["a", "b"])
        .output()
        .expect("failed to execute process");
Alternative implementation:
let output = Command::new("x")
        .args(&["a", "b"])
        .status()
        .expect("failed to execute process");
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. 
std::map< K, V > _mymap;
for (const auto& pair : _mymap) {
    std::cout << pair.first << ": " << pair.second << "\n";
}
Alternative implementation:
auto print(auto&... args) {
  // c++17 fold expression
  (std::cout << ... << args) << std::endl;
}

auto print_map_contents(auto mymap) {
  // c++17 structured binding
  for (auto [k, x] : mymap) {
    print("mymap[", k, "] = ", x);
  }
}

auto main() -> int {
  // map is ordered map, iteration is ascending by key value
  auto map = std::map<std::string, int> {
    {"first entry", 12},
    {"second entry", 42},
    {"third entry", 3},
  };

  print_map_contents(map);
}
 
for (k, x) in mymap {
    println!("({}, {})", k, 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_each(begin(mymap), end(mymap),
    [&s](const auto& kv) { s.insert(kv.second); });
 
for (k, x) in mymap.iter().sorted_by_key(|x| x.1) {
	println!("[{},{}]", k, x);
}
Alternative implementation:
let mut items: Vec<_> = mymap.iter().collect();
items.sort_by_key(|item| item.1);
for (k, x) in items {
    println!("[{},{}]", k, x);
}
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. 
let b = x == y;
115
Compare dates
Set boolean b to true if date d1 is strictly before date d2 ; false otherwise. 
let b = d1 < 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, "");
Alternative implementation:
let s2 = str::replace(s1, w, "");
117
Get list size
Set n to the number of elements of the list x. 
size_t n = x.size();
 
let n = x.len();
118
List to set
Create the set y from the list x.
x may contain duplicates. y is unordered and has no repeated values. 
std::unordered_set<T> y (x.begin (), x.end ());
Alternative implementation:
std::set<T> y (x.begin (), x.end ());

let y: HashSet<_> = x.into_iter().collect();
119
Deduplicate list
Remove duplicates from the list x.
Explain if the original order is preserved. 
std::sort(x.begin(), x.end());
auto last = std::unique(x.begin(), x.end());
x.erase(last, x.end());
Alternative implementation:
std::vector<std::string> x = {"one", "two", "two", "one", "three"};
std::unordered_set<std::string> t;
for (auto e : x)
    t.insert(e);
 
x.sort();
x.dedup();
Alternative implementation:
let dedup: Vec<_> = x.iter().unique().collect();
120
Read integer from stdin
Read an integer value from the standard input into the variable n 
std::cin >> n;
 
fn get_input() -> String {
    let mut buffer = String::new();
    std::io::stdin().read_line(&mut buffer).expect("Failed");
    buffer
}

let n = get_input().trim().parse::<i64>().unwrap();
Alternative implementation:
let mut input = String::new();
io::stdin().read_line(&mut input).unwrap();
let n: i32 = input.trim().parse().unwrap();
Alternative implementation:
let n: i32 = std::io::stdin()
    .lock()
    .lines()
    .next()
    .expect("stdin should be available")
    .expect("couldn't read from stdin")
    .trim()
    .parse()
    .expect("input was not an integer");
Alternative implementation:
let n: i32 = read!();
121
UDP listen and read
Listen UDP traffic on port p and read 1024 bytes into the buffer b. 
let mut b = [0 as u8; 1024];
let sock = UdpSocket::bind(("localhost", p)).unwrap();
sock.recv_from(&mut b).unwrap();
122
Declare an enumeration
Create an enumerated type Suit with 4 possible values SPADES, HEARTS, DIAMONDS, CLUBS. 
enum class Suit {
    SPADES, HEARTS, DIAMONDS, CLUBS
};
Alternative implementation:
enum class Color : char{
	Red, Black, Green
};
 
enum Suit {
    Spades,
    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());
 
assert!(is_consistent);
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. 
template<typename T>
int binarySearch(const std::vector<T> &a, const T &x)
{
    if(a.size() == 0) return -1;

    size_t lower = 0;
    size_t upper = a.size() - 1;

    while(lower <= upper)
    {
        auto mid = lower + (upper-lower) / 2;

        if(x == a[mid])
        {
            return (int)mid;
        }
        else if(x > a[mid])
        {
            lower = mid + 1;
        }
        else
        {
            upper = mid - 1;
        }
    }

    return -1;
}

a.binary_search(&x).unwrap_or(-1);
125
Measure function call duration
measure the duration t, in nanoseconds, of a call to the function foo. Print this duration. 
auto start = std::chrono::steady_clock::now();
foo();
auto end = std::chrono::steady_clock::now();
auto t = std::chrono::duration_cast<std::chrono::nanoseconds>(end-start).count();
 
let start = Instant::now();
foo();
let duration = start.elapsed();
println!("{}", duration);
126
Multiple return values
Write a function foo that returns a string and a boolean value. 
std::tuple<std::string, bool> foo() {
  return std::make_tuple("someString", true);
}
Alternative implementation:
auto foo()
{
    return std::make_tuple("Hello", true);
}
 
fn foo() -> (String, bool) {
    (String::from("bar"), true)
}
127
Source code inclusion
Import the source code for the function foo body from a file "foobody.txt". 
void _foo() {
#include "foobody.txt"
}

fn main() {
    include!("foobody.txt");
}
128
Breadth-first traversing of a tree
Call a function f on every node of a tree, in breadth-first prefix order 
void bfs(Node& root, std::function<void(Node*)> f) {
  std::deque<Node*> node_queue;
  node_queue.push_back(&root);
  while (!node_queue.empty()) {
    Node* const node = node_queue.front();
    node_queue.pop_front();
    f(node);
    for (Node* const child : node->children) {
      node_queue.push_back(child);
    }
  }
}
 
struct Tree<V> {
    children: Vec<Tree<V>>,
    value: V
}

impl<V> Tree<V> {
    fn bfs(&self, f: impl Fn(&V)) {
        let mut q = VecDeque::new();
        q.push_back(self);

        while let Some(t) = q.pop_front() {
            (f)(&t.value);
            for child in &t.children {
                q.push_back(child);
            }
        }
    }
}
129
Breadth-first traversal in a graph
Call the function f on every vertex accessible from the vertex start, in breadth-first prefix order 
struct Vertex<V> {
	value: V,
	neighbours: Vec<Weak<RefCell<Vertex<V>>>>,
}

// ...

fn bft(start: Rc<RefCell<Vertex<V>>>, f: impl Fn(&V)) {
	let mut q = vec![start];
	let mut i = 0;
	while i < q.len() {
	    let v = Rc::clone(&q[i]);
	    i += 1;
	    (f)(&v.borrow().value);
	    for n in &v.borrow().neighbours {
	        let n = n.upgrade().expect("Invalid neighbour");
	        if q.iter().all(|v| v.as_ptr() != n.as_ptr()) {
	            q.push(n);
	        }
	    }
	}
}
130
Depth-first traversal in a graph
Call th function f on every vertex accessible from the vertex v, in depth-first prefix order 
void dfs(Node& root, std::function<void(Node*)> f) {
  std::stack<Node*> queue;
  queue.push(&root);
  std::unordered_set<const Node*> visited;
  while (!queue.empty()) {
    Node* const node = queue.top();
    queue.pop();
    f(node);
    visited.insert(node);
    for (Node* const neighbor : node->neighbors) {
      if (visited.find(neighbor) == visited.end()) {
        queue.push(neighbor);
      }
    }
  }
}
 
struct Vertex<V> {
	value: V,
	neighbours: Vec<Weak<RefCell<Vertex<V>>>>,
}

// ...

fn dft_helper(start: Rc<RefCell<Vertex<V>>>, f: &impl Fn(&V), s: &mut Vec<*const Vertex<V>>) {
	s.push(start.as_ptr());
	(f)(&start.borrow().value);
	for n in &start.borrow().neighbours {
		let n = n.upgrade().expect("Invalid neighbor");
		if s.iter().all(|&p| p != n.as_ptr()) {
			Self::dft_helper(n, f, s);
		}
	}
}
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. 
if (c1)
    f1();
else if (c2)
    f2();
else if (c3)
    f3();
Alternative implementation:
if (c1)
{
    f1();
}
else if (c2)
{
    f2();
}
else if (c3)
{
    f3();
}
Alternative implementation:
(c1 && (f1(), true))
 || (c2 && (f2(), true))
 || (c3 && (f3(), true));
 
if c1 { f1() } else if c2 { f2() } else if c3 { f3() }
Alternative implementation:
match true {
    _ if c1 => f1(),
    _ if c2 => f2(),
    _ if c3 => f3(),
    _ => (),
}
132
Measure duration of procedure execution
Run the procedure f, and return the duration of the execution of f. 
std::clock_t start = std::clock();
f();
double duration = (std::clock() - start) / (double) CLOCKS_PER_SEC;
Alternative implementation:
#include <chrono>

double measure() {
    using namespace std::chrono;
    const auto start{ high_resolution_clock::now() };
    f();
    const auto elapsed{ high_resolution_clock::now() - start };
    const double seconds{ duration_cast<duration<double>>(elapsed).count() };
    return seconds;
}

let start = Instant::now();
f();
let duration = start.elapsed();
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. 
auto ok = std::search(std::begin(s), std::end(s), std::begin(word), std::end(word),
    [](auto c, auto d){
        return std::tolower(c) == std::tolower(d);
    }
) != std::end(s);
 
let re = Regex::new(&format!("(?i){}", regex::escape(word))).unwrap();
let ok = re.is_match(&s);
Alternative implementation:
let re =
    RegexBuilder::new(&regex::escape(word))
    .case_insensitive(true)
    .build().unwrap();

let ok = re.is_match(s);
Alternative implementation:
let ok = s.to_ascii_lowercase().contains(&word.to_ascii_lowercase());
134
Create a new list
Declare and initialize a new list items, containing 3 elements a, b, c. 
std::vector<T> items = {a, b, c};
Alternative implementation:
list items { a, b, c };
 
let items = vec![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. 
erase(items, x);

if let Some(i) = items.iter().position(|item| *item == x) {
    items.remove(i);
}
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. 
items.remove(x);
 
items = items.into_iter().filter(|&item| item != x).collect();
Alternative implementation:
items.retain(|&item| item != 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. 
bool b = false;
if (! s.empty() && std::all_of(s.begin(), s.end(), [](char c){return std::isdigit(c);})) {
    b = true;
}
 
let chars_are_numeric: Vec<bool> = s.chars()
				.map(|c|c.is_numeric())
				.collect();
let b = !chars_are_numeric.contains(&false);
Alternative implementation:
let b = s.chars().all(char::is_numeric);
Alternative implementation:
let b = s.bytes().all(|c| c.is_ascii_digit());
138
Create temp file
Create a new temporary file on the filesystem. 
let temp_dir = TempDir::new("prefix")?;
let temp_file = File::open(temp_dir.path().join("file_name"))?;
139
Create temp directory
Create a new temporary folder on filesystem, for writing. 
let tmp = TempDir::new("prefix")?;
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.erase(k);

m.remove(&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. 
void print_seq(const auto& ...xs)
{
 (std::for_each(std::begin(xs), std::end(xs), 
           [](const auto& x) { std::cout << x; }), ...);
}

 std::list xs { "hi", "there", "world" }, ys { "lo" "thar" };

 print_seq(xs, ys);
 
for i in item1.iter().chain(item2.iter()) {
    print!("{} ", i);
}
142
Hexadecimal digits of an integer
Assign to string s the hexadecimal representation (base 16) of integer x.

E.g. 999 -> "3e7" 
template <typename I>
std::string n2hexstr(I w, size_t hex_len = sizeof(I)<<1) {
    static const char* digits = "0123456789ABCDEF";
	std::string str(hex_len, '-');
    for (size_t i=0, j=(hex_len-1)*4 ; i<hex_len; ++i,j-=4)
       str[i] = digits[(w>>j) & 0x0f];
	return str;
}
Alternative implementation:
std::ostringstream stream;
stream << std::hex << x;
s = stream.str();
Alternative implementation:
using namespace std;
stringstream f;
f << hex << x;
string s {f.str()};
 
let s = format!("{: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 izip!(&items1, &items2) {
    println!("{}", pair.0);
    println!("{}", pair.1);
}
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. 
bool b = std::filesystem::exists(fp);
 
let b = std::path::Path::new(fp).exists();
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. 
eprintln!("[{}] {}", humantime::format_rfc3339_seconds(std::time::SystemTime::now()), msg);
146
Convert string to floating point number
Extract floating point value f from its string representation s 
float f = std::stof(s);
 
let f = s.parse::<f32>().unwrap();
Alternative implementation:
let f: f32 = s.parse().unwrap();
147
Remove all non-ASCII characters
Create string t from string s, keeping only ASCII characters 
 copy_if(begin(src), end(src), back_inserter(dest),
         [](const auto c) { return static_cast<unsigned char>(c) <= 0x7F; });
 
let t = s.replace(|c: char| !c.is_ascii(), "");
Alternative implementation:
let t = s.chars().filter(|c| c.is_ascii()).collect::<String>();
148
Read list of integers from stdin
Read a list of integer numbers from the standard input, until EOF. 
vector<int> v;
for(int t;;){
	cin >> t;
	if(cin.eof())
		break;
	v.push_back(t);
}
 
let mut string = String::new();
io::stdin().read_to_string(&mut string)?;
let result = string
    .lines()
    .map(i32::from_str)
    .collect::<Result<Vec<_>, _>>();
150
Remove trailing slash
Remove the last character from the string p, if this character is a forward slash / 
 if('/' == s.back())
  s.pop_back();
 
if p.ends_with('/') { p.pop(); }
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! 
let p = if ::std::path::is_separator(p.chars().last().unwrap()) {
    &p[0..p.len()-1]
} else {
    p
};
Alternative implementation:
p = p.strip_suffix(std::path::is_separator).unwrap_or(p);
152
Turn a character into a string
Create string s containing only the character c. 
string s { 'c' };
 
let s = c.to_string();
153
Concatenate string with integer
Create the string t as the concatenation of the string s and the integer i. 
auto t = s + std::to_string (i);

let t = format!("{}{}", s, i);
Alternative implementation:
let t = format!("{s}{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. 
using namespace std;
struct rgb {
    int r, g, b;
    rgb(string s) {
        int x {stoi(s, nullptr, 16)};
        r = (x & 0xff'00'00) >> 16;
        g = (x &    0xff'00) >>  8;
        b = (x &       0xff);
    }
};
rgb x {c1.substr(1)}, y {c2.substr(1)};
int r {(x.r + y.r) / 2},
    g {(x.g + y.g) / 2},
    b {(x.b + y.b) / 2},
    i {(r << 16) + (g << 8) + b};
string c;
sprintf(c.data(), "#%06x", i);
Alternative implementation:
using namespace std;
union rgb {
    uint32_t x;
    struct { uint8_t b, g, r; };
    rgb(string s) {
        x = stoi(s, nullptr, 16);
    }
};
rgb x {c1.substr(1)}, y {c2.substr(1)};
int r {(x.r + y.r) / 2},
    g {(x.g + y.g) / 2},
    b {(x.b + y.b) / 2},
    i {(r << 16) + (g << 8) + b};
string c;
sprintf(c.data(), "#%06x", i);
Alternative implementation:
using namespace std;
union rgb {
    uint32_t x;
    struct {
        uint8_t b: 8;
        uint8_t g: 8;
        uint8_t r: 8;
    };
    rgb(string s) {
        x = stoi(s, nullptr, 16);
    }
};
rgb x {c1.substr(1)}, y {c2.substr(1)};
int r {(x.r + y.r) / 2},
    g {(x.g + y.g) / 2},
    b {(x.b + y.b) / 2},
    i {(r << 16) + (g << 8) + b};
ostringstream f;
f << hex << setw(6) << setfill('0') << i;
string c {'#' + f.str()};
 
"Too long for text box, see online demo"
155
Delete file
Delete from filesystem the file having path filepath. 
auto r = std::filesystem::remove(filepath);
 
let r = fs::remove_file(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. 
let s = format!("{:03}", i);
157
Declare constant string
Initialize a constant planet with string value "Earth". 
// using char*
constexpr char planet[] = "Earth";

// using string
const std::string planet{"Earth"};
Alternative implementation:
using namespace std;
string const planet {"Earth"};
 
const PLANET: &str = "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. 
let mut rng = &mut rand::thread_rng();
let y = x.choose_multiple(&mut rng, k).cloned().collect::<Vec<_>>();
159
Trie
Define a Trie data structure, where entries have an associated value.
(Not all nodes are entries) 
struct Trie {
    val: String,
    nodes: Vec<Trie>
}
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. 
#ifdef __x86_64
f64();
#else
#ifdef _M_AMD64
f64();
#else
f32();
#endif
#endif
Alternative implementation:
if constexpr(sizeof(nullptr) == 8) {
  f64();
} else {
  f32();
}
 
match std::mem::size_of::<&char>() {
    4 => f32(),
    8 => f64(),
    _ => {}
}
Alternative implementation:
#[cfg(target_pointer_width = "64")]
f64();

#[cfg(target_pointer_width = "32")]
f32();
161
Multiply all the elements of a list
Multiply all the elements of the list elements by a constant c 
for (auto &it : elements)
	it *= c;
Alternative implementation:
std::transform(std::begin(elements), std::end(elements), std::begin(elements), [c](auto i){
        return i*c;
    });
 
let elements = elements.into_iter().map(|x| c * x).collect::<Vec<_>>();
Alternative implementation:
elements.iter_mut().for_each(|x| *x *= c);
162
Execute procedures depending on options
execute bat if b is a program option and fox if f is a program option. 
if let Some(arg) = ::std::env::args().nth(1) {
    if &arg == "f" {
        fox();
    } else if &arg = "b" {
        bat();
    } else {
	eprintln!("invalid argument: {}", arg),
    }
} else {
    eprintln!("missing argument");
}
Alternative implementation:
if let Some(arg) = ::std::env::args().nth(1) {
    match arg.as_str() {
        "f" => fox(),
        "b" => box(),
        _ => eprintln!("invalid argument: {}", arg),
    };
} else {
    eprintln!("missing argument");
}
163
Print list elements by group of 2
Print all the list elements, two by two, assuming list length is even. 
for pair in list.chunks(2) {
    println!("({}, {})", pair[0], pair[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).expect("failed to open URL");
165
Last element of list
Assign to the variable x the last element of the list items. 
std::vector<int> items;
int last = items.back();
Alternative implementation:
auto x = *std::crbegin(items);
 
let x = items[items.len()-1];
Alternative implementation:
let x = items.last().unwrap();
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. 
auto ab = a;
ab.insert (ab.end (), b.begin (), b.end ());

let ab = [a, b].concat();
167
Trim prefix
Create the string t consisting of the string s with its prefix p removed (if s starts with p). 
std::string t = s.starts_with(p) ? s.substr(p.size()) : s;
 
let t = s.trim_start_matches(p);
Alternative implementation:
let t = if s.starts_with(p) { &s[p.len()..] } else { s };
Alternative implementation:
let t = s.strip_prefix(p).unwrap_or(s);
168
Trim suffix
Create string t consisting of string s with its suffix w removed (if s ends with w). 
let t = s.trim_end_matches(w);
Alternative implementation:
let t = s.strip_suffix(w).unwrap_or(s);
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. 
auto utf8len(std::string_view const& s) -> size_t
{
	std::setlocale(LC_ALL, "en_US.utf8");

	auto n = size_t{};
	auto size = size_t{};
	auto mb = std::mbstate_t{};

	while(size < s.length())
	{
		size += mbrlen(s.data() + size, s.length() - size, &mb);
		n += 1;
	}

	return n;
}
 
let n = s.chars().count();
170
Map size
Set n to the number of elements stored in mymap.

This is not always equal to the map capacity. 
auto n = mymap.size();
 
let n = mymap.len();
171
Add an element at the end of a list
Append the element x to the list s. 
s.emplace_back(x);
Alternative implementation:
s.push_back(x);
 
s.push(x);
172
Insert an entry in a map
Insert value v for key k in map m. 
m[k] = v;
 
m.insert(k, v);
173
Format a number with grouped thousands
Number will be formatted with a comma separator between every group of thousands. 
println!("{}", 1000.separated_string());
174
Make HTTP POST request
Make a HTTP request with method POST to the URL u 
let client = reqwest::blocking::Client::new();
let mut response = client.post(u).body("abc").send()?;
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). 
using namespace std;
ostringstream x;
x << hex << setfill('0');
auto const& w {setw(2)};
for (byte& b : a)
    x << w << int(b);
string s {x.str()};
Alternative implementation:
using namespace std;
string s (2 * n, 0);
for (int i {}; i not_eq n; ++i)
    sprintf(&s[i * 2], "%02x", a[i]);
 
let s = a.encode_hex::<String>();
Alternative implementation:
let mut s = String::with_capacity(2 * n);
for byte in a {
    write!(s, "{:02X}", byte)?;
}
Alternative implementation:
let s = HEXLOWER.encode(&a);
Alternative implementation:
fn byte_to_hex(byte: u8) -> (u8, u8) {
    static HEX_LUT: [u8; 16] = [b'0', b'1', b'2', b'3', b'4', b'5', b'6', b'7', b'8', b'9', b'a', b'b', b'c', b'd', b'e', b'f'];

    let upper = HEX_LUT[(byte >> 4) as usize];
    let lower = HEX_LUT[(byte & 0xF) as usize];
    (lower, upper)
}

let utf8_bytes: Vec<u8> = a.iter().copied().flat_map(|byte| {
        let (lower, upper) = byte_to_hex(byte);
        [upper, lower]
    }).collect();
let s = unsafe { String::from_utf8_unchecked(utf8_bytes) };
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). 
let a: Vec<u8> = Vec::from_hex(s).expect("Invalid Hex String");
Alternative implementation:
let a: Vec<u8> = decode(s).expect("Hex string should be valid");
Alternative implementation:
let a: Vec<u8> = HEXLOWER_PERMISSIVE.decode(&s.into_bytes()).unwrap();
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. 
let d = Path::new("/path/to/D");
let l: Vec<PathBuf> = d
    .read_dir()?
    .filter_map(|f| f.ok())
    .filter(|f| match f.path().extension() {
        None => false,
        Some(ex) => ex == "jpg" || ex == "jpeg" || ex == "png"
    })
    .map(|f| f.path())
    .collect();
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. 
struct Rect {
    x1: i32,
    x2: i32,
    y1: i32,
    y2: i32,
}

impl Rect {
    fn contains(&self, x: i32, y: i32) -> bool {
        return self.x1 < x && x < self.x2 && self.y1 < y && y < self.y2;
    }
}
179
Get center of a rectangle
Return the center c of the rectangle with coördinates(x1,y1,x2,y2) 
template <typename T> struct Point {
  T x = {};
  T y = {};
};
template <typename T> struct Rectangle {
  Point<T> upper_left;
  Point<T> lower_right;
};
template <typename T>
Point<T> get_center(const Rectangle<T>& rect) {
  return {
    .x = (rect.upper_left.x + rect.lower_right.x) / 2,
    .y = (rect.upper_left.y + rect.lower_right.y) / 2,
  };
}
 
struct Rectangle {
    x1: f64,
    y1: f64,
    x2: f64,
    y2: f64,
}

impl Rectangle {
    pub fn center(&self) -> (f64, f64) {
	    ((self.x1 + self.x2) / 2.0, (self.y1 + self.y2) / 2.0)
    }
}
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. 
auto directory_contents(auto path) {
  auto iterator = std::filesystem::directory_iterator(path);
  return std::vector<std::filesystem::path>(
    std::ranges::begin(iterator),
    std::ranges::end(iterator)
  );
}

auto main(int argc, char** argv) -> int {
  auto path = argc >= 2
    ? std::filesystem::path(argv[1])
    : std::filesystem::current_path();

  for (auto entry : directory_contents(path)) {
    std::cout << entry.string() << std::endl;
  }
}
 
let x = std::fs::read_dir(d).unwrap();
Alternative implementation:
let x = std::fs::read_dir(d)?.collect::<Result<Vec<_>, _>>()?;
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. 
fn main() {
    let x = "fn main() {\n    let x = ";
    let y = "print!(\"{}{:?};\n    let y = {:?};\n    {}\", x, x, y, y)\n}\n";
    print!("{}{:?};
    let y = {:?};
    {}", x, x, y, y)
}
Alternative implementation:
fn main(){print!("{},{0:?})}}","fn main(){print!(\"{},{0:?})}}\"")}
184
Tomorrow
Assign to t a string representing the day, month and year of the day after the current date. 
let t = chrono::Utc::now().date().succ().to_string();
185
Execute function in 30 seconds
Schedule the execution of f(42) in 30 seconds. 
sleep(Duration::new(30, 0));
f(42);
186
Exit program cleanly
Exit a program cleanly indicating no error to OS 
exit(0);
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. 
let c = a.dot(&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. 
template <typename SeqT>
auto transform_copy_if(const SeqT i, auto p, auto f)
{
 using namespace std;

 SeqT o;

 for_each(begin(i), end(i),
          [&](const auto& x) {
              if(p(x))
               o.push_back(f(x));
          }
         );

 return o;
}
Alternative implementation:
constexpr auto transform_matches(const auto& x, auto p, auto t) {
    auto result = x 
	| std::views::filter(p)
        | std::views::transform(t);

    std::vector<std::ranges::range_value_t<decltype(result)>> y;
    std::copy(result.begin(), result.end(), std::back_inserter(y));
    return y;
}
 
let y = x.iter()
	.filter(P)
        .map(T)
	.collect::<Vec<_>>();
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. 
extern "C" void foo(double *a, int n);
 
extern "C" {
    /// # Safety
    ///
    /// `a` must point to an array of at least size 10
    fn foo(a: *mut libc::c_double, n: libc::c_int);
}

let mut a = [0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0];
let n = 10;
unsafe {
    foo(a.as_mut_ptr(), n);
}
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 (std::any_of(a.begin(), a.end(), [x](auto y) { return y > x; }))
    f();
 
if a.iter().any(|&elem| elem > 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. 
let a = Decimal::from_str("1234567890.123456789012345").unwrap();
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). 
let a = DMatrix::<u8>::from_fn(n, m, |_, _| rand::thread_rng().gen());
let b = a.transpose();
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). 
fn foo(a: Vec<Vec<usize>>) {
    println!(
        "Length of array: {}",
        a.clone()
            .into_iter()
            .flatten()
            .collect::<Vec<usize>>()
            .len()
    );

    let mut sum = 0;
    for (i, j) in izip!(&a[0], &a[1]) {
        sum += i * j
    }

    println!("Sum of all products of indices: {}", sum);
}
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. 
fn foo(el: &mut i32) {
    *el = 42;
}
a.iter_mut().take(m).step_by(2).for_each(foo);
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. 
std::ifstream file (path);
for (std::string line; std::getline(file, line); lines.push_back(line)) {}
 
let lines = BufReader::new(File::open(path).unwrap())
	.lines()
	.collect::<Vec<_>>();
198
Abort program execution with error condition
Abort program execution with error condition x (where x is an integer value) 
process::exit(x);
199
Truncate a file at the current file position
Truncate a file F at the given file position. 
let pos = f.stream_position()?;
f.set_len(pos)?;
200
Return hypotenuse
Compute the hypotenuse h of the triangle where the sides adjacent to the square angle have lengths x and y. 
auto h = std::hypot(x, y);
 
fn hypot(x:f64, y:f64)-> f64 {
    let num = x.powi(2) + y.powi(2);
    num.powf(0.5)
}
Alternative implementation:
let h = x.hypot(y);
201
Euclidean norm
Calculate n, the Euclidean norm of data, where data is a list of floating point values. 
fn euclidean(data: Vec<f64>) -> f64 {
    let mut n = 0.0;
    for i in data {
        n += i*i;
    }
    return sqrt(n as f64)
}
let n = euclidean(data);
202
Sum of squares
Calculate the sum of squares s of data, an array of floating point values. 
let s = data.iter().map(|x| x.powi(2)).sum::<f32>();
203
Calculate mean and standard deviation
Calculate the mean m and the standard deviation s of the list of floating point values data. 
let sum: f64 = data.iter().sum();
let m = sum / (data.len() as f64);
let sum_of_squares: f64 = data.iter().map(|item| (item - m) * (item - m)).sum();
let s = (sum_of_squares / (list.len() as f64)).sqrt();
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
 
let sign = if a < 0.0 { a = -a; -1 } else { 1 };
let exponent = (a + f64::EPSILON).log2().ceil() as i32;
let fraction = a / 2.0f64.powi(exponent);
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". 
const char* tmp = std::getenv("FOO");
std::string foo = tmp ? std::string(tmp) : "none";
 
let foo = match env::var("FOO") {
    Ok(val) => val,
    Err(_e) => "none".to_string(),
};
Alternative implementation:
let foo = env::var("FOO").unwrap_or("none".to_string());
Alternative implementation:
let foo = match env::var("FOO") {
    Ok(val) => val,
    Err(_e) => "none".to_string(),
};
Alternative implementation:
if let Ok(tnt_root) = env::var("TNT_ROOT") {
     //
}
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. 
match str {
    "foo" => foo(),
    "bar" => bar(),
    "baz" => baz(),
    "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. 
let a = vec![0; 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. 
using namespace std;
valarray a {1, 2}, b {3, 4},
         c {5, 6}, d {7, 8};
a = e * (a + b * c + cos(d));
 
for i in 0..a.len() {
    a[i] = e * (a[i] + b[i] * c[i] + d[i].cos());
}
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). 
#include <string>
using namespace std::string_literals;
#include <vector>
#include <memory>

struct t {
    std::string s;
    std::vector<int> n;
};

auto v = std::make_unique<t>(
    "Hello, world!"s,
    decltype(t::n){1, 4, 9, 16, 25}
);
v.reset();

// Automatically:
void fn(){
    auto v = std::make_unique<t>(
        "Hello, world!"s,
        decltype(t::n){1, 4, 9, 16, 25}
    );
}

struct T {
	s: String,
	n: Vec<usize>,
}

fn main() {
	let v = T {
		s: "Hello, world!".into(),
		n: vec![1,4,9,16,25]
	};
}
211
Create folder
Create the folder at path on the filesystem 
namespace fs = std::filesystem;
fs::create_directory(path);
 
fs::create_dir(path)?;
Alternative implementation:
fs::create_dir_all(path)?;
212
Check if folder exists
Set the boolean b to true if path exists on the filesystem and is a directory; false otherwise. 
auto b = std::filesystem::is_directory(path);
 
let b: bool = Path::new(path).is_dir();
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. 
for x in strings
    .iter()
    .combinations(2)
    .filter(|x| x[0].to_lowercase() == x[1].to_lowercase())
{
    println!("{:?} == {:?}", x[0], x[1])
}
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. 
let out = s.pad_to_width_with_char(m, c);
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. 
if let Some(columns_short) = m.checked_sub(s.width()) {
    let padding_width = c
        .width()
        .filter(|n| *n > 0)
        .expect("padding character should be visible");
    // Saturate the columns_short
    let padding_needed = columns_short + padding_width - 1 / padding_width;
    let mut t = String::with_capacity(s.len() + padding_needed);
    t.extend((0..padding_needed).map(|_| c)
    t.push_str(&s);
    s = t;
}
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". 
let n = (m + s.len())/2;
s = format!("{s:X>n$}");
s = format!("{out:X<m$}");
Alternative implementation:
s = s.pad(m, c, Alignment::Middle, true);
217
Create a Zip archive
Create a zip-file with filename name and add the files listed in list to that zip-file. 
let path = std::path::Path::new(_name);
let file = std::fs::File::create(&path).unwrap();
let mut zip = zip::ZipWriter::new(file); zip.start_file("readme.txt", FileOptions::default())?;                                                          
zip.write_all(b"Hello, World!\n")?;
zip.finish()?;
Alternative implementation:
fn zip(_name: &str, _list: Vec<&str>) -> zip::result::ZipResult<()>
{
    let path = std::path::Path::new(_name);
    let file = std::fs::File::create(&path).unwrap();
    let mut zip = zip::ZipWriter::new(file);
    for i in _list.iter() {
        zip.start_file(i as &str, FileOptions::default())?;
    }
    zip.finish()?;
    Ok(())
}
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. 
let unique_a = a.iter().collect::<HashSet<_>>();
let unique_b = b.iter().collect::<HashSet<_>>();

let c = unique_a.intersection(&unique_b).collect::<Vec<_>>();
Alternative implementation:
let set_a: HashSet<_> = a.into_iter().collect();
let set_b: HashSet<_> = b.into_iter().collect();
let c = set_a.intersection(&set_b);
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. 
auto t = s;
t.erase(std::ranges::unique(t, 
  [](char const &lhs,char const &rhs)
  { return lhs == rhs && ::std::iswspace(lhs); }
).begin(), t.end());
Alternative implementation:
using namespace std;
regex r {R"(\s+)"};
string t {regex_replace(s, r, " ")};
 
let re = Regex::new(r"\s+").unwrap();
let t = re.replace_all(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. 
std::tuple<float, std::string, bool> t(2.5f, "foo", false);
Alternative implementation:
auto t = std::make_tuple(2.5f, std::string("foo"), false);
 
let t = (2.5, "hello", -1);
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. 
let t: String = s.chars().filter(|c| c.is_digit(10)).collect();
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. 
auto it = std::find(items.begin(), items.end(), x);
i = it == items.end() ? std::distance(items.begin(), it) : -1;
 
let opt_i = items.iter().position(|y| *y == x);


let i = match opt_i {
   Some(index) => index as i32,
   None => -1
};
Alternative implementation:
let i = items.iter().position(|y| *y == x).map_or(-1, |n| n as i32);
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. 
bool found = false;
for (const auto item : items) {
  if (item == "baz") {
    baz();
    found = true;
    break;
  }
}
if (!found) {
  DoSomethingElse();
}
Alternative implementation:
if (std::any_of(items.begin(), items.end(), condition))
    baz();
else
    DoSomethingElse();
 
let mut found = false;
for item in items {
    if item == &"baz" {
        println!("found it");
        found = true;
        break;
    }
}
if !found {
    println!("never found it");
}
Alternative implementation:
if let None = items.iter().find(|&&item| item == "rockstar programmer") {
        println!("NotFound");
    };
Alternative implementation:
items
    .iter()
    .find(|&&item| item == "rockstar programmer")
    .or_else(|| {
        println!("NotFound");
        Some(&"rockstar programmer")
    });
Alternative implementation:
if 'search: loop {
    for i in items {
        if i == &"qux" {
            println!("found it");
            break 'search false;
        }
    }
    break 'search true
} {
    println!("not found!");
}
Alternative implementation:
'label: {
	for &item in items {
		if item == "baz" {
			println!("found");
			break 'label;
		}
	}
	println!("not found");
}
224
Add element to the beginning of the list
Insert the element x at the beginning of the list items. 
items.emplace_front(x);
Alternative implementation:
items.push_front(x);
 
items.push_front(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 
void f(std::optional<int> x = {}) {
  std::cout << (x ? "Present" + std::to_string(x.value()) : "Not Present");
}
Alternative implementation:
void f(std::optional<int> x = {}) {
  if (x) {
    std::cout << "Present" << x.value();
  } else {
    std::cout << "Not present";
  }
}
 
fn f(x: Option<()>) {
    match x {
        Some(x) => println!("Present {}", x),
        None => println!("Not present"),
    }
}
226
Delete last element from list
Remove the last element from the list items. 
items.pop_back();
 
items.pop();
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). 
let y = x.clone();
228
Copy a file
Copy the file at path src to dst. 
fs::copy(src, dst).unwrap();
230
Timeout
Cancel an ongoing processing p if it has not finished after 5s. 
timeout(Duration::from_secs(5), p()).await;
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. 
let b = std::str::from_utf8(&bytes).is_ok();
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. 
int main(int argc, char** argv) {
    char* x;
    bool b {};
    while (x = *argv++)
        if (not strcmp(x, "-v")) {
            b = true;
            break;
        }
    printf("%s", b ? "true" : "false");
}
 
let matches = App::new("My Program")
                .arg(Arg::with_name("verbose")
                    .short("v")
                    .takes_value(false))
                .get_matches();
                   
if matches.is_present("verbose") {
    println!("verbose is true")
} else {
    println!("verbose is false")
}
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. 
int main(int argc, char** argv) {
    using namespace std;
    string s;
    char* x;
    while (x = *argv++)
        if (not strcmp(x, "-country"))
            if (x = *argv) {
                s = x;
                break;
            }
    if (s.empty()) s = "Canada";
    printf("%s", s.data());
}
234
Encode bytes to base64
Assign to the string s the standard base64 encoding of the byte array data, as specified by RFC 4648. 
let s = base64::encode(data);
235
Decode base64
Assign to byte array data the bytes represented by the base64 string s, as specified by RFC 4648. 
let bytes = base64::decode(s).unwrap();
237
Xor integers
Assign to c the result of (a xor b) 
int c = a ^ b;
Alternative implementation:
int c {a xor b};
 
let c = 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. 
std::array<std::byte, a.size()> c;
for (auto ia = a.begin(), ib = b.begin(); auto & rc : c) {
  rc = *ia++ ^ *ib++;
}
Alternative implementation:
int constexpr n {size(a)};
array<int, n> c;
for (int i {}; i != n; ++i)
    c[i] = a[i] xor b[i];
 
let c: Vec<_> = a.iter().zip(b).map(|(x, y)| x ^ y).collect();
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. 
std::regex re("\\b\\d{3}\\b");
auto it = std::sregex_iterator(s.begin(), s.end(), re);
std::string x = it == std::sregex_iterator() ? "" : it->str();
Alternative implementation:
regex r {R"(\b\d{3}\b)"};
sregex_iterator i {s.begin(), s.end(), r}, n;
string x {i not_eq n ? i->str() : ""};
 
let re = Regex::new(r"\b\d\d\d\b").expect("failed to compile regex");
let x = re.find(s).map(|x| x.as_str()).unwrap_or("");
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. 
let mut tmp: Vec<_> = a.iter().zip(b).collect();
tmp.as_mut_slice().sort_by_key(|(&x, _y)| x);
let (aa, bb): (Vec<i32>, Vec<i32>) = tmp.into_iter().unzip();
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. 
::std::thread::yield_now();
busywork();
242
Iterate over a set
Call a function f on each element e of a set x. 
std::for_each(x.begin(), x.end(), f);
Alternative implementation:
for (auto e : x)
    f(e);
 
for item in &x {
    f(item);
}
243
Print list
Print the contents of the list or array a on the standard output. 
std::copy(a.begin(), a.end(), std::ostream_iterator<int>(std::cout, "\n"));
 
println!("{:?}", a)
244
Print a map
Print the contents of the map m to the standard output: keys and values. 
for (auto entry : m) {
	std::cout << entry.first << ": " << entry.second << "\n";
}
 
println!("{:?}", m);
245
Print value of custom type
Print the value of object x having custom type T, for log or debug. 
std::cout << x;
 
println!("{:?}", x);
246
Count distinct elements
Set c to the number of distinct elements in the list items. 
let c = items.iter().unique().count();
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. 
std::list<Foo> x;
x.remove_if(std::not_fn(p));
Alternative implementation:
std::erase_if(x, std::not_fn(p));
 
let mut j = 0;
for i in 0..x.len() {
    if p(x[i]) {
        x[j] = x[i];
        j += 1;
    }
}
x.truncate(j);
Alternative implementation:
x.retain(p);
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). 
let d = if s { -1.0 } else { 1.0 } * m as f64 * 2.0f64.powf(e as f64);
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. 
int a, b, c = b = a = 123;
Alternative implementation:
auto [a, b, c, _] = "abc";
 
let (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. 
let mut rng = rand::thread_rng();
let x = m.values().choose(&mut rng).expect("m is empty");
251
Parse binary digits
Extract integer value i from its binary string representation s (in radix 2)
E.g. "1101" -> 13 
int i = std::stoi(s, nullptr, 2);
Alternative implementation:
auto f {[] (int a, int b) {
    return a * 2 + (b - '0');
}};
int i {accumulate(s.begin(), s.end(), 0, f)};
 
let i = i32::from_str_radix(s, 2).expect("Not a binary number!");
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 = condition() ? "a" : "b";
Alternative implementation:
std::string x {"ba"[condition()]};
 
x = if condition() { "a" } else { "b" };
253
Print stack trace
Print the stack frames of the current execution thread of the program. 
let bt = Backtrace::new();
println!("{:?}", bt);
254
Replace value in list
Replace all exact occurrences of "foo" with "bar" in the string list x 
std::replace(x.begin(), x.end(), "foo", "bar");
 
for v in &mut x {
    if *v == "foo" {
        *v = "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. 
println!("{:?}", x);
256
Count backwards
Print the numbers 5, 4, ..., 0 (included), one line per number. 
for (int i = 5; i >= 0; --i) {
	std::cout << i;
}
 
(0..=5).rev().for_each(|i| println!("{}", i));
Alternative implementation:
for i in (0..=5).rev() {
    println!("{}", i);
}
257
Traverse list backwards
Print each index i and value x from the list items, from the last down to the first. 
int i {items.size()};
while (i--)
    printf("%d %s\n", i, items[i]);
 
for (i, item) in items.iter().enumerate().rev() {
    println!("{} = {}", i, *item);
}
Alternative implementation:
  for (i, x) in items.iter().rev().enumerate() {
    println!("{i} = {x}");
  }
258
Convert list of strings to list of integers
Convert the string values from list a into a list of integers b. 
let b: Vec<i64> = a.iter().map(|x| x.parse::<i64>().unwrap()).collect();
Alternative implementation:
let b: Vec<i32> = a.iter().flat_map(|s| s.parse().ok()).collect();
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). 
using namespace std;
vector<string> parts;
regex r {R"([\Q,-_\E])"};
sregex_token_iterator i {
    s.begin(), s.end(), r, -1
}, n;
while (i not_eq n)
    parts.push_back(*i++);
 
let parts: Vec<_> = s.split(&[',', '-', '_'][..]).collect();
260
Create an empty list of strings
Declare a new list items of string elements, containing zero elements 
std::vector<std::string> items;
 
let items: Vec<String> = vec![];
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. 
let format = format_description!("[hour]:[minute]:[second]");
let x = d.format(&format).expect("Failed to format the time");
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 
let t = n.trailing_zeros();
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. 
using namespace std;
double x {log2(2)};
auto log2d = [&x] (double n) {
    return floor(log(n) / x);
};
auto log2u = [&x] (double n) {
    return ceil(log(n) / x);
};
for (int i {1}; i <= 12; ++i) {
    printf("%f, ", log2d(i));
    printf("%f\n", log2u(i));
}
 
fn log2d(n: f64) -> f64 {
    n.log2().floor()
}

fn log2u(n: f64) -> f64 {
    n.log2().ceil()
}

fn main() {
    for n in 1..=12 {
        let f = f64::from(n);
        println!("{} {} {}", n, log2d(f), log2u(f));
    }
}
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. 
fn foo(matrix: &[Vec<i32>]) {
    let iter = matrix.iter();
    let (vertical, _) = iter.size_hint();
    let horizontal = iter
        .max()
        .expect("empty array!")
        .len();
    println!("{horizontal} by {vertical}");
}

fn main() {
    let matrix = vec![
        vec![1, 2, 3],
        vec![4, 5, 6],
    ];
    foo(&matrix);
}
Alternative implementation:
fn foo<const X: usize, const Y: usize>(_: [[i32;X];Y]) {
    println!("{} {}", Y, X);
}

let a = [
    [1, 2, 3],
    [4, 5, 6],
];
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. 
let p = i.count_ones() % 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.reserve(v.size() * n);
for (size_t i = 0; i < n; ++i)
  s.append(v);
 
let s = v.repeat(n);
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. 
fn foo(x: &dyn Any) {
    if let Some(s) = x.downcast_ref::<String>() {
        println!("{}", s);
    } else {
        println!("Nothing")
    }
}

fn main() {
    foo(&"Hello, world!".to_owned());
    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. 
struct Vector {
    x: f32,
    y: f32,
    z: f32,
}

impl Mul for Vector {
    type Output = Self;

    fn mul(self, rhs: Self) -> Self {
        Self {
            x: self.y * rhs.z - self.z * rhs.y,
            y: self.z * rhs.x - self.x * rhs.z,
            z: self.x * rhs.y - self.y * rhs.x,
        }
    }
}
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. 
let e = t::bike;
let s = format!("{:?}", e);

println!("{}", s);
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." 
template<typename T>
void tst(T)
{
    using namespace std;

    // Strip away any references and pointer
    typedef remove_const<remove_pointer<decay<T>::type>::type>::type D;

    if(is_same<D, foo>::value)
    {
        cout << "same type" << endl;
    }
    else if(is_base_of<foo, D>::value)
    {
        cout << "extends type" << endl;
    }
    else
    {
        cout << "not related" << endl;
    }
}
 
fn type_of<T>(_: &T) -> &str {
    std::any::type_name::<T>()
}


if type_of(&x) == type_of(&foo) {
    println!("x & foo -> same type");
} else {
    println!("x & foo -> 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(int i = 1; i <= 100; i++)
{
    if((i % 15) == 0) std::cout << "FizzBuzz" << std::endl;
    else if((i % 5) == 0) std::cout << "Buzz" << std::endl;
    else if((i % 3) == 0) std::cout << "Fizz" << std::endl;
    else std::cout << i << std::endl;
}
Alternative implementation:
for (int n=1; n<=100; n++) {
        string out="";
        if (n%3==0) {
            out=out+"Fizz";
        }
        if (n%5==0) {
            out=out+"Buzz";
        }
        if (out=="") {
            out=out+std::to_string(n);
        }
        cout << out << "\n";
    }
 
for i in 1..101 {
    match i {
        i if (i % 15) == 0 => println!("FizzBuzz"),
        i if (i % 3) == 0 => println!("Fizz"),
        i if (i % 5) == 0 => println!("Buzz"),
        _ => println!("{i}"),
    }
}
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) 
let b = fs::read_dir(p).unwrap().count() == 0;
274
Remove all white space characters
Create the string t from the string s, removing all the spaces, newlines, tabulations, etc. 
auto t = s;
t.erase(std::ranges::remove_if(t, [](const char c) { return std::isspace(c); }).begin(),t.end()) ;
 
let t: String = s.chars().filter(|c| !c.is_whitespace()).collect();
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). 
const size_t n = s.length() / 8;

vector<uint8_t> a(n);

for(size_t block = 0; block < n; block++)
{
    uint8_t acc = 0;
    const size_t start = block * 8;
    for(size_t offset = start; offset < start + 8; offset++)
    {
        acc = (acc << 1) + (s[offset] - '0');
    }

    a[block] = acc;
}
 
let a: Vec<u8> = s.as_bytes()
    .chunks(8)
    .map(|chunk| unsafe {
        let chunk_str = std::str::from_utf8_unchecked(chunk);
        u8::from_str_radix(chunk_str, 2).unwrap_unchecked()
    })
    .collect();
276
Insert an element in a set
Insert an element e into the set x. 
x.insert(e);
 
x.insert(e);
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.erase(e);
 
x.remove(e);
Alternative implementation:
x.take(e)
278
Read one line from the standard input
Read one line into the string line.

Explain what happens if EOF is reached. 
int main() {
    for (std::string line; std::getline(std::cin, line);) {
        std::cout << line << std::endl;
    }
    return 0;
}
 
let mut buffer = String::new();
let mut stdin = io::stdin();
stdin.read_line(&mut buffer).unwrap();
279
Read list of strings from the standard input
Read all the lines (until EOF) into the list of strings lines. 
let lines = std::io::stdin().lock().lines().map(|x| x.unwrap()).collect::<Vec<String>>();
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. 
for (auto it = m.begin(); it != m.end();) {
  if (!p(it->second)) {
    it = m.erase(it);
  } else {
    ++it;
  }
}
 
m.retain(|_, &mut v| 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). 
let mut map: HashMap<Point, String> = HashMap::new();
map.insert(Point { x: 42, y: 5 }, "Hello".into());
283
Split with a custom string separator
Build the list parts consisting of substrings of input string s, separated by the string sep. 
let parts = s.split(sep);
Alternative implementation:
let parts = s.split(sep).collect::<Vec<&str>>();
Alternative implementation:
let parts: Vec<&str> = s.split(sep).collect();
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. 
std::vector<int> a(n, 0);
 
let a = vec![0; 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 = std::numeric_limits<float>::quiet_NaN();
b = std::numeric_limits<float>::signaling_NaN();
 
let a: f64 = f64::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 s.chars().enumerate() {
    println!("Char {} is {}", i, c);
}
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. 
let n = s.len();
288
Check if set contains a value
Set the boolean b to true if the set x contains the element e, false otherwise. 
b = x.find(e) != x.end();
 
let b = x.contains(&e);
289
Concatenate two strings
Create the string s by concatenating the strings a and b. 
std::string a{"Hello"};
std::string b{"world"};
auto s = a + b;

let s = format!("{}{}", a, b);
Alternative implementation:
let 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].sort_by(c);
291
Remove sublist
Delete all the elements from index i (included) to index j (excluded) from the list items. 
items.drain(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.
 
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. 
let mut s: Vec<T> = vec![];
s.push(x);
let y = s.pop().unwrap();
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.
 
std::array a{1, 12, 42};

{
  std::stringstream _ss;
  auto iter = a.begin();
  _ss << *iter++;
  for (; iter != a.cend(); _ss << std::string_view(", ") << *iter++);
  std::cout << _ss.view();
}
Alternative implementation:
::std::cout << (a
  | ::std::views::transform([](int const i){return ::std::to_string(i);})
  | ::std::views::join_with(::std::string(", "))
  | ::std::ranges::to<::std::string>());
 
let a = [1, 12, 42];
println!("{}", a.map(|i| i.to_string()).join(", "))
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.
 
using namespace std::literals;

enum T {
  case_0,
  case_1,
  case_2,
  case_4 = 4
};

std::optional<T> TryStrToEnum(std::string_view s) {
  if (str == "case_0"sv) return case_0;
  if (str == "case_1"sv) return case_1;
  if (str == "case_2"sv) return case_2;
  if (str == "case_4"sv) return case_4;
  return std::nullopt;
}
Alternative implementation:
using namespace std;
enum struct T { x, y, z };
map<string, T> m {
    {"x", T::x},
    {"y", T::y},
    {"z", T::z}
};
T TryStrToEnum(string s) {
    auto p {m.find(s)};
    if (p not_eq m.end())
        return p->second;
    throw exception();
}
 
enum MyEnum {
    Foo,
    Bar,
    Baz,
}

impl FromStr for MyEnum{
    type Err =  String;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        match s.to_lowercase().as_str() {
            "foo" => Ok(MyEnum::Foo),
            "bar" => Ok(MyEnum::Bar),
            "baz" => Ok(MyEnum::Baz),
            _ => Err("Could not convert str to enum".to_string())
        }
    }
}
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. 
fn fib(n: i32) -> i32 {
    if n < 2 {
        1
    } else {
        fib(n - 2) + fib(n - 1)
    }
}
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 = std::format("Our sun has {} planets", x);
 
let s = format!("Our sun has {} planets", x);
Alternative implementation:
let s = format!("Our sun has {x} planets");
304
Encode string into UTF-8 bytes
Create the array of bytes data by encoding the string s in UTF-8. 
let data = s.into_bytes();
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. 
x.reserve(200);
307
XOR encrypt/decrypt string
Create a function that XOR encrypts/decrypts a string 
fn xor(s: Vec<u8>, key: &[u8]) -> Vec<u8> {
    let mut b = key.iter().cycle();
    s.into_iter().map(|x| x ^ b.next().unwrap()).collect()
}
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"

 
using namespace std;
string s;
int x {n}, y, c;
do {
    y = x % b;
    c = y > 9 ? 'a' - 10 : '0';
    s = char(c + y) + s;
} while (x /= b);
 
let s = radix(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. 
let y = x.clone();
310
Fill array with random bytes
Fill the byte array a with randomly generated bytes. 
let mut rng = rand::thread_rng();
rng.fill(&mut a);
312
Test for list equality
Set b to true if the lists p and q have the same size and the same elements, false otherwise. 
bool b = ::std::ranges::equal(p, q);
 
b = p == q;
313
Test for map equality
Set b to true if the maps m and n have the same key/value entries, false otherwise. 
b = m == n;
314
Fill array with value
Set all the elements in the array x to the same value v 
x.fill(v);
Alternative implementation:
fill(begin(x), end(x), v);
 
x.fill(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. 
using namespace std;
template <typename F, typename T>
auto m(F f) {
    return [f] (T x) {
        map<T, T> static m;
        auto static n {m.end()};
        auto i {m.find(x)};
        if (i not_eq n) return i->second;
        return m[x] = f(x);
    };
}
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 
fn random_string(n: usize) -> String {
    rand::thread_rng()
        .sample_iter(Alphanumeric)
        .take(n)
        .map(char::from)
        .collect()
}
Alternative implementation:
fn random_string(n: usize) -> String {
    Alphanumeric.sample_string(&mut rand::thread_rng(), n)
}
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. 
let mut rng = rand::thread_rng();
let x = rng.gen_range(0..18);
319
Generator function
Write a function g that behaves like an iterator.
Explain if it can be used directly in a for loop. 
using namespace std;
template <typename T>
auto x(function<T(T)> f, T t = {}) {
    return [=] {
        T static x = t;
        return x = f(x);
    };
}
function f {[] (int x) { return x + 1; }};
function g {x(f)};
320
Test if string is empty
Set b to true if the string s is empty, false otherwise 
b = s.empty();
 
b = s.is_empty()
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. 
let c = s.chars().nth(i).expect("s is too short");
322
replace value of variable with new one and return old value 
std::exchange(x, v);
 
std::mem::replace(&mut x, v);
324
Read HTTP response header
Set the string c to the (first) value of the header "cache-control" of the HTTP response res. 
let c = res.headers()
    .get(CACHE_CONTROL)
    .expect("cache-control header exists")
    .to_str()
    .expect("cache-control header contains only visible ascii");
325
Create a queue
Create a new queue q, then enqueue two elements x and y, then dequeue an element into the variable z. 
let mut q = VecDeque::new();
q.push_back(x);
q.push_back(x);
let z = q.pop_front(); 
println!("1st item ~> {}",z.unwrap());
326
Milliseconds since Unix epoch
Assign to t the number of milliseconds elapsed since 00:00:00 UTC on 1 January 1970. 
auto t = std::chrono::milliseconds(std::chrono::system_clock::now().time_since_epoch()).count();
 
let t: u128 = SystemTime::now()
    .duration_since(UNIX_EPOCH)
    .expect("we run this after `UNIX_EPOCH`")
    .as_millis();
327
Convert string to lower case
Assign to t the value of the string s, with all letters mapped to their lower case. 
let t = s.to_lowercase()
328
Convert string to upper case
Assign to t the value of the string s, with all letters mapped to their upper case. 
let t = s.to_uppercase();
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. 
let v = m.get(&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. 
let a = m.into_values().collect::<Vec<_>>();
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();
332
List of the keys of a map
Create the list k containing all the keys of the map m 
m.keys().collect::<Vec<_>>()
335
List to map
Create the map m containing all the elements e of the list a, using as key the field e.id. 
let mut m = HashMap::new();

for e in a {
	m.entry(e.id).or_insert_with(Vec::new).push(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 
let x = b.pow(n)
339
Clear a byte array
Set all the elements of the byte array a to zero 
a.fill(0);
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. 
let c = s.chars().last().unwrap();
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. 
let i = x.rfind(y);
342
Leap year?
Determine if the current year is a leap year. 
let leap_year = year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);
343
Rename file
Rename the file at path1 into path2 
std::filesystem::rename(path1, path2);
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/Decimal
https://en.wikipedia.org/wiki/Fraction 
using namespace std;
string s {to_string(a)}, f;
regex p {"\\."};
sregex_token_iterator r {
    begin(s), end(s), p, -1
};
int i {stoi(*r)}, n {stoi(*++r)},
    d {int(pow(10, r->length()))},
    v {gcd(n, d)};
if (v) {
    n /= v;
    d /= v;
}
if (not n) f = to_string(i);
else {
    f = to_string(n) + '/' + to_string(d);
    if (i) f = to_string(i) + ' ' + f;
}
349
Convert a Fraction into a Decimal value
Parse a fraction 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
https://en.wikipedia.org/wiki/Decimal 
using namespace std;
int x (f.find(' ')), y (f.find('/'));
double a;
if (y == -1) a = stoi(f);
else {
    int n {stoi(f.substr(x + 1, y))},
        d {stoi(f.substr(y + 1))};
    a = double(n) / d;
    if (x not_eq -1)
        a += stoi(f.substr(0, x));
}
Alternative implementation:
using namespace std;
regex r {"[ /]"};
sregex_token_iterator i {
    f.begin(), f.end(), r, -1
}, n;
int x {stoi(*i++)}, y {stoi(*i++)};
double a {
    i not_eq n ?
        x + y / stod(*i) :
        x / double(y)
};
355
Absolute value
Assign to y the absolute value of the number n 
let y = n.abs();
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). 
using namespace std;
vector<int> items;
int x, y, n (s.length()), i;
for (x = 0; x not_eq n; ++x)
    if (not isspace(s[x])) {
        for (y = x + 1; y not_eq n; ++y)
            if (isspace(s[y])) break;
        i = stoi(s.substr(x, y - x));
        items.push_back(i);
        x = y - 1;
    }
 
let items = s
    .split_whitespace()
    .map(|x| x.parse::<i32>().unwrap())
    .collect::<Vec<i32>>();
Alternative implementation:
let items = s
    .split_whitespace()
    .map(|x| x.parse::<i32>().unwrap())
    .collect::<Vec<i32>>();
357
Swap elements of list
Swap the elements at indices i, j in the list items 
items.swap(i, j);
358
Format a value in unit notation
Create the end-user text, s, specifying the value a, in units of x.

For example, "0 files", "1 file", or "1,000 files". 
std::string s {a == 1 ? "file" : "files"};
360
Create a formatted list
Create the end-user text, s, specifying the contents of collection a.

For example, "A", "A and B", "A, B, and C", etc. 
int n {size(a)};
string s;
if (n == 1) s = a[0];
else if (n == 2)
    s = a[0] + " and "s + a[1];
else {
    int i {};
    s = a[i++];
    --n;
    while (i not_eq n)
        s += ", "s + a[i++];
    s += ", and "s + a[n];
}
364
Calculate the magnitude symbol for a value
Calculate the SI (International System of Units) prefix, x, of value a.

For example, 'k', 'M', 'G', 'T', etc.

https://en.wikipedia.org/wiki/Metric_prefix 
using namespace std;
struct units {
    double y;
    vector<char> a;
    units(int x) { y = log(x); }
    void add(char x) { a.push_back(x); }
    pair<int, char> get(int x) {
        int i (log(x) / y);
        return {i, a[i - 1]};
    }
};
units u {1'000};
for (char x : "kMGT") u.add(x);
pair p {u.get(a)};
a /= pow(1'000, p.first);
char x {p.second};
Alternative implementation:
int y (log(a) / log(1'000));
a /= pow(1'000, y);
char x {"kMGT"[y - 1]};
365
Convert an angle to a direction
Convert a degree, x, of 360, to a Compass direction, y.

For example, `123.4 deg` is "South-east". 
using namespace std;
namespace point {
    array a {
        "N", "NE", "E", "SE",
        "S", "SW", "W", "NW"
    };
    string parse(double& x) {
        double b {(x / 45) + .5};
        return a[int(b) % 8];
    }
}
string y {point::parse(a)};
Alternative implementation:
char y {"NESW"[int((x / 90.) + .5) % 4]};
367
Find the next significant digit
Floor a decimal value, x, to the first significant digit.

For example, `1.00123` is `1.001`.

https://en.wikipedia.org/wiki/Significant_figures 
double i, n {modf(x, &i)},
       y {log(n) / log(.1)},
       d {pow(10, floor(y) + 1)};
x = trunc(x * d) / d;
368
Parse a millisecond value
Parse the millisecond value a, to a collection of duration values, m.

For example, `1,000,000 ms` is `{min=16, sec=40}`.

https://en.wikipedia.org/wiki/Gregorian_calendar
https://en.wikipedia.org/wiki/Time 
using namespace std;
struct units : vector<pair<string, double>> {
    void set(string x, double y) {
        if (not empty()) y *= front().second;
        insert(begin(), {x, y});
    }
    auto get(int ms) {
        vector<pair<string, int>> m;
        for (auto& [x, y] : *this)
            if (ms >= y) {
                m.emplace_back(x, ms / y);
                ms %= int(y);
            }
        return m;
    }
};
369
Parse matching brackets
Generate a collection, a, of index-values, given the text, s, containing paired bracket values.

For example, "{ { } } { }" is `(0, 6) (2, 4) (8, 10)`.

https://en.wikipedia.org/wiki/Bracket_(mathematics)
https://en.wikipedia.org/wiki/Scope_(computer_science) 
using namespace std;
struct xy { int x, y; };
vector<xy> a;
int x, y, m (s.size()), n, c;
for (x = 0; x not_eq m; ++x)
    if (s[x] == '{')
        for (y = x + (n = 1); y not_eq m; ++y)
            if ((c = s[y]) == '{') ++n;
            else if (c == '}' and not --n) {
                a.push_back({x, y});
                break;
            }
370
Determine the Numeral value of an Integer
Create the end-user text, s, specifying the numeral value of integer a.

For example, 1 is "1st", 2 is "2nd", 3 is "3rd", etc. 
using namespace std;
string s;
int x {a % 100};
if (x > 9 and x < 14) s = "th";
else switch (a % 10) {
    case 1: s = "st"; break;
    case 2: s = "nd"; break;
    case 3: s = "rd"; break;
    default: s = "th";
}
s = to_string(a) + s;
373
Size of set
Set n to the number of elements of the set x. 
int n (x.size());
374
Trim whitespace
Create the string t consisting of the string s without its leading and trailing whitespace characters. 
using namespace std;
string t {s};
int n (t.length()), x {}, y {n - 1};
while (x not_eq  n && isspace(t[x])) ++x;
while (y not_eq -1 && isspace(t[y])) --y;
t = t.substr(x, (y - x) + 1);