Comparing two arrays in javascript and creating new arrays with missing and new elements

Question

What is the most efficient way to compare two javascript arrays and create two new arrays of missing and new elements? Array elements will be always strings or numbers and it is not 100% sure that they will be sorted in any way.

var old_array = ['11', '13', '14', '18', '22', '23', '25'];
var new_array = ['11', '13', '15', '16', '17', '23', '25', '31'];
var missing_elements = [];
var new_elements = [];

/*
* some magic goes here
* which compares
* new_array with old_array
*/

console.log(missing_elements); // would produce ['14', '18', '22']
console.log(new_elements); // would produce ['15', '16', '17', '31']

Thanks a bunch!

Lukas · Accepted Answer · 2012-12-19 21:55:03Z

3

The code to Drew Noakes' solution:

var old_array = ['11', '13', '14', '18', '22', '23', '25'];
var new_array = ['11', '13', '15', '16', '17', '23', '25', '31'];
var missing_elements = _.difference(old_array, new_array);
var new_elements = _.difference(new_array, old_array);

console.log(missing_elements); // would produce ['14', '18', '22']
console.log(new_elements); // would produce ['15', '16', '17', '31']

answered Dec 19, 2012 at 21:55

Lukas

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Drew Noakes · Accepted Answer · 2012-12-19 21:40:53Z

2

I would use Underscore.js's intersection and difference functions.

answered Dec 19, 2012 at 21:40

Drew Noakes

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3 Comments

errata Over a year ago

Thanks for this tip, +1 to you, but I accepted Lukas' answer just because he added a code snippet.

Drew Noakes Over a year ago

The main thing is that you found a solution that works for you.

Drew Noakes Over a year ago

Also, you asked for the most efficient way. Mine's probably the simplest, but Bergi's may actually be more efficient. Perhaps someone could test this with jsperf.com

Bergi · Accepted Answer · 2012-12-19 21:52:42Z

0

Sorting will be the most efficient (assuming you don't have any other preconditions to base an optimization on):

var old_array_sorted = old_array.slice().sort(); // slicing for not mutating
var new_array_sorted = new_array.slice().sort(); // the original array

for (var o=0, n=0; o<old_array_sorted.length && n<new_array_sorted.length; ) {
    if (old_array_sorted[o] < new_array_sorted[n])
        missing_elements.push ( old_array_sorted[o++] );
    else if (old_array_sorted[o] > new_array_sorted[n])
        new_elements.push( new_array_sorted[n++] );
    else
        n++, o++;
}
[].push.apply(missing_elements, old_array_sorted.slice(o));
[].push.apply(new_elements, new_array_sorted.slice(n));

edited Dec 19, 2012 at 21:52

answered Dec 19, 2012 at 21:44

Bergi

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