Question

I have several numbers in an array

var numArr = [1, 3, 5, 9];

I want to cycle through that array and multiply every unique 3 number combination as follows:

1 * 3 * 5 = 
1 * 3 * 9 = 
1 * 5 * 9 = 
3 * 5 * 9 =

Then return an array of all the calculations

var ansArr = [15,27,45,135];

Anyone have an elegant solution? Thanks in advance.

Answer 1

A general-purpose algorithm for generating combinations is as follows:

function combinations(numArr, choose, callback) {
    var n = numArr.length;
    var c = [];
    var inner = function(start, choose_) {
        if (choose_ == 0) {
            callback(c);
        } else {
            for (var i = start; i <= n - choose_; ++i) {
                c.push(numArr[i]);
                inner(i + 1, choose_ - 1);
                c.pop();
            }
        }
    }
    inner(0, choose);
}

In your case, you might call it like so:

function product(arr) {
    p = 1;
    for (var i in arr) {
        p *= arr[i];
    }
    return p;
}

var ansArr = [];

combinations(
    [1, 3, 5, 7, 9, 11], 3,
    function output(arr) {
        ansArr.push(product(arr));
    });

document.write(ansArr);

…which, for the given input, yields this:

15,21,27,33,35,45,55,63,77,99,105,135,165,189,231,297,315,385,495,693