Search string for a substring - indexOf

Question

I am trying to write my own implementation of indexOf for learning purposes. I have an implementation of as follows. I'm sure a more efficient and shorter implementation exists.

function _indexOf(needle, haystack) {

    var n = needle.split("");
    var h = haystack.split("");

    var n_len = n.length;
    var h_len = h.length;

    for (var i = 0; i < h_len; i++) {

        var j = 0;
        var flag = false;

        while (h[i] == n[j] || flag) {

            if (flag)
                j = 0;

            flag = false;

            i++;
            j++;

            if (h[i] == n[0])
                flag = true;


            if (j >= n_len)
                return i - n_len;

        }

    }

    return -1;

}

Here's a test: assert(_indexOf("ced","ccceccecedeeced")==7) // true

Answer 1

Your code is pretty neat, and well structured. I like the variable names. There are a number of problems to point out, though...

• splitting the string is a huge "cheat".... if you're going to use regex, you may as well just match for the substring.
• splitting the strings is also slow. Just use an index in to the string and the string[indx] operator. You can remove the h and n and h_len and n_len variables as well then.
• your outer loop can terminate at h_len - n_len - there's no need to loop to the end when a match would be impossible

You have some bugs.... like:

1. your needle cannot contain the same letter as the start letter in a another place... For example, your code will fail to find cec. This is because you do weird logic here: if (h[i] == n[0]) ... I am not sure why that's needed.... other than to reset the search?
2. related to the above, you cannot backtrack and reset a search in the event that the search pattern starts part way through a partial match....

Even when you resolve the above issues, your algorithm is considered 'naive'. There are two commonly used algorithms that are considered better. The Rabin–Karp algorithm is simpler to implement because you just create a hash of the needle, and then a matching rolling-hash in the haystack. If you find the hash in the haystack you can confirm a match, and return, in what is effectively \$O(n+m )\$. A more complicated, but also more efficient algorithm is the Boyer-Moore algorithm which creates a "jump table" from the needle, which allows you to 'skip' values in the haystack based on the details in the needle. It is efficient, and, the longer the needle, the more you can skip.

Answer 2

• I don't like the variable names needle and haystack. I'd would use list and item. This makes them a little clearer.

• Put braces around your if statements. Even if you're only ever running one statement, you should still put braces around them. Read this and this. The Apple bug happened because curly braces weren't used. Here's where the bug happened:

  if ((err = SSLHashSHA1.update(&hashCtx, &signedParams)) != 0)
      goto fail;
      goto fail;  /* MISTAKE! THIS LINE SHOULD NOT BE HERE */
  

  While in this small scenario, it's not a huge deal, it's a good practice to get into.

• Add some comments into here to describe what the code is doing. There are no comments here, so some parts of the code could be unclear to some.
• You have a lot of extra blank lines in here. Find the ones can be removed to increase clarity.
• I'm not sure why you're incrementing i. The loop is already incrementing it. This may be a bug.

Anyways, if there's anything else you want me to cover, just ask in the comments, and I'll cover it. Hope this helps!

Answer 3

1) I don't think that splitting a string to an array and accessing its elements is faster than using String's native method charAt.

2) You don't need to iterate through all characters of haystack, unless needle's length is 1.

3) Since indexOf is within String prototype scope you don't need to prefix your function name with underscore, just use indexOf as is.

function indexOf(needle, haystack) {
    var len = needle.length, lim = haystack.length - len;
    for (var i = 0; i < lim; i++) {
        for (var j = 0; j < len && haystack.charAt(i+j) == needle.charAt(j); j++);
        if (j >= len) return i;
    }
    return -1;
}