In Javascript or Underscore, can I count how many instances of a substring within an array?

Question

This must be so simple, yet I can't quite figure out how to count the number of substrings within an array of things. Anything in Underscore for this that I'm missing?

var foo = ["fred", "ted", "medical", "car"]

function howManySubstringsInArray(arr, word){
    var i = arr.length,
    j = 0;
    while (i) if (arr[--i].indexOf(word))++j;
    return j;
}

howManySubstringsInArray(foo, 'ed')
Output => 3

This isn't working.

Check out: http://stackoverflow.com/questions/4009756/how-to-count-string-occurrence-in-string — John K., Mar 28 '13 at 15:18

Alnitak · Accepted Answer · 2013-03-28T15:31:48.080

6

Assuming that you don't care about repeated matches within each array element, you can filter the array to find matching elements:

NB: this is a standard ECMAScript 5 function, not from underscore. There's a shim for older browsers at the above link.

function howManySubstringsInArray(arr, word) {
    return arr.filter(function(v) {
        return v.indexOf(word) >= 0;
    }).length;
}

Strictly speaking this is creating a new array which is slightly wasteful since all we're doing is counting the elements, but for "small" lists it won't matter.

A bare loop would actually work pretty well anyhow - you just need to make sure the loop is structured correctly, and that you actually look at the result of the .indexOf call. Your existing backwards iteration makes the loop conditions far harder to follow than they need to be:

function howManySubstringsInArray(arr, word) {
    var i = 0, n = arr.length, count = 0;
    for ( ; i < n; ++i) {
        if (arr[i].indexOf(word) >= 0) {
             ++count;
        }
    }
    return count;
}

edited Mar 28 '13 at 15:31

answered Mar 28 '13 at 15:18

Alnitak

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That'll miss repeats in the same string, such as "bedded". – T.J. Crowder Mar 28 '13 at 15:19
1

Yes, but the OP is asking for help with the code, knowing it's wrong. – T.J. Crowder Mar 28 '13 at 15:20
Yes, but the OP's code is wrong because he didn't check the result of `indexOf` - he never attempts to iterate over each string. – Alnitak Mar 28 '13 at 15:21
It may be useful if the OP indicates whether repeats in the string are significant. – Jeff Shaver Mar 28 '13 at 15:22
Looks like the OP doesn't care about repeats in the same string (see comment on loganfsmyth's answer). – T.J. Crowder Mar 28 '13 at 15:22
@T.J.Crowder yes, that's what I surmised (and I did consider that before answering, BTW). – Alnitak Mar 28 '13 at 15:22
Thanks guys- repeats within the strings are not significant. Only need to check once per array element. – Hairgami_Master Mar 28 '13 at 15:22
This sure looks sexier than a "clumsy" loop, but I don't like it. It's wasteful just for the sake of looking good. – Jon Mar 28 '13 at 15:26
@Jon fair point, and acknowledged in the update I added at the same time as your comment. Using `reduce` would be less wasteful, albeit still not as efficient as a bare loop. It would also be harder to understand for a novice. – Alnitak Mar 28 '13 at 15:29
@Alnitak: Spot on. I was happy to see both updates. You have my +1. – Jon Mar 28 '13 at 15:31
Many thanks gentlemen. I owe you both a beer or beverage of your choice. Let me know next time you are in North Carolina. – Hairgami_Master Mar 28 '13 at 15:40

loganfsmyth · Answer 2 · 2013-03-28T15:24:43.930

3

Your while loop needs to check the actual index.

 while (i) if (arr[--i].indexOf(word) !== -1) ++j;

You could also use reduce well here:

function howManySubstringsInArray(arr, word){
  return _.reduce(arr, function(count, val){
    return count + (val.indexOf(word) === -1 ? 0 : 1)
  }, 0);
}

edited Mar 28 '13 at 15:24

answered Mar 28 '13 at 15:18

loganfsmyth

156,129
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That'll miss repeats in the same string, such as "bedded". – T.J. Crowder Mar 28 '13 at 15:19
1

Good point gentlemen. I don't envision checking for repeats within the same element, just once per element is fine. – Hairgami_Master Mar 28 '13 at 15:20
@Hairgami_Master: Ah, well, there you are then. – T.J. Crowder Mar 28 '13 at 15:21
don't forget the ES5 built-in `Array.prototype.reduce` – Alnitak Mar 28 '13 at 15:26
@Alnitak He asked about underscore in the title, so he probably already has it. Otherwise he would have to add even more code for the shim. In the general case though, I agree. – loganfsmyth Mar 28 '13 at 15:28
@loganfsmyth Many thanks Logan! I really appreciate your help. I haven't ever used _.reduce before. Cheers. – Hairgami_Master Mar 28 '13 at 15:41

score -1 · Answer 3 · answered Mar 28 '13 at 15:21

-1

function howManySubstringsInArray(arr,str)
{
var count=0;
   $.each(arr,function(i,item)){
   if(item.indexOf(str)!=-1)
    {
      count ++;
    }
  });
 return count;
}

Change $.each to for if you want.

answered Mar 28 '13 at 15:21

Anh Tú

636
7
21

In Javascript or Underscore, can I count how many instances of a substring within an array?

3 Answers3