Pairwise combinations of entries in a javascript array

Question

I'm given an array of entries in javascript, such as :

var entries = ["cat", "dog", "chicken", "pig"];

I'd now like to iterate over all unique pairwise combinations of them. In this example, I'd like to see:

("cat", "dog"),
("cat", "chicken"),
...

In other languages, like scala, this is super easy. You just do

entries.combinations(2)

is there a similar method or function in a library for javascript? Or do I just have to write it myself the ugly way with nested loops?

Answer 1

var arr = ["cat","dog","chicken","pig"].map(function(item,i,arr) {
    return arr.map(function(_item) { if( item != _item) return [item, _item];});
});

This will return the expected results. There are caveats, it does not work in older browsers without shims.

Also the duplicate value is 'undefined' instead of there being 4 arrays of 3. I'm sure there is a more graceful way to handle this.

Array.prototype.map() - MDN

edit

this will give you the proper pairwise combinations.

var arr = ["cat","dog","chicken","pig"].map(function(item,i,arr) {
    var tmp = arr.map(function(_item) { if( item != _item) return [item, _item];});
    return tmp.splice(tmp.indexOf(undefined),1), tmp;
});

Array splice method - MDN

and here is a more readable version of the same code.

var myArray = ["cat", "dog", "chicken", "pig"];
var pairwise = myArray.map(function(item, index, originalArray) {
    var tmp = originalArray.map(function(_item) {
        if (item != _item) {
            return [item, _item];
        }
    });
    tmp.splice(tmp.indexOf(undefined), 1); // because there is now one undefined index we must remove it.
    return tmp;
});

Answer 2

Not as far as I know. I think you have to stick to nested loops.

A similar question has been asked here: Output each combination of an array of numbers with javascript maybe you can find an answer there.