String comparison using pointers

Question

This piece of code works fine. But I'm wondering if it can be done in a more efficient way. More specifically, this part (*(s1 + i)) if it possible to force it to sequence through entire array character by character via pointer, for example, *s1++.

My task to do this function compareStrings without index array []:

int  compareStrings(const char  *s1, const char  *s2)
{
    int i = 0, answer;
    //  i - to sequence through array of characters
    // pointer to character string1 and character string2
    while (*(s1 + i) == *(s2 + i) && *(s1 + i) != '\0'&& *(s2 + i) != '\0')
    {
        i++;
    }

    if ( *(s1 + i) < *(s2 + i) )
        answer = -1;               /* s1 < s2  */
    else if ( *(s1 + i) == *(s2 + i) )
            answer = 0;                 /* s1 == s2 */
        else
            answer = 1;                 /* s1 > s2  */

        return answer;

But I want to change it to s1++ and s2++ instead of *(s1 + i) and *(s2 + i). I've tried to implement this idea with pining an extra pointer to the beginning but I've failed.

int  compareStrings(const char  *s1, const char  *s2)
{
  int answer;
  char **i = s1, **j = s2;
  // i to sequence through array of characters
  while (*(i++) == *(j++) && *(i++) != '\0'&& *(j++) != '\0');

  if (*i < *j)
      answer = -1;               /* s1 < s2  */
  else if (*i == *j)
      answer = 0;                 /* s1 == s2 */
  else
      answer = 1;                 /* s1 > s2  */

  return answer;
}

Answer 1

You don't need pointers to character pointers at all:

int str_cmp(const char* s1, const char* s2)
{
    while (*s1 != '\0' && *s1 == *s2)
    {
        ++s1;
        ++s2;
    }

    if (*s1 == *s2)
    {
        return 0;
    }

    return *s1 < *s2 ? -1 : 1;
}

Also, there is a bug in your second implementation: it returns 1 on compareStrings("hello", "helloo"), when the correct result is -1.

Hope that helps.

Answer 2

There's another huge bug in your code which stems from a quirk of C (and C++):
Plain char can be either signed or unsigned.

Next, do you really want to normalize the return-values to one of -1, 0 and 1, or is negative, zero, positive enough? The latter is what the standard does... and it's less work.

int compareStrings(const char* s1, const char* s2)
{
    while (*s1 && *s1 == *s2) {
        ++s1;
        ++s2;
    }
    return (int)(unsigned char)*s1 - (int)(unsigned char)*s2; // not normalized
    return *s1 == *s2 ? 0 : (unsigned char)*s1 < (unsigned char)*s2 ? -1 : 1; // normalized
}

(The code assumes that an int is bigger than a char.)

Answer 3

Remember when you learned that arrays are nothing but the pointer to the first element of the same array? You can do this the other way around too: *(s1 + 1) is equivalent to s1[1].

In the second example take a look at how you resolve which pointer. **i = s1 so will *(i++) be equal to *(s1 + 1)? Your debugger can tell you the details from here :)