Thread: K&R Page 61 atoi function example

Hi,
I am just trying to see how to run the atoi code on Page 61. I wrote the following but the below code prints just 0.

Can anyone please help? Thank you.

Note: I just want to thank the folks that help us newbies on this website. For beginners like myself, experts on websites like these offer tremendous opportunity to learn. Thank you for taking time to help people like myself who are starting learning C from absolute scratch (Nope, not trying to score brownie points, I sincerely mean it).

Andrew.

Code:

#include <stdio.h>
#include <ctype.h>

#define MAX 5

int atoi(char a[]);

int main()
{
        int val;
        char s[MAX] = {'s', ' ', 'a','b',' '};
        while(val = atoi(s)) {
                printf("%d\n",val);
        }
        return 0;
}

/* atoi: convert s to integer; version 2 */
int atoi(char s[])
{
        int i, n, sign;
        for (i = 0; isspace(s[i]); i++) /* skip white space */
                ;
        sign = (s[i] == '-') ? -1 : 1;
        if (s[i] == '+' || s[i] == '-') /* skip sign */
                i++;
        for (n = 0; isdigit(s[i]); i++) {
                n = 10 * n + (s[i] - '0');
        }
        return sign * n;
}

Code:

int main()
{
        int val;
        char s[MAX] = {'+', '1', '0','0',' '};
        val = atoi(s);
        printf("%d\n",val);
        return 0;
}

Try using above code to test atoi.

Tim S.

Originally Posted by iMalc

Better yet, realise that MAX is defined as 5 and therefore neither example has a null-terminator. That's also not the best way to write a string.
A better example would be:

Code:

int main()
{
        char s[] = "-13572468";
        int val = atoi(s);
        printf("%d\n",val);
        return 0;
}

Yes I know it's longer than 4 characters, but by taking out the MAX from inside the square brackets, the compiler now calculates the correct size for us

iMalc, Thank you.

If my char s[] has a value as below:

Code:

char s[] = "-13572 468";

I see that the value returned is

Code:

-13572

I was expecting that the function is just going to skip all the white spaces, collect the integers and the sign and print the value returned as

Code:

-13572468

based on my understanding of what isspace and isdigit are doing in the above atoi function code.

Andrew

Tim,
Thank you.

If we change isdigit to isalpha, and change the atoi function to something as below:

Code:

    char atoi(char s[])
{
        int i, n, sign;
        for (i = 0; isspace(s[i]); i++) /* skip white space */
                ;
        sign = (s[i] == '-') ? -1 : 1;
        if (s[i] == '+' || s[i] == '-') /* skip sign */
                i++;
        for (i = 0; isalpha(s[i]); i++) {
        //return sign * n ;
                return n;
        }
}

Here is the complete code I am using...really just a modification of your solution:

Code:

#include <stdio.h>
#include <ctype.h>

#define MAX 5

char atoi(char a[]);

int main()
{
        char val;
        char s[MAX] = {'+', '1', '0','0',' '};
//      char s[MAX] = {'b', ' ', 'b',' ',' '};
        val = atoi(s);
        printf("%c\n",val);
        return 0;
}

/* atoi: convert s to integer; version 2 */
char atoi(char s[])
{
        int i, n, sign;
        for (i = 0; isspace(s[i]); i++) /* skip white space */
                ;
        sign = (s[i] == '-') ? -1 : 1;
        if (s[i] == '+' || s[i] == '-') /* skip sign */
                i++;
        for (i = 0; isalpha(s[i]); i++) {
                return n;
        }
}

Andrew