CODEWITHSUNDEEP
catoi
Tom San
Tom San

Reputation: 41

Why doesn't my atoi implementation work with negative numbers?

#include<stdio.h>

int ft_atoi(char *str)
{
    int i;
    int sign;
    int val;
    int nbr;

    i = 0;
    sign = 1;
    val = 0;
    nbr = 0;
    while(str[i] != '\0')
    {
        if (str[i] == '-')
            sign = -sign;
        i++;
    }
    i = 0;
    while(str[i] >= '0' && str[i] <= '9' && str[i] != '\0')
    {
        nbr = (int) (str[i] - '0');
        val = (val * 10) + nbr;
        i++;
    }
    i++;
    return (val * sign);
}

It returns 0 when I try with negative numbers.

Upvotes: 3

Views: 4517

Answers (3)

rtroulak
rtroulak

Reputation: 469

you can just str++; in your if when str is negative

#include <stdio.h>
int ft_atoi(char *str)
{
    int i;
    int sign;
    int val;
    int nbr;



       i = 0;
        sign = 1;
        val = 0;
        nbr = 0;
        while(str[i] != '\0')
        {
            if (str[i] == '-')
            {

                sign = -sign;
                str++;
            }
            i++;
        }
        i = 0;
        while(str[i] >= '0' && str[i] <= '9' && str[i] != '\0')
        {
            nbr = (int) (str[i] - '0');
            val = (val * 10) + nbr;
            i++;
        }
        i++;
        return (val * sign);
    }

Upvotes: 1

M Oehm
M Oehm

Reputation: 29126

After you have scanned the string for minus signs, you reset i to zero, but str[0] isn't a valid digit, because it's a minus sign.

You can change the first loop to advance past the first minus sign and the start parsing the numerical value from where the first loop left off:

int ft_atoi(char *str)
{
    int i = 0;
    int sign = 1;
    int val = 0;

    while (str[i] == '-') {
        sign = -sign;
        i++;
    }

    while(str[i] >= '0' && str[i] <= '9')
    {
        int nbr = (int) (str[i] - '0');

        val = (val * 10) + nbr;
        i++;
    }

    return (val * sign);
}

(There's no need to check that str[i] isn't the null character, because if it is a minus sign or a digit, it can't be the null character. The check is useful for testing negatives, however, e.g. when testing that some character isn't a minus.)

Upvotes: 2

mch
mch

Reputation: 9804

the second while loop breaks if str[0] is '-'.

while(str[i] != '\0')
{
    if (str[i] == '-')
        sign = -sign;
    i++;
}

should be

if (str[0] == '-')
{
    sign = -1;
    str++;
}

Upvotes: 4

Related Questions