ft_strcspn

Subject

Assignment name	: ft_strcspn
Expected files	: ft_strcspn.c
Allowed functions: None
---------------------------------------------------------------

Reproduce exactly the behavior of the function strcspn
(man strcspn).

The function should be prototyped as follows:

size_t	ft_strcspn(const char *s, const char *reject);

Man Page

STRCSPN(3) (simplified)

NAME
    strcspn -- span a string
LIBRARY
    Standard C Library (libc, -lc)
SYNOPSIS
    #include <string.h>
    size_t strcspn(const char *s, const char *charset);
DESCRIPTION
    The strcspn() function spans the initial part of the null-terminated
string s as long as the characters from s do not occur in the null-
terminated string charset (it spans the complement of the charset). In
other words, it computes the string array index of the first character
of s which is also in charset, else the index of the first null character.
RETURN VALUES
    The strcspn() functions return the number of characters spanned.

Commented solution

For this one, I'll give you two different examples, one will be done using indexes in the string s and reject. And the second example will use pointers directly.

Indexes

ft_strcspn.c

// Needed for size_t
#include <stdio.h>

size_t ft_strcspn(const char *s, const char *reject)
{
    size_t i;
    size_t j;
    
    i = 0;
    // Looping over the whole string
    while(s[i++])
    {
        j = 0;
        // Check the current string char against every char in charset
        while (reject[j])
        {
            // If character found, return index in current string
            if (s[i] == reject[j++])
            	return (i);
        }
    }
    // If we spanned the whole string, return i, in that case the
    // rejected char found is the NULL-terminating one
    return (i);
}

Pointers

ft_strcspn.c

#include <stdio.h>

size_t ft_strcspn(const char *s, const char *reject)
{
    // Save a pointer to the start of s
    const char *og = s;
    const char *s2;
    
    // Looping over the whole string 
    while (*s++)
    {
        // Set s2 to be the start of reject
        // that way we keep the reject pointer to its original position
        // Then loop over the whole charset
        s2 = reject;
        while (*s2)
        {
            // If current string charcter found in charset, return the
            // number of characters spanned
            if (*s == *s2++)
                // Here we make use of the original pointer to calculate
                // the number of char. spanned
                return (s - og);
        }
    }
    // Same as above, we use the original pointer to calculate the number
    // of character spanned
    return (s - og);
}