"String" contains function in C

Question

This function checks whether or not a substring needle exists in another string haystack and returns the position if it does or 0 if it doesn't, unless the position is 0 in which case, it won't be located.

Looking for ways to improve this code, specifically better error handling.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

size_t contains(const char * needle, const char *haystack);

int main(void)
{
    char *needle = "test";
    char *haystack = "This is a dinosaurtest.";
    printf("Position: %lu", contains(needle, haystack));

    return EXIT_SUCCESS;
}

size_t contains(const char * needle, const char *haystack)
{
    if(needle == NULL || haystack == NULL)
    {
        return 0;
    }
    long int first_char_pos = -1;
    size_t len_h = strlen(haystack);
    size_t len_n = strlen(needle);
    size_t i, j;
    size_t exist_count = 0;

    // Find the first character. If it doesn't exist, we're done.
    for(i = 0; i < len_h; i++)
    {
            if((haystack[i] == needle[0]) && (first_char_pos == -1))
            {
                first_char_pos = i;
                exist_count++;
            }
    }

    if(first_char_pos == -1)
    {
        return 0;
    }
    printf("First char match index: %li\n", first_char_pos);
    printf("Char: %c\n", haystack[first_char_pos]);
    size_t current_index = (size_t) first_char_pos;
    for(i = first_char_pos; i < len_h; i++)
    {
        if(haystack[i] == needle[exist_count] && (i == (current_index + 1)))
        {
            current_index = i;
            exist_count++;
        }
        printf("Exist count: %lu\n", exist_count); //<--Debugging
        if(exist_count == len_n)
        {
            return first_char_pos;
        }
    }
    return 0;
}

Answer 1

Just a couple of remarks:

• You should add a newline after the last line:

    $ ./nh
    First char match index: 18
    Char: t
    Exist count: 1
    Exist count: 2
    Exist count: 3
    Exist count: 4
    Position: 18 $
  

• I don't know what compiler you use but with when compiled with gcc and -Wall -Wextra -pedantic you get:

  gcc -O2 nh.c -lm -o nh -Wall -Wextra -pedantic
  nh.c: In function ‘contains’:
  nh.c:25:15: warning: unused variable ‘j’ [-Wunused-variable]
   size_t i, j;
             ^
  

• Code formatting should be more consistent. For example, in this line you put a whitespace before needle but don't put a whitespace before haystack:

  size_t contains(const char * needle, const char *haystack);
  

• %lu is not a portable specifier for size_t type, you should use %zu introduced in C99.

• You said:

returns the position if it does or 0 if it doesn't, unless the position is 0 in which case, it won't be located.

This is really not good. For example, with this it returns 0:

char *needle = "This";
char *haystack = "This is a dinosaurtest.";

With this, it also returns zero:

char *needle = "non-existent";
char *haystack = "This is a dinosaurtest.";

You can't tell the difference between success and failure in this two examples. Actually, atoi() has the same problem. I don't know what operating system you use but maybe you could use ssize_t as the return type if it's available and return -1 in case of failure.

Answer 2

Adding on to the previous answer by @Arkadiusz Drabczyk:

A simple, trivial implementation of contains could be done like this:

ssize_t contains(const char * needle, const char *haystack)
{
    char *needle_in_haystack;
    if(!needle || !haystack) return -1;
    needle_in_haystack = strstr(haystack, needle);
    return needle_in_haystack ? needle_in_haystack - haystack : -1;
}

Then, this program (with a few changes as mentioned above) should work:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>

ssize_t contains(const char * needle, const char *haystack)
{
    char *needle_in_haystack;
    if(!needle || !haystack) return -1;
    needle_in_haystack = strstr(haystack, needle);
    return needle_in_haystack ? needle_in_haystack - haystack : -1;
}

int main(void)
{
    char *needle = "test";
    char *haystack = "This is a dinosaurtest.";
    char *haystack2 = "This does not contain the string.";
    printf("Position: %zd\n", contains(needle, haystack));
    printf("Position: %zd\n", contains(needle, haystack2));

    return EXIT_SUCCESS;
}

Output:

Position: 18
Position: -1

Answer 3

Your code doesn't work. It returns 0 for haystack "abbc" and needle "bc", even though haystack contains needle.

Answer 4

You don't need the first loop and all the length calculations. Btw., the function doesn't succeed, if the first char is found, but only the second occourrence of the first char fits with needle.

The task can be reduced to a few lines:

int contains(char *buf, char *needle) {
    char *src, *srch, *srcp;
    for(src=buf; *src; src++) {
        for(srch = needle, srcp = src; *srch && *srcp && *srch == *srcp; srch++, srcp++);
        if(!*srch) return src - buf;
    }
    return -1;
}