Algorithm to get changes between two arrays

Question

I needed to create an algorithm which will (efficiently) take an old array and a new array and give me back the changes between the two (which items added, which removed). It ha

Answer 1

I think the implementation of the diff method is correct, the time complexity of your algorithm is O(n * log(n)) because of the sort methods. If you use arrays, you need to sort them before the comparison and the time complexity of sorting algorithms is at least O(n * log(n)).

If the o and n arrays cannot contain the same value more than once, maybe the use of objects (associative arrays) instead of arrays is a better choice.

Example:

function diff(o, n) { 
    var a = {}; var r = {}; 

    for (var i in o) {
        if (!(i in n)) {
            r[i] = 1;
        }
    }
    for (var i in n) {
        if (!(i in o)) {
            a[i] = 1;
        }
    }
    return {added: a, removed: r};
}

    // how to use
var o = {"a":1, "b":1, "ab":1};
var n = {"a":1, "aa":1};

var diffon = diff (o, n);

    // display the results
var added = "", removed = "";
for (var i in diffon.added) {
    added += i + ", ";
}
for (var i in diffon.removed) {
    removed += i + ", ";
}

alert ("added: " + added);
alert ("removed: " + removed);

The time complexity in that case is O(n).

If you need further details about arrays, associative arrays in JavaScript, I suggest you the following link: Array object