Question

I’m studying some basic functions for my C midterm and I realized I copied down my instructor’s example incorrectly. I’m probably just too tired to think this through right now and a little push would be greatly appreciated.

void replace_last(char s[], int oldc, int newc){
    size_t i, pos-1; /* I have no idea why I wrote pos-1. I know I need it though */
        for(i = 0; s[i] != '\0'; i++){
             if(s[i] == oldc)
                 pos == i;
             if(pos != -1)
                 s[pos] = newc;
         }
}

Answer 1

void replace_last(char s[], int oldc, int newc){
   size_t i, pos = -1;
   for(i = 0; s[i]; i++)
      if(s[i] == oldc)
         pos = i;
   if(pos != -1)
      s[pos] = newc;
}

So your copied code had three problems: pos-1, pos == i, and thirdly, the logic if(pos != -1) s[pos] = newc; needs to be outside the loop.

Edit: Just to round-off the answer, the “non-manual” way of doing this would be, for example:

void replace_last(char s[], int oldc, int newc) {
   if (strrchr(s, oldc))
      *strrchr(s, oldc) = newc;
}

(You could, if you wished, save the return of strrchr() of in a temporary pointer, to avoid the second call to strrchr(), but this is probably unnecessary: gcc with -O1 does this for you automatically.)