Recursively constructing a JavaScript array of all possible combinations while respecting order

Question

I am having difficulty getting the following concept in to code:

Let's say we are given the following array:

[
    'h1,h2',
    'span,style'
]

From this I wish to get the following output:

[
    'h1 span',
    'h1 style',
    'h2 span',
    'h2 style
]

So that we have an array of strings containing all combinations of the original array, which also respects order (so span h1 and style h2 are not valid).

To describe verbose: I have a single level array of strings which are effectively comma separated values. I wish to iterate over this array and split these strings by their comma in to a new array, and for each index in this new array build a new string which contains all the other split values from subsequent indexes in the original array.

I am having difficulty trying to program this in JavaScript. I understand that I will need some recursion, but I'm confused about how to do it. After trying various different and failing methods, I currently have this:

function mergeTagSegments(arr, i, s) {

    i = i || 0;
    s = s || '';

    if(!arr[i]) { return s; }

    var spl = arr[i].split(',');
    for(var j in spl) {
        s += spl[j] + mergeTagSegments(arr, i+1, s);
    }

    return s;

}

This also fails to work as intended.

I feel a little embarrassed that I am unable to complete what I originally thought was such a simple task. But I really hope to learn from this.

Many thanks in advance for your advice and tips.

kaspermoerch · Accepted Answer · 2014-07-28 09:35:45Z

1

Your thinking along the right lines. Recursion is definetly the way to go. I have implemented a suggested solution below, which should give you the desired output.

var values = ['h1,h2', 'span,style'];

function merge(input, index) {
    var nextIndex = index + 1;
    var outputArray = [];
    var currentArray = [];

    if(index < input.length) {
        currentArray = input[index].split(',');
    }

    for(var i = 0, end = currentArray.length; i < end; i++) {               
        if(nextIndex < input.length) {
            merge(input, nextIndex).forEach(function(item) {
                outputArray.push(currentArray[i] + ' ' + item);
            });
        }
        else {
            outputArray.push(currentArray[i]);
        }
    }

    return outputArray;
}

var output = merge(values, 0, '');
console.log(output);

answered Jul 28, 2014 at 9:35

kaspermoerch

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