Most efficient way of finding last char occurrence

Question

Consider this function I wrote real quick to find the last occurrence of a given char in a string and return it's position within the array of chars that physically is the string:

size_t strlstchar(const char *str, const char ch)
{
    char *chptr = strrchr(str, ch);
    return chptr - str;
}

I just typed this up real quick here (haven't compiled or anyting yet) just because I have questions about a few things.

To me this seems like the simplest solution to find which array element holds the last instance of a particular char, but I have no idea how it works. I just made this following the documentation of strrchr, so it's technically strrchr doing all the work. I just can't imagine this being the best way (in terms of performance) to achieve this, and was hoping somebody could give some input on what would be the best way to do this.

Is strrchr an efficient way to do this? Or is strrchr best left for some other use?

Answer 1

The approach you used is perfectly fine - unfortunately, array operations are expensive. Strrchr in most implementations simply steps through the string beginning from its end until it finds a matching character. That's O(n) time. Then you perform a subtraction which is O(1). This is not that bad.

Answer 2

From the docs:

Returns a pointer to the last occurrence of character in the C string str.

So it does exactly what you want. The purpose of its existence is this.

Is strrchr an efficient way to do this?

It is almost certainly written at least as well or better than you could do it yourself.

Or is strrchr best left for some other use?

No. It written exactly for this purpose.

Answer 3

It will be faster if you can supply the length of the string then just loop backwards. When you find the first occurrence of the character return right away.

If you don't know the length just use strrchr.