Question

I’m trying to subtract strings where each ASCII character is treated as a decimal digit. For instance:

"1000000000001" - "0100000000001" = "900000000000"

How would I get started on an implementation of this if my function prototype looked like:

char* get_sub(char* a, char* b)

Answer 1

It doesn’t seems, but it’s a quite complex problem (unless I’m getting too much old).
This works only in N. So it must be true that a >= 0, b >= 0, a >= b. I won’t explain how does it works. As I’ve written, it’s quite complex 🙂 (and I’m not even happy of what I’ve written. I’m sure there is something I haven’t thought)

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

char* get_sub(const char* a, const char* b);

#define MAX(a, b) (((a) > (b)) ? (a) : (b))

int main(int argc, char* argv[])
{
    char *res = get_sub("10000","9999");
    printf("%s\n", res);
    free(res);
    return 0;
}

char* get_sub(const char* a, const char* b)
{
    size_t a1len = strlen(a);
    size_t a2len = strlen(b);

    size_t max = MAX(a1len, a2len);

    /* I'm using calloc to make it easier to debug. You could use malloc, but you'll have to uncomment a line below */
    char *res = (char*)calloc(max + 1, sizeof(char));

    int carry = 0;

    char *pres = res;
    for (const char *pa = a + a1len - 1, *pb = b + a2len - 1; pa >= a || pb >= b; pa--, pb--, pres++)
    {
        int val1 = pa >= a ? (*pa - '0') : 0;
        int val2 = pb >= b ? (*pb - '0') : 0;

        int diff = val1 - carry - val2;

        if (diff >= 0)
        {
            *pres = (char)(diff + '0');
            carry = 0;
        }
        else
        {
            *pres = (char)(10 + diff + '0');
            carry = 1;
        }
    }

    if (carry != 0)
    {
        free(res);
        return (char*)calloc(1, 1);
    }

    /* *pres = '\0'; */ /* Uncomment this line to use malloc */

    pres--;

    while (pres > res && *pres == '0')
    {
        *pres = '\0';
        pres--;
    }

    strrev(res);

    return res;
}