Why indexOf in javascript not working?

Question

I'm not sure what I'm doing wrong here. The first instance that I use indexOf it works perfectly fine, but when I use it the second time it's not returning the result that I'm expecting.

function mutation(arr) {
    //return arr;
    res = "";
    for (var x=0; x<arr[1].split("").length; x++) {
        if (arr[0].indexOf(arr[1].split("")[x]) !== -1) {
            res += "t";
        } else {
            res += "f";
        }
    }
    // res = ttt
    if (res.indexOf("f") !== -1) {
        return true;
    } else {
        return false;
    }
}

mutation(["hello", "hey"]);
// this returns true instead of false
mutation(["floor", "loo"]); 
// returns false instead of true

mutation should return false if an element from arr[1] is not present in arr[0] else return true.

I just pasted your code into the JS console and I got true as well — csga5000, Nov 22 '15 at 05:28
If you are using IE8 and olders, `.indexOf()` doesn't be supported. Reference link: http://stackoverflow.com/questions/3629183/why-doesnt-indexof-work-on-an-array-ie8 — , Nov 22 '15 at 05:32
@splucena check out my edited answer. The problems with your code are outlined there. You had an off by 1 error and a negated indexOf check. — David Zorychta, Nov 22 '15 at 05:41

David Zorychta · Accepted Answer · 2015-11-22T05:39:00.430

3

your code isn't working because when you say:

res.indexOf("f") != -1

this means: "I found an f", but you're treating it as if it means "I did not find an f".

In your case that you want to return false if you find an 'f', but you're returning true. Flip your true and false cases:

if (res.indexOf("f") != -1) {
   return false;
 } else {
   return true;
 }

ALSO your for loop is wrong because x starts at 0, so you need to go to < length not <= length of your string.

for (var x=0; x < arr[1].split("").length; x++) {

and now your code works as you wanted it to.

edited Nov 22 '15 at 05:39

answered Nov 22 '15 at 05:35

David Zorychta

13,039
6
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81

or better yet: `return !res.indexOf("f")` – Samuel Cook Nov 22 '15 at 05:37
@Macmee You're right I patterned my thinking on the first time I use indexOf and expected it to return true when I'm looking for false. – splucena Nov 22 '15 at 06:00

score 2 · Answer 2 · 2015-11-22T05:49:42.180

Just edited your code. Click on the <p> to check:

function mutation(arr) {
  //return arr;
  res = "";
  for (var x=0; x< arr[1].split("").length; x++) {
    res += arr[0].indexOf(arr[1].split("")[x]) > -1 ? 't' : 'f';
  }
 return res.indexOf('f') > -1;
}

$('p').click(function(){
  alert(mutation(["hello", "hey"]));
  alert(mutation(["floor", "loo"]));
})

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<p>Click me</p>

score 0 · Answer 3 · edited May 23 '17 at 11:52

0

If you simplify the logic a bit, that's easier to check:

function mutation(arr) {
  return arr[1].split('').reduce(function(res, x) {
    return arr[0].indexOf(x) >= 0;
  }, true);
}

Thanks Leon for the correction.

edited May 23 '17 at 11:52

Community

1
1

answered Nov 22 '15 at 05:32

Hunan Rostomyan

2,176
2
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does using `true && condition` have any benefit? – Leon Adler Nov 22 '15 at 05:34
@LeonAdler Compared to what? – Hunan Rostomyan Nov 22 '15 at 05:35
This is working. Can you spot what's wrong with my code? – splucena Nov 22 '15 at 05:35
compared to just `condition` - `return arr[0].indexOf(x) >= 0` – Leon Adler Nov 22 '15 at 05:35
@LeonAdler haha! Of course, thanks man. That was stupid of me. – Hunan Rostomyan Nov 22 '15 at 05:36
no problem - expected that I missed some fancy JS performance improvement like `var x = number | 0` ;-) – Leon Adler Nov 22 '15 at 05:37
@splucena Macmee's has already addressed that question, so check his explanation out. – Hunan Rostomyan Nov 22 '15 at 05:42

score 0 · Answer 4 · answered Nov 22 '15 at 06:19

I tried to not chance your logic, the mistake are:

You're trying to compare with all characters on the array[0], not only the first.
If you find a character equals on the first character on array[0] you should return true.

Correct code:

function mutation(arr) {
  res = "";
  for (var x=0; x<=arr[1].split("").length; x++) {
    if (arr[0].split("")[0].indexOf(arr[1].split("")[x]) !== -1) {
      return true;
    }
  }
  return false;
}

Why indexOf in javascript not working?

4 Answers4

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