*ft_atoi

Subject

Assignment name  : ft_atoi
Expected files   : ft_atoi.c
Allowed functions: None
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Write a function that converts the string argument str to an integer (type int)
and returns it.

It works much like the standard atoi(const char *str) function, see the man.

Your function must be declared as follows:

int	ft_atoi(const char *str);

Commented solution

ft_atoi

ft_atoi.c

/* this function returns 1 if the character is one 
 * of the "space" character or 0 if it is not
 */
int ft_isspace(int c)
{
    return ((c >= 9 && c <= 13) || c == 32 ? 1 : 0);
}

/* this function returns 1 if the character is a digit 0-9
 * and 0 if the character is not a digit 0-9
 */
int ft_isdigit(int c)
{
    return (c >= 48 && c <= 57 ? 1 : 0);
}

int ft_atoi(const char *str)
{
    int res = 0;
    int i = 0;
    int s = 1;
    
    /* skipping all the space characters 
    */
    while (ft_isspace(str[i]))
        i++;
    /* if there is a - we set the sign to -1 and we go to
     * the next character
     */
    if (str[i] == '-')
    {
        s = -1;
        i++;
    }
    /* while the number is a digit, we convert it to an int
     */
    while (ft_isdigit(str[i]))
    {
        res *= 10;
        res += str[i] - 48;
        i++;
    }
    /* at the end, we multiply the number by the sign variable
     * and we finally return the int number
     */
    return (res *= s);
}