Check if duplicate array pairs exist using underscore only

Question

I am wondering how I can check if a duplicate pair of values in an array exist as part of a larger array in javascript. You can see there is a duplicate pair of [1,2] - so the function should just return true. i.e

var arr = [[1,2], [3,4], [5,6], [7,8], [9,10], [11,12], [13,14], [1,2]]

I have tried using this logic which gives me a clean array and a "true"

var unique = [];
var done = []; var dup = false;
for(var x = 0; x < arr.length; x++) {
    var myStr = arr[x].toString();

    if(done.indexOf(myStr) != -1) {
        // val already exist, ignore
        dup = true;
        continue;
    }

    done.push(myStr);
    unique.push(arr[x]);
}

But I was wondering if there is something more elegant using Underscore ?

Answer 1

The shortest way would be to use _.uniq and JSON.stringify:

function unique(arr) {
    return _.uniq(arr, JSON.stringify).length === arr.length;
}

But that doesn't short-circuit, so it's somewhat slow compared to the other ways you could do it. Tomalak's second function should be faster.

Answer 2

Well, uniq seems like a good fit

function containsDuplicates(arr) {
  return arr.length !== _.uniq(arr, function (item) { return item.toString(); }).length;
}

You should use Blender's version of this function. It's shorter and safer.

---

BTW, your code should look more like this:

function containsDuplicates(arr) {
    var index = {}, i, str;

    for(i = 0; i < arr.length; i++) {
        // you could use arr[i].toString() here, but JSON.stringify()
        // is a lot safer because it cannot create ambiguous output.
        str = JSON.stringify(arr[i]);
        if (index.hasOwnProperty(str)) {
            return true;
        } else {
            index[str] = true;
        }
    }

    return false;
}

Note that this is probably more efficient than the underscore one-liner.