Question

I tried to implement the subtract method but got there are some bugs.

55-44 is correct
555-44 is not correct, it will return 011
100-44 will cause segmentation fault

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

char* subtract(char *n1,char *n2){
    int n1Len = strlen(n1);
    int n2Len = strlen(n2);


    int diff;  
    int max=n1Len;
    char* res = (char*)malloc (max+2);
    memset(res, '0', max +1); 

    res[max] = '\0';
    int i=n1Len - 1, j = n2Len - 1, k = max;
    for (; i >= 0 && j >=0; --i, --j, --k) {
        if(i >= 0 && j>=0)
        {
            diff = (n1[i] - '0') - (n2[j] - '0');
            if(diff<0)
            {
                int temp=n1[i-1]-'0';
                temp=temp-1;
                n1[i-1]=temp+'0';
                diff+=10;
            }
            res[i]=diff+'0';
        }
        else 
            res[i]=n1[i];

    }
    return res;
}


int main(void) {

    printf("%s\n",  subtract("100","44"));
}

Answer 1

I wrote it in GMP just for kicks.

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

char* subtract(char *n1,char *n2){
  mpz_t n1z, n2z;
  char * res = malloc(strlen(n1) > strlen(n2) ? strlen(n1) + 1 : strlen(n2) + 1);
  mpz_init_set_str(n1z, n1, 10);
  mpz_init_set_str(n2z, n2, 10);
  mpz_sub(n2z, n1z, n2z);
  gmp_sprintf(res, "%Zd", n2z);
  mpz_clear(n1z);
  mpz_clear(n2z);
  return res; 
}

int main(void) {
  printf("%s\n",  subtract("55","44"));
  printf("%s\n",  subtract("555","44"));
  printf("%s\n",  subtract("100","44")); 
}