c,string,algorithm,data-structures , strcmp performance in C


strcmp performance in C

Question:

Tag: c,string,algorithm,data-structures
int myStrCmp (const char *s1, const char *s2) {
    const unsigned char *p1 = (const unsigned char *)s1;
    const unsigned char *p2 = (const unsigned char *)s2;

    while (*p1 != '\0') {
       if (*p2 == '\0') return  1;
       if (*p2 > *p1)   return -1;
       if (*p1 > *p2)   return  1;

       p1++;
       p2++;
     }

     if (*p2 != '\0') return -1;

        return 0;
}

Does this compare have the notation exactly O(n)?. When measured with two strings with s1="abcd" s2="abcd." for the first case and s1="asdsdcvv" s2="asdsdcvv". for the second case, does the second case be exactly two times of the first case?


Answer:

Yes, this is in O(n) in the average and worst case, where n is the length of the shorter of both given strings. You could also express that as O(min(m,n)) with m and n being the lengths of both strings, respectively.

But no, O(n) doesn't mean that it needs exactly linear time. It means that starting from a minimum problem size, all bigger problems will be below some linear function.

If you wanted to know more details, you would need a very detailed model of your machine, which seems pretty impossible with modern machines (caching, branch prediction, pipelining, hyper-threading etc.) and Operating Systems (unless you are using a very simple one without multi-tasking).


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