JavaScript - Generating combinations from n arrays with m elements

Question

I\'m having trouble coming up with code to generate combinations from n number of arrays with m number of elements in them, in JavaScript. I\'ve seen similar questions about

Answer 1

Another implementation with ES6 recursive style

Array.prototype.cartesian = function(a,...as){
  return a ? this.reduce((p,c) => (p.push(...a.cartesian(...as).map(e => as.length ? [c,...e] : [c,e])),p),[])
           : this;
};

console.log(JSON.stringify([0,1].cartesian([0,1,2,3], [[0],[1],[2]])));

Answer 2

var f = function(arr){
    if(typeof arr !== 'object'){
        return false;
    }

    arr = arr.filter(function(elem){ return (elem !== null); }); // remove empty elements - make sure length is correct
    var len = arr.length;

    var nextPerm = function(){ // increase the counter(s)
        var i = 0;

        while(i < len)
        {
            arr[i].counter++;

            if(arr[i].counter >= arr[i].length){
                arr[i].counter = 0;
                i++;
            }else{
                return false;
            }
        }

        return true;
    };

    var getPerm = function(){ // get the current permutation
        var perm_arr = [];

        for(var i = 0; i < len; i++)
        {
            perm_arr.push(arr[i][arr[i].counter]);
        }

        return perm_arr;
    };

    var new_arr = [];

    for(var i = 0; i < len; i++) // set up a counter property inside the arrays
    {
        arr[i].counter = 0;
    }

    while(true)
    {
        new_arr.push(getPerm()); // add current permutation to the new array

        if(nextPerm() === true){ // get next permutation, if returns true, we got them all
            break;
        }
    }

    return new_arr;
};

Answer 3

Just for fun, here's a more functional variant of the solution in my first answer:

function cartesian() {
    var r = [], args = Array.from(arguments);
    args.reduceRight(function(cont, factor, i) {
        return function(arr) {
            for (var j=0, l=factor.length; j<l; j++) {
                var a = arr.slice(); // clone arr
                a[i] = factor[j];
                cont(a);
            }
        };
    }, Array.prototype.push.bind(r))(new Array(args.length));
    return r;
}

Alternative, for full speed we can dynamically compile our own loops:

function cartesian() {
    return (cartesian.cache[arguments.length] || cartesian.compile(arguments.length)).apply(null, arguments);
}
cartesian.cache = [];
cartesian.compile = function compile(n) {
    var args = [],
        indent = "",
        up = "",
        down = "";
    for (var i=0; i<n; i++) {
        var arr = "$"+String.fromCharCode(97+i),
            ind = String.fromCharCode(105+i);
        args.push(arr);
        up += indent+"for (var "+ind+"=0, l"+arr+"="+arr+".length; "+ind+"<l"+arr+"; "+ind+"++) {\n";
        down = indent+"}\n"+down;
        indent += "  ";
        up += indent+"arr["+i+"] = "+arr+"["+ind+"];\n";
    }
    var body = "var res=[],\n    arr=[];\n"+up+indent+"res.push(arr.slice());\n"+down+"return res;";
    return cartesian.cache[n] = new Function(args, body);
}

Answer 4

After doing a little research I discovered a previous related question: Finding All Combinations of JavaScript array values

I've adapted some of the code from there so that it returns an array of arrays containing all of the permutations:

function(arraysToCombine) {
    var divisors = [];
    for (var i = arraysToCombine.length - 1; i >= 0; i--) {
       divisors[i] = divisors[i + 1] ? divisors[i + 1] * arraysToCombine[i + 1].length : 1;
    }

    function getPermutation(n, arraysToCombine) {
       var result = [], 
           curArray;    
       for (var i = 0; i < arraysToCombine.length; i++) {
          curArray = arraysToCombine[i];
          result.push(curArray[Math.floor(n / divisors[i]) % curArray.length]);
       }    
       return result;
    }

    var numPerms = arraysToCombine[0].length;
    for(var i = 1; i < arraysToCombine.length; i++) {
        numPerms *= arraysToCombine[i].length;
    }

    var combinations = [];
    for(var i = 0; i < numPerms; i++) {
        combinations.push(getPermutation(i, arraysToCombine));
    }
    return combinations;
}

I've put a working copy at http://jsfiddle.net/7EakX/ that takes the array you gave earlier ([[0,1], [0,1,2,3], [0,1,2]]) and outputs the result to the browser console.

Answer 5

const charSet = [["A", "B"],["C", "D", "E"],["F", "G", "H", "I"]];
console.log(charSet.reduce((a,b)=>a.flatMap(x=>b.map(y=>x+y)),['']))

Answer 6

You could take an iterative approach by building sub arrays.

var parts = [[0, 1], [0, 1, 2, 3], [0, 1, 2]],
    result = parts.reduce((a, b) => a.reduce((r, v) => r.concat(b.map(w => [].concat(v, w))), []));

console.log(result.map(a => a.join(', ')));

.as-console-wrapper { max-height: 100% !important; top: 0; }