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JS Java
1
Print Hello World
Print a literal string on standard output 
document.write("Hello World!");
Alternative implementation:
console.log('Hello World');
 
System.out.println("Hello World");
2
Print Hello 10 times
Loop to execute some code a constant number of times 
for (let i = 0; i < 10; i++) {
  console.log("Hello");
}
Alternative implementation:
let count = 0;
while (count < 10) {
  count++; 
  console.log('Hello');
};
Alternative implementation:
[...Array(10)].forEach(() => console.log('Hello'))
Alternative implementation:
console.log( 'Hello\n'.repeat(10) )
 
for(int i=0;i<10;i++)
  System.out.println("Hello");
Alternative implementation:
System.out.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) 
var bli = function() { 
	console.log('Hello World!!!');
}
Alternative implementation:
function bli() { 
	console.log('Hello World!!!');
}
Alternative implementation:
let dog = 'Poodle';
const dogAlert = () => alert(dog);
 
void finish(String name){
  System.out.println("My job here is done. Goodbye " + name);
}
4
Create a function
Create a function which returns the square of an integer 
function square(x) { 
	return x * x;
}
Alternative implementation:
const square = (x) => x * x;
Alternative implementation:
const square = n => n**2
Alternative implementation:
const square = (number) => Math.pow(number, 2);
 
int square(int x){
  return x*x;
}
Alternative implementation:
Function<Integer,Integer> squareFunction = x -> x * x;
5
Create a 2D Point data structure
Declare a container type for two floating-point numbers x and y 
var p = { x: 1.122, y: 7.45 };
Alternative implementation:
const point = { x: 1, y: 2 };
 
class Point{
  double x;
  double y;
}
6
Iterate over list values
Do something with each item x of an array-like collection items, regardless indexes. 
items.forEach((x) => {
    doSomething(x);
});
Alternative implementation:
for (x of items) {
	doSomething(x);
}
Alternative implementation:
items.forEach(doSomething)
Alternative implementation:
for (var i = 0; i<items.length; i++) {
  var x = items[i];
  doSomething(x);
}
Alternative implementation:
items.map(doSomething)
 
for (Item x: items) {
    doSomething(x);
}
Alternative implementation:
for(int i=0;i<items.length;i++){
	doSomething( items[i] );
}
Alternative implementation:
items.stream().forEach(item -> doSomething(item));
7
Iterate over list indexes and values
Print each index i with its value x from an array-like collection items 
items.forEach((val, idx) => {
  console.log("index=" + idx + ", value=" + val);
});
Alternative implementation:
for (var i in items) {
   console.log("index=" + i + ", value=" + items[i]);
}
 
for (int i = 0; i < items.length; i++) {
    T x = items[i];
    System.out.printf("Item %d = %s%n", i, x);
}
Alternative implementation:
for (int i = 0; i < items.size(); i++) {
    T x = items.get(i);
    System.out.printf("Item %d = %s%n", i, x);
}
8
Initialize a new map (associative array)
Create a new map object x, and provide some (key, value) pairs as initial content. 
const x = {one: 1, two:2}
Alternative implementation:
const x = new Map();
x.set("one", 1);
x.set("two", 2);
Alternative implementation:
const x = new Map([["one",1],["two",2]]);
 
Map<String,Integer> x = new HashMap<>();
x.put("one", 1);
x.put("two", 2);
Alternative implementation:
final Map<String, Integer> x = new HashMap<String, Integer>() {{
    put("one", 1);
    put("two", 2);
    put("three", 3);
}};
9
Create a Binary Tree data structure
The structure must be recursive because left child and right child are binary trees too. A node has access to children nodes, but not to its parent. 
class Node {
  constructor (data) {
    this.data = data
    this.left = null
    this.right = null
  }
}
 
class BinTree<T extends Comparable<T>>{
   T value;
   BinTree<T> left;
   BinTree<T> right;
}
Alternative implementation:
interface Tree<T>{
    boolean search(T x);
}
class Nil<T extends Comparable<? super T>> implements Tree<T>{
    public boolean search(T x) { return false; }
}
class Node<T extends Comparable<? super T>> implements Tree<T> {
 T item;  Tree<T> left; Tree<T> right;
 public Node(T i, Tree<T> l, Tree<T> r) {
     item=i;  left=l;  right=r; }
 public boolean search(T x) {
     int cmp = x.compareTo(item);
     return cmp==0 || (cmp<0 && left.search(x)) || (cmp>0 && right.search(x));
 }
10
Shuffle a list
Generate a random permutation of the elements of list x 
for (var i = x.length - 1; i > 0; i--) {
    var j = Math.floor(Math.random() * (i + 1));
    var temp = x[j];
    x[j] = x[i];
    x[i] = temp;
}
 
Collections.shuffle(x);
11
Pick a random element from a list
List x must be non-empty. 
x[Math.floor(Math.random() * x.length)]
Alternative implementation:
const x = [1, 'two', 3, 4, 'five', 'six', 7, 8, 'nine' ];

const randomElement = x[~~(Math.random() * x.length)];
 
x.get((int)(Math.random()*x.size()))
Alternative implementation:
x.get(ThreadLocalRandom.current().nextInt(0, x.size()))
12
Check if list contains a value
Check if list contains a value x.
list is an iterable finite container. 
return list.indexOf(x) !== -1;
Alternative implementation:
return list.includes(x);
 
boolean contains(int[] list, int x){
  for(int y:list)
    if( y==x )
      return true;
  return false;
}
Alternative implementation:
boolean <T> contains(T[] list, T x){
  if( x==null){
    for(T y:list)
      if( y==null )
        return true;
  }else{
    for(T y:list)
      if( x.equals(y) )
        return true;
  }
  return false;
}
Alternative implementation:
list.contains(x)
13
Iterate over map keys and values
Access each key k with its value x from an associative array mymap, and print them. 
Object.entries(mymap).forEach(([key, value]) => {
	console.log('key:', key, 'value:', value);
});
Alternative implementation:
for (const key in mymap) {
    console.log('key:', key, 'value:', mymap[key]);
}
 
for (Map.Entry<Object, Object> entry : mymap.entrySet()) {
    Object k = entry.getKey();
    Object x = entry.getValue();
    System.out.println("Key=" + k + ", Value=" + x);
}
Alternative implementation:
mymap.forEach((k,x) -> System.out.println("Key=" + k + ", Value=" + x));
14
Pick uniformly a random floating point number in [a..b)
Pick a random number greater than or equals to a, strictly inferior to b. Precondition : a < b. 
a + (b-a) * Math.random();
Alternative implementation:
a + Math.random() * (b - a)
 
double pick(double a, double b){
	return a + (Math.random() * (b-a));
}
15
Pick uniformly a random integer in [a..b]
Pick a random integer greater than or equals to a, inferior or equals to b. Precondition : a < b. 
function pick(a, b) {
    return a + Math.floor(Math.random() * (b - a + 1));
}
 
int pick(int a, int b){
	return a + new Random().nextInt(b - a + 1);
}
16
Depth-first traversing of a binary tree
Call a function f on every node of binary tree bt, in depth-first infix order 
function dfs(bt) {
	if (bt === undefined) return;
	dfs(bt.left);
	f(bt);
	dfs(bt.right);
}
 
void dfs(BinTree bt) {
	if (bt.left != null) {
		dfs(bt.left);
        }
	f(bt);
	if (bt.right != null) {
		dfs(bt.right);
        }
}
Alternative implementation:
class BinTree {
	// ...

	void dfs() {
		if( left != null )
			left.dfs();
		f(this);
		if( right != null )
			right.dfs();
	}
}
Alternative implementation:
class BinTree<T> {
	// ...

	void dfs(Consumer<BinTree<T>> f) {
		if( left != null )
			left.dfs(f);
		f.accept(this);
		if( right != null )
			right.dfs(f);
	}
}
17
Create a Tree data structure
The structure must be recursive. A node may have zero or more children. A node has access to its children nodes, but not to its parent. 
class Node {
  constructor (value, children = []) {
    this.value = value
    this.children = children
  }
}
 
class Tree<K,V> {
  K key;
  V deco;
  List<Tree<K,V>> children = new ArrayList<>();
}
18
Depth-first traversing of a tree
Call a function f on every node of a tree, in depth-first prefix order 
function DFS(f, root) {
	f(root)
	if (root.children) {
		root.children.forEach(child => DFS(f, child))
	}
}
19
Reverse a list
Reverse the order of the elements of list x.
This may reverse "in-place" and destroy the original ordering. 
x = x.reverse();
 
static <T> void reverse(List<T> x){
	int n = x.size();
	for(int i=0;i<n/2;i++){
		T tmp = x.get(i);
		x.set(i, x.get(n-i-1));
		x.set(n-i-1, tmp);
	}
}
Alternative implementation:
Collections.reverse(x);
Alternative implementation:
Collections.reverse(x);
20
Return two values
Implement a function search which looks for item x in a 2D matrix m.
Return indices i, j of the matching cell.
Think of the most idiomatic way in the language to return the two values at the same time. 
function search(m, x) {
    for (var i = 0; i < m.length; i++) {
        for (var j = 0; j < m[i].length; j++) {
            if (m[i][j] == x) {
                return [i, j];
            }
        }
    }
    return false;
}
 
static class Position{
	int i;
	int j;
}

Position search(int[][] m, int x){
	for(int i=0;i<m.length;i++)
		for(int j=0;j<m[i].length;j++)
			if(m[i][j] == x){
				Position pos= new Position();
				pos.i = i;
				pos.j = j;
				return pos;
			}
	return null;
}
21
Swap values
Swap values of variables a and b 
var tmp = a;
a = b;
b = tmp;
Alternative implementation:
[a, b] = [b, a];
 
T tmp = a;
a = b;
b = tmp;
22
Convert string to integer
Extract integer value i from its string representation s (in radix 10) 
let i = parseInt(s, 10)
Alternative implementation:
const i = Number(s);
Alternative implementation:
const i = +s
 
int i = Integer.parseInt(s);
Alternative implementation:
int i = new Integer(s).intValue();
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. 
num.toFixed(2)
 
String s = String.format("%.2f", x);
24
Assign to string the japanese word ネコ
Declare a new string s and initialize it with the literal value "ネコ" (which means "cat" in japanese) 
s = "ネコ";
 
String 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" 
{
  // in file worker.js
  onmessage = ({data}) => {
    console.log (`Hello, ${data}`)
  }
}
{
  // in file main.js
  const worker = new Worker ('worker.js')
  worker.postMessage ('Alan')
}
 
https://gist.github.com/f7c174c53efaba4d8575
26
Create a 2-dimensional array
Declare and initialize a matrix x having m rows and n columns, containing real numbers. 
var x = [];
for (var i = 0; i < m; i++) {
  x[i] = [];
}
Alternative implementation:
const x = new Array(m).fill(new Array(n).fill(Math.random()));
 
double[][] x = new double[m][n];
27
Create a 3-dimensional array
Declare and initialize a 3D array x, having dimensions boundaries m, n, p, and containing real numbers. 
const x = new Array(m).fill(
  new Array(n).fill(
    new Array(p).fill(Math.random())
  )
)
 
double[][][] x = new double[m][n][p];
28
Sort by a property
Sort elements of array-like collection items in ascending order of x.p, where p is a field of the type Item of the objects in items. 
items.sort(function(a,b) {
  return compareFieldP(a.p, b.p);
});
 
Arrays.sort(items, new Comparator<Item>(){
	public int compare(Item a, Item b){
		return a.p - b.p;
	}
});
Alternative implementation:
Collections.sort(items, new Comparator<Item>(){
	@Override
	public int compare(Item a, Item b){
		return a.p - b.p;
	}
});
Alternative implementation:
items.stream().sorted(Comparator.comparing(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. 
let new_list = items.filter(function(val,idx,ary) { return idx != i });
Alternative implementation:
items.splice(i,1);
 
items.remove(i);
30
Parallelize execution of 1000 independent tasks
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. 
for (let i = 1; i <= 1000; i++) setTimeout(() => f(i), 0);
 
final ExecutorService executor = Executors.newFixedThreadPool(NB_THREADS);
for (int i = 1; i <= 1000; i++) {
  executor.submit(() -> f(i));
}
executor.shutdown();
31
Recursive factorial (simple)
Create the recursive function f which returns the factorial of the non-negative integer i, calculated from f(i-1) 
function f(n) {
   return n<2 ? 1 : n * f(n-1);
}
Alternative implementation:
const fact = n => n === 0 ? 1 : n * fact(n-1)
 
int f(int i) {
    if (i == 0)
        return 1;
    else
        return i * f(i - 1);
}
32
Integer exponentiation by squaring
Create function exp which calculates (fast) the value x power n.
x and n are non-negative integers. 
function exp(x, n) {
   if (n===0) return 1;
   if (n===1) return x;
   return n%2 ? x * exp(x*x, (n-1)/2)
              : exp(x*x, n/2);
}
Alternative implementation:
const exp = Math.pow;
 
int exp(int x, int n){
	if(n==0)
		return 1;
	if(n==1)
		return x;
	if(n%2==0)
		return exp(x*x, n/2);
	else
		return x * exp(x*x, (n-1)/2);
}
33
Atomically read and update variable
Assign to the variable x the new value f(x), making sure that no other thread may modify x between the read and the write. 
let x = f(x)
 
synchronized(lock){
  x = f(x);
}
34
Create a set of objects
Declare and initialize a set x containing objects of type T. 
let x = new Set();
 
Set<T> x = new HashSet<T>();
Alternative implementation:
Set<T> x = new HashSet<T>();
x.add(a);
x.add(b);
35
First-class function : compose
Implement a function compose (A -> C) with parameters f (A -> B) and g (B -> C), which returns composition function g ∘ f 
function compose(f,g){
    return function(x){
        return g(f(x));
    };
}
 
public Function<A, C> compose(Function<A, B> f, Function<B, C> g) {
   return x -> g.apply(f.apply(x));
}
Alternative implementation:
Function<Integer, Integer> addOne = i-> i + 1;
Function<Integer, String> toString = i-> i.toString();
Function<Integer, String> printIncremented = toString.compose(addOne);
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. 
function compose(f,g){
    return function(x){
        return g(f(x));
    };
}
Alternative implementation:
const compose = f => g => x => g(f(x));
 
    default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
        Objects.requireNonNull(before);
        return (V v) -> apply(before.apply(v));
    }
37
Currying
Transform a function that takes multiple arguments into a function for which some of the arguments are preset. 
function curry (fn, scope) {
   
    scope = scope || window;
    
    // omit curry function first arguments fn and scope
    var args = Array.prototype.slice.call(arguments, 2);
    
    return function() {
	var trueArgs = args.concat(Array.prototype.slice.call(arguments, 0));
        fn.apply(scope, trueArgs);
    };
}
Alternative implementation:
const curry = (fn, ...initialArgs) => (...args) => fn(...initialArgs, ...args);

const add = (a, b) => a + b;

const add5 = curry(add, 5);

const result = add5(1) // 6
 
IntBinaryOperator simpleAdd = (a, b) -> a + b;
IntFunction<IntUnaryOperator> curriedAdd = a -> b -> a + b;
System.out.println(simpleAdd.applyAsInt(4, 5));
System.out.println(curriedAdd.apply(4).applyAsInt(5));
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. 
let t = s.substring(i, j);
Alternative implementation:
let t = s.slice(i, j);
 
String t = s.substring(i,j);
39
Check if string contains a word
Set boolean ok to true if string word is contained in string s as a substring, or to false otherwise. 
var ok = s.indexOf(word) !== -1;
Alternative implementation:
var ok = s.includes(word);
 
boolean 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. 
class Graph{
  List<Vertex> vertices;

  static class Vertex{
    int id;
    List<Vertex> neighbours;
  }
}
Alternative implementation:
class Graph{
  Set<Vertex> vertices;

  static class Vertex{
    int id;
    Set<Vertex> neighbours;
  }
}
41
Reverse a string
Create string t containing the same characters as string s, in reverse order.
Original string s must remain unaltered. Each character must be handled correctly regardless its number of bytes in memory. 
var t = s.split("").reverse().join("");
 
String t = new StringBuilder(s).reverse().toString();
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. 
OUTER:
for (var i in a) {
   for (var j in b) {
      if (a[i] === b[j]) {
         continue OUTER;
      }
   }
   console.log(a[i] + " not in the list");
}
 
mainloop: for(int v:a){
	for(int w:b){
		if(v==w)
			continue mainloop;
	}
	System.out.println(v);
}
43
Break outer loop
Look for a negative value v in 2D integer matrix m. Print it and stop searching. 
OUTER:
for (var i in m) {
   for (var j in m[i]) {
      if (m[i][j] < 0) {
         console.log("Negative value found: "+m[i][j]);
         break OUTER;
      }
   }
}
 
mainloop: for(int i=0;i<m.length;i++)
	for(int j=0;j<m[i].length;j++)
		if(m[i][j]<0){
			System.out.println(m[i][j]);
			break mainloop;
		}
44
Insert element in list
Insert element x at position i in list s. Further elements must be shifted to the right. 
s.splice(i, 0, x);
 
s.add(i, x);
45
Pause execution for 5 seconds
Sleep for 5 seconds in current thread, before proceeding with next instructions. 
setTimeout(function(){
	// Instructions after delay
},5000);
Alternative implementation:
const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));

// use inside an async function
await sleep(5000);

Thread.sleep(5000);
Alternative implementation:
TimeUnit.SECONDS.sleep(5);
46
Extract beginning of string (prefix)
Create string t consisting of the 5 first characters of string s.
Make sure that multibyte characters are properly handled. 
let t = s.substring(0,5);
 
String t = s.substring(0,5);
47
Extract string suffix
Create string t consisting in the 5 last characters of string s.
Make sure that multibyte characters are properly handled. 
var t = s.slice(-5);
 
int n = s.length();
String t = s.substring(n-5,n);
48
Multi-line string literal
Assign to variable s a string literal consisting in several lines of text, including newlines. 
let s = "This is a very long string which needs \n" +
        "to wrap across multiple lines because \n" +
        "otherwise my code is unreadable.";
Alternative implementation:
let s = "This is a very long string which needs \
to wrap across multiple lines because \
otherwise my code is unreadable.";
Alternative implementation:
let s = `This is a very long string which needs 
to wrap across multiple lines because 
otherwise my code is unreadable.`;
 
String s = "This is a very long string which needs \n" +
           "to wrap across multiple lines because \n" +
           "otherwise my code is unreadable.";
Alternative implementation:
String s = """
This is a very long string which needs
to wrap across multiple lines because
otherwise my code is unreadable.
""";
49
Split a space-separated string
Build list chunks consisting in substrings of input string s, separated by one or more space characters. 
let chunks = s.split(/ +/);
 
String[] chunks = s.split("\\s+");
50
Make an infinite loop
Write a loop that has no end clause. 
while (true) {
	// to infinity
}
Alternative implementation:
for(;;) {
	console.log('Oops')
}
 
for(;;){
	// Do something
}
Alternative implementation:
while(true) {
	// Do something	
}
51
Check if map contains key
Determine whether map m contains an entry for key k 
k in m
Alternative implementation:
m.hasOwnProperty(k)
Alternative implementation:
_m.has(_k)
 
m.containsKey(k)
52
Check if map contains value
Determine whether map m contains an entry with value v, for some key. 
Object.values(m).includes(v)
Alternative implementation:
[...m.values()].includes(v)
 
m.containsValue(v)
53
Join a list of strings
Concatenate elements of string list x joined by the separator ", " to create a single string y. 
y = x.join(", ");
 
String y = String.join(", ", x);
54
Compute sum of integers
Calculate the sum s of the integer list or array x. 
var s = x.reduce((a, b) => a + b, 0);
Alternative implementation:
var s = x.reduce((a, b) => a + b)
 
int s = 0;
for (int i : x) {
  s += i;
}
55
Convert integer to string
Create the string representation s (in radix 10) of integer value i. 
var s = i.toString();
Alternative implementation:
var s = i + "";
 
String s=((Integer)i).toString();
Alternative implementation:
String s = Integer.toString(i);
Alternative implementation:
String s = String.valueOf(i);
Alternative implementation:
String s = "" + 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". 
{
  // in file worker.js
  onmessage = f
}
{
  // in file main.js
  for (let i = 0; i < 1000; i++) {
    new Worker ('worker.js')
      .postMessage (i)
  }
}
 
class Task implements Runnable {
	int i;
	Task(int i) {
		this.i = i;
	}
	@Override
	public void run() {
		f(i);
	}
}

ExecutorService executor = Executors.newFixedThreadPool(4);
for (int i = 1; i <= 1000; i++) {
	Task task = new Task(i);
	executor.submit(task);
}
executor.shutdown();
executor.awaitTermination(10L, TimeUnit.MINUTES);
System.out.println("Finished");
Alternative implementation:
ExecutorService executor = Executors.newFixedThreadPool(4);
for (int i = 1; i <= 1000; i++) {
	Task task = new Task(i);
	executor.submit(() -> f(i));
}
executor.awaitTermination(10L, TimeUnit.MINUTES);
System.out.println("Finished");
57
Filter list
Create the list y containing the items from the list x that satisfy the predicate p. Respect the original ordering. Don't modify x in-place. 
y = x.filter(p);
 
var y = x.stream().filter(p).collect(Collectors.toList());
58
Extract file content to a string
Create string lines from the content of the file with filename f. 
fs.readFile(f, (err, lines) => {
    if (err) {
        // Handle error...
    }

    // Work with `lines` here.
}
 
byte[] encoded = Files.readAllBytes(Paths.get(f));
String lines = new String(encoded, StandardCharsets.UTF_8);
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"). 
console.error(util.format("%d is negative", x));
Alternative implementation:
console.error(x, "is negative");
Alternative implementation:
console.error(`${x} is negative`);
 
System.err.format("%d is negative\n",x);
60
Read command line argument
Assign to x the string value of the first command line parameter, after the program name. 
const x = process.argv[2]
 
String x = args[0];
61
Get current date
Assign to variable d the current date/time value, in the most standard type. 
var d = Date.now();
Alternative implementation:
var d = new Date();
 
Instant d = Instant.now();
62
Find substring position
Set i to the position of string y inside string x, if exists. 
let i = x.indexOf(y);
 
int i = x.indexOf(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. 
var x2 = x.replace(new RegExp(y, 'g'), z);
 
String x2 = x.replace(y, z);
64
Big integer : value 3 power 247
Assign to x the value 3^247 
let x = 3n ** 247n;
 
BigInteger x = new BigInteger("3", 10).pow(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%" 
const s = Math.round (x * 1000) / 10 + '%'
 
String s = new DecimalFormat("0.0%").format(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 = x**n
 
BigInteger z = x.pow(n);
67
Binomial coefficient "n choose k"
Calculate binom(n, k) = n! / (k! * (n-k)!). Use an integer type able to handle huge numbers. 
const fac = x => x ? x * fac (x - 1) : x + 1
const binom = (n, k) => fac (n) / fac (k) / fac (n - k >= 0 ? n - k : NaN)
 
static BigInteger binom(int N, int K) {
    BigInteger ret = BigInteger.ONE;
    for (int k = 0; k < K; k++) {
        ret = ret.multiply(BigInteger.valueOf(N-k))
                 .divide(BigInteger.valueOf(k+1));
    }
    return ret;
}
68
Create a bitset
Create an object x to store n bits (n being potentially large). 
let x = new Buffer (Math.ceil (n / 8))
 
BitSet x = new BitSet(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. 
seed (s)
 
Random r = new Random(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. 
Math.random ()
 
Random rand = new Random(System.currentTimeMillis());
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. 
console.log(process.argv.slice(2).join(" "));
 
public class Echo {
    public static void main(final String... args) {
        out.println(join(" ", args));
    }
}
73
Create a factory
Create a factory named fact for any sub class of Parent and taking exactly one string str as constructor parameter. 
class Parent {
    constructor(str) {}
    fact(new_class, str) {
        if (new_class.prototype instanceof Parent) {
            return new new_class(str)
        }
    }
}

class Child extends Parent {}
74
Compute GCD
Compute the greatest common divisor x of big integers a and b. Use an integer type able to handle huge numbers. 
const gcd = (a, b) => b === 0 ? a : gcd (b, a % b)
 
BigInteger 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. 
const gcd = (a, b) => b === 0 ? a : gcd (b, a % b)
let x = (a * b) / gcd(a, b)
 
BigInteger a = new BigInteger("123456789");
BigInteger b = new BigInteger("987654321");
BigInteger x = a.multiply(b).divide(a.gcd(b));
76
Binary digits from an integer
Create the string s of integer x written in base 2.

E.g. 13 -> "1101" 
let s = x.toString(2);
 
String s = Integer.toBinaryString(x);
77
Complex number
Declare a complex x and initialize it with value (3i - 2). Then multiply it by i. 
var x = math.complex(-2, 3);
x = math.multiply(x, math.i);
 
Complex x = new Complex(-2.0, 3.0);
x = x.multiply(Complex.I);
78
"do while" loop
Execute a block once, then execute it again as long as boolean condition c is true. 
do {
   something();
} while (c);
 
do {
	someThing();
	someOtherThing();
} while(c);
79
Convert integer to floating point number
Declare floating point number y and initialize it with the value of integer x . 
let y = x + .0
 
int x = 10
float y = (float)x;
System.out.println(y);
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). 
let y = BigInt (x | 0)
 
int y = (int)x;
81
Round floating point number to integer
Declare integer y and initialize it with the rounded value of floating point number x .
Ties (when the fractional part of x is exactly .5) must be rounded up (to positive infinity). 
var y = Math.round(x);
 
long y = Math.round(x);
82
Count substring occurrences
Find how many times string s contains substring t.
Specify if overlapping occurrences are counted. 
let n = 0 // the number of occurences
let acc = s
let i
while ((i = acc.indexOf (t)) + 1) {
  n++
  acc = acc.slice (i + 1)
}
 
int count = StringUtils.countMatches(s, t);
83
Regex with character repetition
Declare regular expression r matching strings "http", "htttp", "httttp", etc. 
const r = /htt+p/
 
Pattern r = Pattern.compile("htt+p");
84
Count bits set in integer binary representation
Count number c of 1s in the integer i in base 2.

E.g. i=6 → c=2 
const c = i.toString(2).replace(/[^1]/g, '').length
 
int c = Integer.bitCount(i);
85
Check if integer addition will overflow
Write boolean function addingWillOverflow which takes two integers x, y and return true if (x+y) overflows.

An overflow may be above the max positive value, or below the min negative value. 
function addingWillOverflow(x, y) {
  return x > 0 && y > 0 && x > Number.MAX_SAFE_INTEGER - y
}

public boolean addingWillOverflow(int x, int y){
    boolean willOverFlow = false;
    if(x > 0 && y > 0){
        if(y > (Integer.MAX_VALUE - x)){
            willOverFlow = true;
        }
    }
    if(x < 0 && y < 0){
       if(y < (Integer.MIN_VALUE - x)){
           willOverFlow = true;
       }
    }
    return willOverFlow;
}
86
Check if integer multiplication will overflow
Write boolean function multiplyWillOverflow 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. 
static boolean multiplyWillOverflow(int x, int y) {
	return Integer.MAX_VALUE/x < y;
}
87
Stop program
Exit immediatly.
If some extra cleanup work is executed by the program runtime (not by the OS itself), describe it. 
process.exit()
 
System.exit(0);
88
Allocate 1M bytes
Create a new bytes buffer buf of size 1,000,000. 
let buf = new Buffer (1e6)
 
byte[] buf = new byte[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 new Error ('x is invalid')
 
throw new IllegalArgumentException("Invalid value for x:" + 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. 
const Foo = function Counter () {
  let n = 0
  Object.defineProperty (this, 'value', {get: () => n++})
}
{
  const counter = new Foo ()
  counter.value // 0
  counter.value // 1
}
91
Load JSON file into struct
Read from file data.json and write its content into object x.
Assume the JSON data is suitable for the type of x. 
const x = JSON.parse(fs.readFileSync('./data.json'));
Alternative implementation:
const x = require('./data.json');
92
Save object into JSON file
Write the contents of the object x into the file data.json. 
fs.writeFileSync('data.json', JSON.stringify(x));
93
Pass a runnable procedure as parameter
Implement procedure control which receives one parameter f, and runs f. 
function control(f){
	f();
}
 
static void control(Runnable f) {
    f.run();
}
94
Print type of 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. 
console.log(typeof x);
Alternative implementation:
console.log (x == null ? x + '' : x.constructor.name);
 
System.out.println(((Object)x).getClass().getName());
95
Get file size
Assign to variable x the length (number of bytes) of the local file at path. 
let x = read(path).length
 
long x = new File(path).length();
96
Check string prefix
Set boolean b to true if string s starts with prefix prefix, false otherwise. 
var b = s.startsWith(prefix);
Alternative implementation:
var b = (s.lastIndexOf(prefix, 0) === 0);
 
boolean b = s.startsWith(prefix);
97
Check string suffix
Set boolean b to true if string s ends with string suffix, false otherwise. 
var b = s.endsWith(suffix);
 
boolean b = s.endsWith(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 
new Date (ts * 1000)
 
Date d = new Date(ts * 1000);
99
Format date YYYY-MM-DD
Assign to string x the value of fields (year, month, day) of date d, in format YYYY-MM-DD. 
let x = d.toISOString().slice(0, 10 /* 'yyyy-mm-dd'.length */)
Alternative implementation:
const today = new Date();
const isoDate = today.toISOString().slice(0, 10);
 
String x = new SimpleDateFormat("yyyy-MM-dd").format(d);
Alternative implementation:
String x = String.format("%1$tY-%1$tm-%1$td", d)
100
Sort by a comparator
Sort elements of array-like collection items, using a comparator c. 
items.sort(c);
 
Arrays.sort(items, c);
Alternative implementation:
Collections.sort(items, c);
101
Load from HTTP GET request into a string
Make an HTTP request with method GET to URL u, then store the body of the response in string s. 
$.get(u, function(data){
  s = data;
});
Alternative implementation:
var xmlHttp = new XMLHttpRequest();
xmlHttp.onreadystatechange = function() { 
	if (xmlHttp.readyState == 4 && xmlHttp.status == 200)
		s = xmlHttp.responseText;
}
xmlHttp.open("GET", u, true);
xmlHttp.send(null);
Alternative implementation:
fetch(u)
  .then(res => res.text())
  .then(text => s = text)
Alternative implementation:
const res = await fetch(u)
s = await res.text()
105
Current executable name
Assign to string s the name of the currently executing program (but not its full path). 
var s = process.argv0;
 
String s = System.getProperty("sun.java.command");
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) 
let dir = process.cwd ()
 
String path = this.getClass().getClassLoader().getResource("").getPath();
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.) 
const dir = path.resolve();
Alternative implementation:
const dir = __dirname;
108
Determine if variable name is defined
Print the value of variable x, but only if x has been declared in this program.
This makes sense in some languages, not all of them. (Null values are not the point, rather the very existence of the variable.) 
if (typeof x !== 'undefined') {
    console.log(x);
}
Alternative implementation:
try {
	console.log(x);
} catch (e) {
	if (!e instanceof ReferenceError) {
		throw e;
	}
}
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. 
const blank = s == null || s.trim() === ''
 
boolean blank = StringUtils.isBlank(s);
Alternative implementation:
boolean blank = s==null || s.trim().isEmpty();
Alternative implementation:
boolean blank = s==null || s.isBlank();
111
Launch other program
From current process, run program x with command-line parameters "a", "b". 
exec(`${x} a b`);
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. 
[...mymap.entries()].sort().map(([_, x]) => console.log(x))
 
SortedMap<K, V> mymap = new TreeMap<>();
...
for(Map.Entry<K, V> e: mymap.entrySet())
	System.out.println("Key=" + e.getKey() + ", Value=" + e.getValue());
Alternative implementation:
List<K> keys = new ArrayList<>(mymap.keySet());
Collections.sort(keys);
for(K k: keys)
	System.out.println("Key=" + k + ", Value=" + mymap.get(k));
Alternative implementation:
var map = Map.of("a", 1, "d", 4, "c", 3, "b", 2);
new TreeMap<>(map).entrySet().forEach(System.out::println);
Alternative implementation:
mymap.entrySet().stream().sorted(Entry.comparingByKey()).forEach(System.out::println);
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.
Note that multiple entries may exist for the same value x. 
Object.entries(mymap)
  .sort((a, b) => a[1] - b[1])
  .forEach(([key, value]) => {
    console.log('key:', key, 'value:', value);
  });
114
Test deep equality
Set boolean b to true if objects x and y contain the same values, recursively comparing all referenced elements in x and y.
Tell if the code correctly handles recursive types. 
const b = JSON.stringify(x) === JSON.stringify(y);
Alternative implementation:
const arrayDeepEqual = (a, b) => a.length === b.length && a.every((x, i) => deepEqual(x, b[i]))

const deepEqual = (a, b) =>
  Array.isArray(a) && Array.isArray(b)
    ? arrayDeepEqual(a, b)
    : typeof a == 'object' && a && typeof b == 'object' && b
    ? arrayDeepEqual(Object.entries(a), Object.entries(b))
    : Number.isNaN(a) && Number.isNaN(b) || a === b

const b = deepEqual(x, y)
Alternative implementation:
const b = isDeepStrictEqual(x, y)
Alternative implementation:
const b = _.isEqual(x, y);
115
Compare dates
Set boolean b to true if date d1 is strictly before date d2 ; false otherwise. 
let b = d1 < d2
 
boolean before = (d1.compareTo(d2) == -1);
116
Remove occurrences of word from string
Remove all occurrences of string w from string s1, and store the result in s2. 
var regex = RegExp(w, 'g');
var s2 = s1.replace(regex, '');
Alternative implementation:
const s2 = s1.split(w).join('')
 
String s2 = s1.replace(w, "");
117
Get list size
Set n to the number of elements of the list x. 
var n = x.length;
 
int n = x.size();
Alternative implementation:
int n = x.length;
118
List to set
Create set y from list x.
x may contain duplicates. y is unordered and has no repeated values. 
var y = new Set(x);
 
Set<T> y = new HashSet<>(x);
Alternative implementation:
Set<T> y = new HashSet<T>(x);
119
Deduplicate list
Remove duplicates from the list x.
Explain if the original order is preserved. 
x = [...new Set(x)];
Alternative implementation:
x = Array.from(new Set(x));
Alternative implementation:
const seen = new Set();
x = x.filter( v => {
  if(seen.has(v))
    return false;
  seen.add(v);
  return true;
});
 
Set<T> uniques = new HashSet<>(x);
x.clear();
x.addAll(uniques);
Alternative implementation:
x = new ArrayList<T>(new HashSet<T>(x));
Alternative implementation:
final HashSet<T> seen = new HashSet<T>();
final Iterator<T> listIt = x.iterator();
while (listIt.hasNext()) {
  final T curr = listIt.next();
  if (seen.contains(curr)) {
    listIt.remove();
  } else {
    seen.add(curr);
  }
}
120
Read integer from stdin
Read an integer value from the standard input into variable n 
const {createInterface} = require('readline')

const rl = createInterface ({
  input: process.stdin,
  output: process.stdout
})

rl.question('Input an integer: ', response => {
  let n = parseInt (response)
  // stuff to be done with n goes here

  rl.close()
})
 
Scanner in = new Scanner(System.in);
n = in.nextInt();
121
UDP listen and read
Listen UDP traffic on port p and read 1024 bytes into buffer b. 
int len = 1024;
int p = 8888;
byte[] b = new byte[len];
try (DatagramSocket socket = new DatagramSocket(p)) {
  DatagramPacket packet = new DatagramPacket(b, len);
  socket.receive(packet);
}
122
Declare enumeration
Create an enumerated type Suit with 4 possible values SPADES, HEARTS, DIAMONDS, CLUBS. 
const spades = 0
const hearts = 1
const diamonds = 2
const clubs = 3
 
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. 
console.assert(_isConsistent);

assert isConsistent() : "State consistency violated";
124
Binary search for a value in sorted array
Write function binarySearch which returns the index of an element having value x in sorted array a, or -1 if no such element. 
function binarySearch(a, x, i = 0) {
  if (a.length === 0) return -1
  const half = (a.length / 2) | 0
  return (a[half] === x) ?
    i + half :
    (a[half] > x) ?
    binarySearch(a.slice(0, half), x, i) :
    binarySearch(a.slice(half + 1), x, half + i + 1)
}
 
static int binarySearch(final int[] arr, final int key) {
    final int index = Arrays.binarySearch(arr, key);
    return index < 0 ? - 1 : index;
}
125
Measure function call duration
measure the duration t, in nanoseconds, of a call to the function foo. Print this duration. 
console.time('foo');
foo();
console.timeEnd('foo');
126
Multiple return values
Write a function foo that returns a string and a boolean value. 
const foo = () => ({string: 'string', bool: true})
Alternative implementation:
const foo = () => ['string', true];
 
static Object[] returnAnything() {
    return new Object[]{"a string", true};
}

public static void main (String[] args) {
    Object[] array = returnAnything();
    System.out.println(array[0]);
    System.out.println(array[1]);
}
128
Breadth-first traversing of a tree
Call a function f on every node of a tree, in breadth-first prefix order 
static void breadthFirstSearch(Node root, Consumer<Node> f) {
    Queue<Node> queue = new LinkedList<>();
    queue.offer(root);

    while(!queue.isEmpty()) {
        Node polled = queue.poll();
        f.accept(polled);
        polled.children.forEach(a -> queue.offer(a));
    }
}
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. 
c1 ? f1 : c2 ? f2 : f3
Alternative implementation:
switch (true) {
  case c1:
    f1();
    break;
  case c2:
    f2();
    break;
  case c3:
    f3();
    break;
}
Alternative implementation:
if (c1) {
  f1();
} else if (c2) {
  f2();
} else if (c3) {
  f3();
}
 
if (c1) {
   f1();
} else if (c2) {
   f2();
} else if (c3) { 
   f3();
}
132
Measure duration of procedure execution
Run procedure f, and return the duration of the execution of f. 
function clock(f) {
  var start = new Date().getTime();
  f();
  var end = new Date().getTime();
  return end - start;
}
 
long clock(Runnable f) {
    long t0 = System.currentTimeMillis();
    f.run();
    long t1 = System.currentTimeMillis();
    return t1 - t0;
}
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. 
var lowerS = s.toLowerCase();
var lowerWord = word.toLowerCase();
var ok = lowerS.indexOf(lowerWord) !== -1;
 
ok = s.toLowerCase().contains(word.toLowerCase());
134
Create a new list
Declare and initialize a new list items, containing 3 elements a, b, c. 
const items = [a, b, c];
Alternative implementation:
const items = new Array(a, b, c);
 
List<T> items = new ArrayList<>();
items.add(a);
items.add(b);
items.add(c);
Alternative implementation:
List<T> items = Arrays.asList(a, b, c);
Alternative implementation:
var items = List.of(a, b, c);
135
Remove item from list, by its value
Remove at most 1 item from list items, having 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. 
const idx = items.indexOf(x)
if (idx !== -1) items.splice(idx, 1)
 
items.stream().findFirst().filter(item -> "x".equals(item)).ifPresent(removeIndex -> items.remove(removeIndex));
136
Remove all occurrences of a value from a list
Remove all occurrences of value x from list items.
This will alter the original list or return a new list, depending on which is more idiomatic. 
const newlist = items.filter(y => x !== y)
 
items.removeAll(Collections.singleton(x));
137
Check if string contains only digits
Set boolean b to true if string s contains only characters in range '0'..'9', false otherwise. 
var b = /^[0-9]+$/.test(s);
 
boolean b = s.matches("[0-9]*");
138
Create temp file
Create a new temporary file on the filesystem. 
const tempFile = tempy.file()
Alternative implementation:
const resultOfCallback = tempy.file.task(tempFile => {
 // do something with tempFile
})
 
File tempFile = File.createTempFile("prefix-", "-suffix");
tempFile.deleteOnExit();
139
Create temp directory
Create a new temporary folder on filesystem, for writing. 
const tempDir = await Deno.makeTempDir();
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.delete(k)
 
map.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. 
for (let item of items1) console.log(item)
for (let item of items2) console.log(item)
Alternative implementation:
items1.concat(items2).forEach(console.log)
 
items1.forEach(System.out::println);
items2.forEach(System.out::println);
Alternative implementation:
for (String item : items1) {
	System.out.println(item);
}
for (String item : items2) {
	System.out.println(item);
}
Alternative implementation:
Stream.concat(
	items1.stream(), 
	items2.stream()
).forEach(System.out::println);
142
Hexadecimal digits of an integer
Assign to string s the hexadecimal representation (base 16) of integer x.

E.g. 999 -> "3e7" 
const s = x.toString(16)
 
String s = Integer.toHexString(x);
143
Iterate alternatively over two lists
Iterate alternatively over the elements of the list items1 and items2. For each iteration, print the element.

Explain what happens if items1 and items2 have different size. 
const shorter = _items1.length > _items2.length ? _items2 : _items1;
const longer = _items1.length <= _items2.length ? _items2 : _items1;
shorter.map((m, i) => {
  console.log(m);
  console.log(longer[i]);
});
 
Iterator<String> iter1 = items1.iterator();
Iterator<String> iter2 = items2.iterator();
while (iter1.hasNext() || iter2.hasNext()) {
	if (iter1.hasNext()) {
		System.out.println(iter1.next());
	}
	if (iter2.hasNext()) {
		System.out.println(iter2.next());
	}
}
Alternative implementation:
IntStream.range(0, Math.max(items1.size(), items2.size()))
	.boxed()
	.flatMap(idx -> Stream.of(
		items1.size() > idx ? items1.get(idx) : null,
		items2.size() > idx ? items2.get(idx) : null
	))
	.filter(Objects::nonNull)
	.forEach(System.out::println);
144
Check if file exists
Set boolean b to true if file at path fp exists on filesystem; false otherwise.

Beware that you should never do this and then in the next instruction assume the result is still valid, this is a race condition on any multitasking OS. 
const b = fs.existsSync(fp);
 
boolean b = new File(fb).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. 
console.log(Date(), msg);
Alternative implementation:
console.error(Date(), msg);
 
Logger LOGGER = Logger.getLogger(MyClass.class.getName());

LOGGER.info(msg);
146
Convert string to floating point number
Extract floating point value f from its string representation s 
let f = +s
 
String floatString = "14.5";
float x = Float.parseFloat(floatString);
double y = Double.parseFloat(floatString);
147
Remove all non-ASCII characters
Create string t from string s, keeping only ASCII characters 
const t = [...s].filter(c => c.charCodeAt(0) <= 0x7f).join('')
Alternative implementation:
const t = s.replace(/[^\x00-\x7f]/gu, '')
 
String t = s.replaceAll("[^\\x00-\\x7F]", "");
148
Read list of integers from stdin
Read a list of integer numbers from the standard input, until EOF. 
process.stdin.on('data', processLine)

function processLine (line) {
  const string = line + ''
  console.log(string)
}
Alternative implementation:
const ints = await new Promise(resolve => {
  let string = ''
  process.stdin
    .on('data', data => string += data.toString())
    .on('close', () => resolve(string.split('\n').map(line => Number.parseInt(line))))
})
149
Rescue the princess
As an exception, this content is not under license CC BY-SA 3.0 like the rest of this website.
150
Remove trailing slash
Remove last character from string p, if this character is a slash /. 
const slashscrape = p => (
  p.slice (-1) === '/' ?
    p.slice (0, -1) :
    p
)
 
if (p.endsWith("/")) {
    p = p.substring(0, p.length() - 1);
}
Alternative implementation:
p = p.replaceAll("/$", "");
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! 
p = p.endsWith(path.sep) ? p.slice(0, -path.sep.length) : p
152
Turn a character into a string
Create string s containing only the character c. 
let s = c
 
String s = String.valueOf(c);
Alternative implementation:
String s = Character.toString(c);
Alternative implementation:
String s = c + "";
Alternative implementation:
String s = = new Character(c).toString();
153
Concatenate string with integer
Create string t as the concatenation of string s and integer i. 
let t = s + i;
Alternative implementation:
let t = `${s}${i}`
 
String t = 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. 
var c = "#";
for(var i = 0; i<3; i++) {
  var sub1 = c1.substring(1+2*i, 3+2*i);
  var sub2 = c2.substring(1+2*i, 3+2*i);
  var v1 = parseInt(sub1, 16);
  var v2 = parseInt(sub2, 16);
  var v = Math.floor((v1 + v2) / 2);
  var sub = v.toString(16).toUpperCase();
  var padsub = ('0'+sub).slice(-2);
  c += padsub;
}
 
String r1 = c1.substring(1,3);
String g1 = c1.substring(3,5);
String b1 = c1.substring(5,7);

String r2 = c2.substring(1,3);
String g2 = c2.substring(3,5);
String b2 = c2.substring(5,7);

String r = String.format("%02X", (Integer.parseInt(r1, 16)+Integer.parseInt(r2, 16))/2 );
String g = String.format("%02X", (Integer.parseInt(g1, 16)+Integer.parseInt(g2, 16))/2 );
String b = String.format("%02X", (Integer.parseInt(b1, 16)+Integer.parseInt(b2, 16))/2 );

String c = "#" + r + g + b;
Alternative implementation:
StringBuilder sb = new StringBuilder("#");
for(int i=0;i<3;i++) {
  String sub1 = c1.substring(1+2*i,3+2*i);
  String sub2 = c2.substring(1+2*i,3+2*i);
  int v1 = Integer.parseInt(sub1, 16);
  int v2 = Integer.parseInt(sub2, 16);
  int v = (v1 + v2)/2;
  String sub = String.format("%02X", v);
  sb.append(sub);
}
String c = sb.toString();
155
Delete file
Delete from filesystem the file having path filepath. 
try {
  fs.unlinkSync(filepath);
} catch (err) {
  console.error(err);
}
Alternative implementation:
await unlink(filepath)
 
new File(filepath).delete();
156
Format integer with zero-padding
Assign to string s the value of integer i in 3 decimal digits. Pad with zeros if i < 100. Keep all digits if i ≥ 1000. 
let s = i.toString();
if(i<100)
  s = ('00'+i).slice(-3);
 
String s = String.format("%03d", i);
157
Declare constant string
Initialize a constant planet with string value "Earth". 
const planet = 'Earth'
 
static final String planet = "Earth";
158
Random sublist
Create a new list y from randomly picking exactly k elements from list x.

It is assumed that x has at least k elements.
Each element must have same probability to be picked.
Each element from x must be picked at most once.
Explain if the original ordering is preserved or not. 
const idx = x.map((item, i) => i);
while (y.length < k) {
  const i = parseInt(Math.random() * idx.length, 10);
  y.push(x[[idx[i]]]);
  idx.splice(i, 1);
}
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. 
const is64Bit = os.arch() === 'x64' || process.env.hasOwnProperty('PROCESSOR_ARCHITEW6432');
is64Bit ? _f64() : _f32();

switch(System.getProperty("sun.arch.data.model")) {
  case "64": 
    f64(); 
    break;
  case "32":
    f32();
    break;
}
161
Multiply all the elements of a list
Multiply all the elements of the list elements by a constant c 
elements = elements.map(x => x * c)
 
elements.stream().map(e -> e*c).collect(Collectors.toList())
162
Execute procedures depending on options
execute bat if b is a program option and fox if f is a program option. 
const args = process.argv.slice(2)
if (args.includes('b')) bat()
else if (args.includes('f')) fox()
163
Print list elements by group of 2
Print all the list elements, two by two, assuming list length is even. 
for (let index = 0; index < list.length; index = index + 2) {
  console.log(list[index], list[index + 1])
}
 
for(int i = 0; i < list.length; i += 2) {
  System.out.println(list[i] + ", " + list[i + 1]);
}
165
Last element of list
Assign to variable x the last element of list items. 
const x = items[items.length - 1];
Alternative implementation:
const x = items.at(-1);
 
int x = items[items.length - 1];
166
Concatenate two lists
Create list ab containing all the elements of list a, followed by all elements of list b. 
var ab = a.concat(b);
 
var ab = new ArrayList<>(a);
ab.addAll(b);
167
Trim prefix
Create string t consisting of string s with its prefix p removed (if s starts with p). 
if (s.indexOf(p) === 0) {
    t = s.substring(p.length);
}
 
String t = s.replaceFirst("^" + p, "");
168
Trim suffix
Create string t consisting of string s with its suffix w removed (if s ends with w). 
const t = s.endsWith(w) ? s.slice(0, -w.length) : s
169
String length
Assign to integer n the number of characters of string s.
Make sure that multibyte characters are properly handled.
n can be different from the number of bytes of s. 
const n = s.length;
 
int n = s.length();
170
Get map size
Set n to the number of elements stored in mymap.

This is not always equal to the map capacity. 
const length = map.size
 
int n = mymap.size();
171
Add an element at the end of a list
Append element x to the list s. 
s.push(x);
Alternative implementation:
s = [...s, x];
 
s.add(x);
172
Insert an entry in a map
Insert value v for key k in map m. 
m.set(k, v);
Alternative implementation:
m[k] = v;
 
m.put(k, v);
173
Format a number with grouped thousands
Number will be formatted with a comma separator between every group of thousands. 
new Intl.NumberFormat().format(1000);
 
String.format("%,d", 1000000);
174
Make HTTP POST request
Make a HTTP request with method POST to URL u 
fetch(u, {
        method: "POST",
	body: JSON.stringify(data)
})
175
Bytes to hex string
From 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). 
const s = Buffer.from(a).toString('hex')
Alternative implementation:
const s = a.map(n => n.toString(16).padStart(2, '0')).join('')
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). 
s
  .split('')
  .map((el, ix, arr) => ix % 2 ? null : el + arr[ix + 1])
  .filter(el => el !== null)
  .map(x => parseInt(x, 16))
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. 
const pointInRect = ({x1, y1, x2, y2}, {x, y}) => (
  (x > x1 && x < x2) && (y > y1 && y < y2)
)
179
Get center of a rectangle
Return the center c of the rectangle with coördinates(x1,y1,x2,y2) 
const center = ({x1, y1, x2, y2}) => ({x: (x1 + x2) / 2, y: (y1 + y2) / 2})
Alternative implementation:
class Point {
  constructor (x, y) {
    this.x = x
    this.y = y
  }
}
const center = ({x1, y1, x2, y2}) => new Point ((x1 + x2) / 2, (y1 + y2) / 2)
 
double[] c = {(x1+x2)/2,(y1+y2)/2};
180
List files in directory
Create list x containing the contents of directory d.

x may contain files and subfolders.
No recursive subfolder listing. 
const x = fs.readdirSync(d)

final File directory = new File(d);
final File[] x = directory.listFiles();
182
Quine program
Output the source of the program. 
c=console.log
q=decodeURIComponent('%22')
l=[
"c=console.log",
"q=decodeURIComponent('%22')",
"l=[",
"]",
"for(i=0;i<3;i++)c(l[i])",
"for(i=0;i<7;i++)c(q+l[i]+q+',')",
"for(i=3;i<7;i++)c(l[i])",
]
for(i=0;i<3;i++)c(l[i])
for(i=0;i<7;i++)c(q+l[i]+q+',')
for(i=3;i<7;i++)c(l[i])
183
Make HTTP PUT request
Make a HTTP request with method PUT to URL u 
fetch(u, {
        method: "PUT",
	body: JSON.stringify(data)
})
184
Tomorrow
Assign to variable t a string representing the day, month and year of the day after the current date. 
var nextDate = new Date(new Date().getTime() + 24 * 60 * 60 * 1000);
var day = nextDate.getDate()
var month = nextDate.getMonth() + 1
var year = nextDate.getFullYear()
var t = `${day}/${month}/${year}`;
Alternative implementation:
var tomorrow = new Date();
tomorrow.setDate(tomorrow.getDate() + 1);
Alternative implementation:
var now = new Date()
var year = now.getFullYear()
var month = now.getMonth()
var day = now.getDate()

var tomorrow = new Date(0)
tomorrow.setFullYear(year, month, day + 1)
tomorrow.setHours(0, 0, 0, 0)

var shortDateFormat = Intl.DateTimeFormat(undefined, { dateStyle: "short" })
var t = shortDateFormat.format(tomorrow)
 
String t = LocalDate.now().plusDays(1).toString();
185
Execute function in 30 seconds
Schedule the execution of f(42) in 30 seconds. 
let id = setTimeout(f, 30000, 42);
 
ScheduledExecutorService executorService = Executors.newSingleThreadScheduledExecutor();
executorService.schedule(() -> _f(42), delayInSeconds, TimeUnit.SECONDS);
186
Exit program cleanly
Exit a program cleanly indicating no error to OS 
process.exit()
 
System.exit(0);
187
Disjoint Set
Disjoint Sets hold elements that are partitioned into a number of disjoint (non-overlapping) sets. 
public class DisjointSetElement {
  private DisjointSetElement representative = this;

  public DisjointSetElement getRepresentative() {
    if (representative != this) {
      representative = representative.getRepresentative();
    }
    return representative;
  }

  public void union(DisjointSetElement other) {
    other.getRepresentative().representative = getRepresentative();
  }
}
189
Filter and transform list
Produce a new list y containing the result of function T applied to all elements e of list x that match the predicate P. 
y = x.filter(e => P(e)).map(e => T(e))
 
x.stream().filter(P).map(T).collect(Collectors.toList());
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(a.some(item => item > x)){
	f()
}
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). 
const a = [[1, 2, 3], [4, 5, 6]]
const m = a[0].length
const b = Array.from({ length: m }, (_, n) => a.map(row => row[n]))
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). 
/**
 * @param {Array<Array>} arry
 *
 * @return {Array<Array>}
 */
function foo(arry) {
  let len = 0;
  let sum = 0;

  arry.forEach(function(base, i) {
    len += base.length;

    base.forEach(function(a, j) {
      sum += a * (i + 1) * (j + 1);
    });
  });

  console.log('Array size:', arry.length, ',', len);

  return sum;
}

foo(arry2d);
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. 
fs.readFileSync(path).split("\n")
 
List<String> lines = Files.readAllLines(new File(path).toPath());
198
Abort program execution with error condition
Abort program execution with error condition x (where x is an integer value) 
process.exit(x);
200
Return hypotenuse
Returns the hypotenuse h of the triangle where the sides adjacent to the square angle have lengths x and y. 
const h = Math.sqrt(a**2 + b**2)

// Before ES7
const h = Math.sqrt(a*a + b*b)

// Before ES6
var h = Math.sqrt(a*a + b*b)
201
Euclidean norm
Calculate n, the Euclidean norm of data (an array or list of floating point values). 
const n = Math.hypot(...data)
Alternative implementation:
var n = Math.hypot.apply(null, data)
202
Sum of squares
Calculate the sum of squares s of data, an array of floating point values. 
s = data.reduce((a, c) => a + c ** 2, 0)
 
Arrays.stream(data).map(i -> i * i).sum()
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
 
function frexp(a) {
    exponent = ( Math.floor(Math.log(a, 2)) + 1 )
    mantissa = ( a * Math.pow(2, -a) )

    return [ mantissa, 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 foo = process.env["FOO"] || "none";
Alternative implementation:
const foo = process.env.FOO ?? 'none'
 
String foo = S