javascript,jquery,arrays,performance,underscore.js , Universal function for getting all unique pairs, trebles etc from an array in javascript

Question:

Tag: javascript,jquery,arrays,performance,underscore.js

I am looking to create a function in javascript, which would allow me to pass a long array, together with one argument. what I'm looking for is something like this:

``````var ar = [1,2,3,4];
var pairs = superAwesomeFunction(ar,2) //=> [[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]];
var trebles = superAwesomeFunction(ar,3) //=> [[1,2,3],[1,2,4],[1,3,4],[2,3,4]
``````

ideally, the function would have no limit on the folding argument. I wrote a piece of code that looks like this, which is working fine, but it's not really useful as it isn't universal, and I would need a lot of such functions, which seems silly.

``````  getAll2Folds: function (ar) {
var combinations = [],
numOdds = ar.length;
for (var i = 0; i < numOdds; i++) {
for (var j = i + 1; j < numOdds; j++) {
combinations.push([ar[i], ar[j]]);
}
}
return combinations;
},

getAll3Folds: function (ar) {
var combinations = [],
numOdds = ar.length;
for (var i = 0; i < numOdds; i++) {
for (var j = i + 1; j < numOdds; j++) {
for (var k = j + 1; k < numOdds; k++) {
combinations.push([ar[i], ar[j], ar[k]]);
}
}
}
return combinations;
};

},
``````

... (not so great :|)

``````  getAll8Folds: function (ar) {
var combinations = [],
numOdds = ar.length;
for (var i = 0; i < numOdds; i++) {
for (var j = i + 1; j < numOdds; j++) {
for (var k = j + 1; k < numOdds; k++) {
for (var l = k + 1; l < numOdds; l++) {
for (var m = l + 1; m < numOdds; m++) {
for (var n = m + 1; n < numOdds; n++) {
for (var o = n + 1; o < numOdds; o++) {
for (var p = o + 1; p < numOdds; p++) {
combinations.push([ar[i], ar[j], ar[k], ar[l], ar[m], ar[n], ar[o], ar[p]]);
}
}
}
}
}
}
}
}
return combinations;
``````

}

I'm free to use underscore, jquery or whatever tool i want, but can't find an elegant solution, which would also be performant. ideas?

Thanks

Basically `combine()` takes an array with the values to combine and a size of the wanted combination results sets.

The inner function `c()` takes an array of previously made combinations and a start value as index of the original array for combination. The return is an array with all made combinations.

The first call is allways `c([], 0)`, because of an empty result array and a start index of 0.

``````var arr = [1, 2, 3, 4, 5, 6, 7];

function combine(a, size) {

function c(part, start) {
var result = [], i, l, p;
for (i = start, l = arr.length; i < l; i++) {
// get a copy of part
p = part.slice(0);
// add the iterated element to p
p.push(a[i]);
// test if recursion can go on
if (p.length < size) {
// true: call c again and concat it to the result
result = result.concat(c(p, i + 1));
} else {
// false: push p to the result, stop recursion
result.push(p);
}
}
return result;
}

return c([], 0);
}

out(JSON.stringify(combine(arr, 2), null, 4), true);
out(JSON.stringify(combine(arr, 3), null, 4), true);
out(JSON.stringify(combine(arr, 4), null, 4), true);
out(JSON.stringify(combine(arr, 5), null, 4), true);
out(JSON.stringify(combine(arr, 6), null, 4), true);

// just for displaying the result
function out(s, pre) {
var descriptionNode = document.createElement('div');
descriptionNode.className = 'description';
if (pre) {
var preNode = document.createElement('pre');
preNode.innerHTML = s + '<br>';
descriptionNode.appendChild(preNode);
} else {
descriptionNode.innerHTML = s + '<br>';
}
document.getElementById('out').appendChild(descriptionNode);
}``````
``<div id="out"></div>``

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