ft_substr

Subject

FT_SUBSTR (simplified)

NAME
    ft_substr -- extract a substring from a string
SYNOPSIS
    char *ft_substr(const char *s, unsigned int start, size_t len);
DESCRIPTION
    Allocate (with malloc(3)) and return a new string from the string s.
    This new string starts at index 'start' and has a maximum size of 'len'.
PARAMETERS
    s: string from which to extract the new string
    start: start index of the new string in the string 's'
    len: maximum size of the new string
RETURN VALUES
    ft_substr() returns the new string; NULL if the memory allocation failed.
AUTHORIZED EXTERNAL FUNCTIONS
    malloc(3)

Understandable explanation

ft_substr returns a substring of the string s passed as parameter.

Here's an example

ft_substr("Bonjour comment ca va?", 5, 8);
=> "ur comme"

Hints

First we have to check if the start index is greater than the length of the string or not.

We also have to check if the start plus the len is greater than the length of the whole string.

Then we allocate enough memory for the substring, and copy from s[start] until we reach len characters copied into our new string.

And finally we can return the substring.

Commented solution

ft_substr

ft_substr.c

#include "libft.h"

char *ft_substr(const char *s, unsigned int start, size_t len)
{
    size_t i;
    char *str;
    
    /* we first check that the string we received is not null
     * if it's the case we return NULL as we won't be able to take
     * a substring out of something empty
     */
    if (!s)
        return (NULL);
    /* if the start index is greater than the length of the original 
     * string we return an allocated empty string that can be freed
     * later on
     */
    if (start > ft_strlen(s))
        return (ft_strdup(""));
    /* if the len we have to copy is greater than the length of the string
     * starting at index start, that means we have to stop after we read
     * the whole string and that the allocated size has not to be 
     * equal to the len received as parameter but only the size we will
     * copy plus one for the NUL-terminating character
     * this let's us only allocate memory that is really necessary
     */
    if (len > ft_strlen(s + start))
        len = ft_strlen(s + start);
    /* we then allocate the memory
    */
    str = ft_calloc(len + 1, sizeof(char));
    if (!str)
        return (NULL);
    i = 0;
    /* we then loop over the string received as parameter from index
     * start + i and copy each character into our new string index i
     */
    while (i < len)
    {
        str[i] = s[start + i];
        i++;
    }
    /* we finally return the substring
     */
    return (str);
}