Javascript - Generating all combinations of elements in a single array (in pairs)

Question

I\'ve seen several similar questions about how to generate all possible combinations of elements in an array. But I\'m having a very hard time figuring out how to write an a

Answer 1

In my case, I wanted to get the combinations as follows, based on the size range of the array:

function getCombinations(valuesArray: String[])
{

var combi = [];
var temp = [];
var slent = Math.pow(2, valuesArray.length);

for (var i = 0; i < slent; i++)
{
    temp = [];
    for (var j = 0; j < valuesArray.length; j++)
    {
        if ((i & Math.pow(2, j)))
        {
            temp.push(valuesArray[j]);
        }
    }
    if (temp.length > 0)
    {
        combi.push(temp);
    }
}

combi.sort((a, b) => a.length - b.length);
console.log(combi.join("\n"));
return combi;
}

Example:

// variable "results" stores an array with arrays string type
let results = getCombinations(['apple', 'banana', 'lemon', ',mango']);

Output in console:

The function is based on the logic of the following documentation, more information in the following reference: https://www.w3resource.com/javascript-exercises/javascript-function-exercise-3.php

Answer 2

A simple way would be to do a double for loop over the array where you skip the first i elements in the second loop.

let array = ["apple", "banana", "lemon", "mango"];
let results = [];

// Since you only want pairs, there's no reason
// to iterate over the last element directly
for (let i = 0; i < array.length - 1; i++) {
  // This is where you'll capture that last value
  for (let j = i + 1; j < array.length; j++) {
    results.push(`${array[i]} ${array[j]}`);
  }
}

console.log(results);

Rewritten with ES5:

var array = ["apple", "banana", "lemon", "mango"];
var results = [];

// Since you only want pairs, there's no reason
// to iterate over the last element directly
for (var i = 0; i < array.length - 1; i++) {
  // This is where you'll capture that last value
  for (var j = i + 1; j < array.length; j++) {
    results.push(array[i] + ' ' + array[j]);
  }
}

console.log(results);

Answer 3

Here are some functional programming solutions:

Using EcmaScript2019's flatMap:

var array = ["apple", "banana", "lemon", "mango"];

var result = array.flatMap(
    (v, i) => array.slice(i+1).map( w => v + ' ' + w )
);

console.log(result);

Before the introduction of flatMap (my answer in 2017), you would go for reduce or [].concat(...) in order to flatten the array:

var array = ["apple", "banana", "lemon", "mango"];

var result = array.reduce( (acc, v, i) =>
    acc.concat(array.slice(i+1).map( w => v + ' ' + w )),
[]);

console.log(result);

Or:

var array = ["apple", "banana", "lemon", "mango"];

var result = [].concat(...array.map( 
    (v, i) => array.slice(i+1).map( w => v + ' ' + w ))
);

console.log(result);

Answer 4

Try this: https://jsfiddle.net/e2dLa9v6/

var array = ["apple", "banana", "lemon", "mango"];
var result = [];

for(var i=0;i<array.length-1;i++){
    for(var j=i+1;j<array.length;j++){
    result.push(array[i]+" "+array[j]);
  }
}
for(var i=0;i<result.length;i++){
    alert(result[i]);
}

Answer 5

Using map and flatMap the following can be done (flatMap is only supported on chrome and firefox)

var array = ["apple", "banana", "lemon", "mango"]
array.flatMap(x => array.map(y => x !== y ? x + ' ' + y : null)).filter(x => x)

Answer 6

Although solutions have been found, I post here an algorithm for general case to find all combinations size n of m (m>n) elements. In your case, we have n=2 and m=4.

const result = [];
result.length = 2; //n=2

function combine(input, len, start) {
  if(len === 0) {
    console.log( result.join(" ") ); //process here the result
    return;
  }
  for (let i = start; i <= input.length - len; i++) {
    result[result.length - len] = input[i];
    combine(input, len-1, i+1 );
  }
}

const array = ["apple", "banana", "lemon", "mango"];    
combine( array, result.length, 0);