Javascript: Multiple Array looping

Question

TL DR; What is the most efficient way to sort and compare values in a multiple arrays?

Okay, so we'll assume a few constants to make this whole thing simple var a = [1, 2, 3, 4, 5, 6], b = [0, 9, 8, 7, 6, 88, 99, 77], i, j;

Now if I wanted to see if any value in a is equal to any other value in b I'd have to sort through one of these arrays 6 times. That's a lot of work, and it would seem there should be a more efficient way to do this. For those needing a visual aide here you are ( and yes I know about -- and ++ I just don't like to use 'em ):

for (i = a.length - 1; i > -1; i -= 1) {
    for (j = b.length - 1; j > -1; j -= 1) {
        if (a[i] === b[j]) {
            return b[j];
        }
    }
}

The B array gets ran through once for EACH element in A. Again, certainly there is a more efficient way to complete this task?

-Akidi

Wtf is TL DR? I see that everywhere.

Anthony Forloney
– Anthony Forloney

2010-03-10 05:26:42 +00:00
Commented Mar 10, 2010 at 5:26 — Anthony Forloney
– Anthony Forloney, Commented Mar 10, 2010 at 5:26

Ben Zotto · Accepted Answer · 2010-03-10 05:31:05Z

3

Depends on the size of your input arrays (several tradeoffs there)-- your nested loops are simplest for small inputs like your examples.

If you have huge arrays, and have control over their construction, consider keeping a map (Object in JS) around acting as a lookup set (if you're creating a in a loop anyways, you can build the set in parallel.)

var setA = {};
for (int i = 0; i < a.length; i++) {
    setA[a[i]] = true;
}

Then you can see whether something exists in the set by just checking setA[?].

Likewise with B, or with both together, etc, depending on your needs.

answered Mar 10, 2010 at 5:31

Ben Zotto

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2 Comments

Akidi Over a year ago

Appreciate the feedback, computer crashed so unfortunately wasn't able to commend sooner. Would vote your answer useful ( as well as the guy below ) but apparently need 15 rep to do that. Oh well.

Akidi Over a year ago

Finally got 15 rep ^_^ So voted both up like I said I would several months ago.

NilColor · Accepted Answer · 2010-03-10 09:29:36Z

1

Maybe something like this may help?

var a = [1, 2, 3, 9, 5, 0], b = [0, 9, 8, 7, 6, 88, 99, 77];
var a_dict = {}, l = a.length;

for (var i=0; i < l; i++) {
  a_dict[a[i]] = 1;
}

l = b.length;
for (var i=0; i < l; i++) {
  if(!!a_dict[b[i]]){
    console.log(b[i]);
  }
}

You can convert one array to "dict-like" object and compare others with it...

answered Mar 10, 2010 at 9:29

NilColor

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