Exercise 3.4 - itoa to handle largest negative integer¶

Question¶

In a two’s complement number representation, our version of itoa does not handle the largest negative number, that is, the value of n equal to -(2wordsize-1). Explain why not. Modify it to print that value correctly, regardless of the machine on which it runs.

The previous version of itoa was this

#include <stdio.h>
#include <string.h>

#define MAXLINE 1000

#define abs(x) ((x) > 0 ? (x) : -(x))

void reverse(char s[]) {
    int c, i, j;

    for (i = 0, j = strlen(s) - 1; i < j; i++, j--)
        c = s[i], s[i] = s[j], s[j] = c;
}

void itoa(int n, char s[]) {
    int i, sign;

    sign = n;

    i = 0;

    do {
        s[i++] = abs(n % 10) + '0';

    } while ((n /= 10) != 0);

    if (sign < 0)
        s[i++] = '-';

    s[i] = '\0';

    reverse(s);
}

int main(void) {
    int number;
    char str[MAXLINE];

    /*  number=-2345645; */

    number = -2147483648;

    printf("Integer %d printed as\n String:", number);

    itoa(number, str);

    printf("%s", str);

    return 0;
}

Explanation¶

In this version of itoa, which involves a largest negative number, we first store the number itself in an integer called sign. Then get numbers from unittest by doing n%10, get the unsigned number by doing a abs value and get character by adding it to 0.

Thus we go about converting each digit starting from unit place to a character. Once this process is over. We check if we were converting negative number, by checking if the sign is less than 0, if it was, we add a - to the string.

And then we do a simple reverse of the string to get our itoa.