How to fix the output of the following C program? - c

Question

I have a problem with output of the following C program. It does not get me the correct output.I transform this program from the sum of two big numbers.
So I get the output with some "errors" like "232423-3-4-34--3424" instead of "23242334343424". How do i fix this?
#include<stdio.h>
int main() {
int num1[255], num2[255], dif[255];
char s1[255], s2[255];
int l1, l2;
printf("Enter Number1:\n");
scanf("%s", &s1);
printf("Enter Number2:\n");
scanf("%s", &s2);
for (l1 = 0; s1[l1] != '\0'; l1++)
num1[l1] = s1[l1] - '0';
for (l2 = 0; s2[l2] != '\0'; l2++)
num2[l2] = s2[l2] - '0';
int carry = 0;
int k = 0;
int i = l1 - 1;
int j = l2 - 1;
for (; i >= 0 && j >= 0; i--, j--, k++) {
dif[k] = (num1[i] - num2[j] - carry) % 10;
carry = (num1[i] - num2[j] - carry) / 10;
}
if (l1 > l2) {
while (i >= 0) {
dif[k++] = (num1[i] - carry) % 10;
carry = (num1[i--] - carry) / 10;
}
} else if (l1 < l2) {
while (j >= 0) {
dif[k++] = (num2[j] - carry) % 10;
carry = (num2[j--] - carry) / 10;
}
} else {
if (carry > 0)
dif[k++] = carry;
}
printf("Result:");
for (k--; k >= 0; k--)
printf("%d", dif[k]);
return 0;
}

Answer 1

Your result includes negative numbers because the code doesn't correctly implement modulo-10 arithmetic. In lines like this:
dif[k] = (num1[i] - num2[j] - carry) % 10;
If subtraction num1[i] - num2[j] - carry is less than 0, you want to store the result of 10-subtraction. There are languages where the % operator works like that in, but in C it returns a negative number, so -1 % 10 yields -1 and not 9.
To fix the issue, the code needs to be more explicit, e.g.:
int sub = num1[i] - num2[j] - carry;
if (sub >= 0) {
dif[k] = sub;
carry = 0;
} else {
dif[k] = 10 - sub;
carry = 1;
}