Question

So I’m trying to make my own strstr function, with the following implemenation:

 char *mystrstr(char *haystack, char *needle);
   // find the first occurrence of string needle
   // in string haystack
   // identical to strstr in <string.h>
   // running time O(mystrlen(needle)*mystrlen(haystack))

Here is what i have:

  char *mystrstr(char *haystack, char *needle)
{
 if (haystack == needle) { return haystack; }
 int i = 0; int j = 0; 
 while (haystack[i] != '\0') {
   if (j == mystrlen(needle)) {return haystack + (i - mystrlen(needle)); }  
   if (haystack [i] == needle [j]) {
   j++; i++; 
   }   
   else {  j = 0; i++; }

   }
 if (j == mystrlen(needle)) {return haystack + (i - mystrlen(needle)); } 
 return NULL;
}  

My problem is that when i set j = 0, i dont want to iterate i. But i eventually need to iterate “i” to cause the loop to break. Any suggestions ?

Answer 1

Assuming you don’t want to get sophisticated (e.g., Knuth-Morris-Pratt or a Boyer-Moore variant) I think I’d do it by stepping through each possible point in the “haystack”, and comparing the N characters of the needle to the next N characters in the haystack. If they’re equal, you’ve found a position.

Edit. In pseudo-code, I’d do something like this:

boolean check_pos check_for check_in
    length = getlength(check_for)
    for i = 1 to length
       if (check_for[i] != check_in[i])
            return false
end check_pos

int my_strstr haystack needle
    length = getlength(haystack) - getlength(needle)

    for i = 1 to length
        if (check_pos(needle, haystack+i)
            return i
    return -1
end my_strstr