Can add very large numbers but not small numbers in C

Question

I have a C program that can add large numbers and it works fine but I also need to do add smaller numbers but I get a odd output and am not sure where the error is coming from.

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

int main() {
int num1[255], num2[255], sum[255], m = 0, z, x = 0;
char s1[255], s2[255], in[255];
int l1, l2;

printf("enter the input as a+b=\n");
scanf("%s", in);
for (z = 0; in[z] != '+'; z++) {
    s1[z] = in[z];
    m++;
}

for (z = m + 1; z < strlen(in) - 1; z++) {
    s2[x] = in[z];
    x++;
}

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++) {
    sum[k] = (num1[i] + num2[j] + carry) % 10;
    carry = (num1[i] + num2[j] + carry) / 10;
}

if (l1 > l2) {
    while (i >= 0) {
        sum[k++] = (num1[i] + carry) % 10;
        carry = (num1[i--] + carry) / 10;
    }

    if (carry != 0) {
        sum[k] = carry;
        k = k + 1;
    }
} else if (l1 < l2) {
    while (j >= 0) {
        sum[k++] = (num2[j] + carry) % 10;
        carry = (num2[j--] + carry) / 10;
    }

    if (carry != 0) {
        sum[k] = carry;
        k = k + 1;
    }
} else {

    if (carry > 0)
        sum[k++] = carry;
}

printf("%s+%s=\n", s1, s2);

for (k--; k >= 0; k--)
    printf("%d", sum[k]);
return 0;
}

If I enter a values such as:

enter the input as a+b=

9999999999999999999999999999+1=

I get this output which is what I wanted it to be

9999999999999999999999999999+1=

10000000000000000000000000000

When I try something smaller like:

123+456=

The output is different

123ó+456=

16-5-6

I don't know where these numbers are coming from and this keeps occurring with any number less than 9999999999999999999999999999+1= It should work for any number not just a single set.

Answer 1

for (l1 = 0; s1[l1] != '\0'; l1++)
//           ^^^^^^^^^^^^^^

Here's a problem: You're checking for '\\0' in s1 , but you never set any element of s1 to '\\0' , so this will access uninitialized memory and potentially go out of bounds of the array.

You have the same problem with s2 .

There may be other problems, but this is where I stopped reading.

---

General comments:

• Never use scanf .
• If you do, always check its return value to make sure it was successful.
• Never use %s with scanf . It's a buffer overflow waiting to happen.
• Your code makes very stringent assumption about what the input looks like. If the user doesn't type exactly what your code is expecting, fun things will happen (eg the first for (z = 0; in[z] != '+'; z++) loop will go out of bounds if there is no + in the string). You should probably verify your assumptions and not blindly assume everything is OK.