CODEWITHSUNDEEP
carrayspointers
Mike
Mike

Reputation: 71

Pointer to subscript -- strstr function

I have an exercise where I should implement the strstr function from the standard library. The implementation of strstr seems to work, however, while it points to 'B', from "Foo Bar Baz", it returns "Bar Baz". How could I change the returned subscript so that it only returns the first occurrence of "B" of "Bar" in "Foo Bar Baz"?

From the manual:

If needle is an empty string, haystack is returned; if needle occurs nowhere in haystack, NULL is returned; otherwise a pointer to the first character of the first occurrence of needle is returned.

#include <stdio.h>

char *ft_strstr(char *str, char *to_find) 
{
  int i;
  int n;

  i = 0;
  n = 0;
  if (to_find == NULL || str == NULL)
    return str;
  if (to_find != NULL) 
  {
    while (str[i] != '\0' && to_find[n] != '\0') 
    {
      while (str[i] == to_find[n]) 
      {
        i++;
        n++;
      }
      i++;
    }
    return str[i - n - 1];
  } 
  else
    return NULL;
}

int main() {
  char *largestring = "Foo Bar Baz";
  char *smallstring = "Bar";
  char *ptr;

  ptr = ft_strstr(largestring, smallstring);
  printf("%s", *ptr);
  return 0;
}

Upvotes: 0

Views: 612

Answers (2)

klutt
klutt

Reputation: 31409

You're trying to mimic the standard strstr function, but that function does not do what you're asking for. It actually does return a pointer to the first occurrence of str in to_find. Your current approach is actually correct.

However, there is two bugs. One is this line in ft_strstr:

return str[i - n - 1];

that should be:

return &str[i - n - 1];

The other one is in main:

printf("%s", *ptr);

that should be

printf("%s", ptr);

Fix these two things and your code should work as it should.

If needle is an empty string, haystack is returned; if needle occurs nowhere in haystack, NULL is returned; otherwise a pointer to the first character of the first occurrence of needle is returned.

The pointer will still always (unless it's a null pointer) point somewhere in the haystack. I think that's what's confusing you.

Maybe this code snippet will help you:

char *largestring = "Foo Bar Baz";
char *smallstring = "Bar";
char *ptr = strstr(largestring, smallstring);
int index = ptr - largestring;

The variable index will now contain the index of the first occurrence, in your case it will be 4. (Given that it was found. It's possibly undefined behavior otherwise.)

Upvotes: 0

Weather Vane
Weather Vane

Reputation: 34585

The difference in what you report is not what is returned, but what you do with it. But first you have some errors:

  • return str[i - n - 1]; should be return &str[i - n - 1];
  • printf("%s", *ptr); should be printf("%s", ptr);
  • You are returning the first argument if the second is NULL, you should explicity return NULL for failure, otherwise the caller will think a substring was found.
  • You must check the return value is not NULL (not found) before you dereference it.

You can print the result, and, just the first character with

printf("%s\n", ptr);
printf("%c\n", *ptr);

Upvotes: 1

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