Subtraction without minus sign in C

Question

How can I subtract two integers in C without the - operator?

Answer 1

int a = 34;
int b = 50;

You can convert b to negative value using negation and adding 1:

int c = a + (~b + 1);

printf("%d\n", c);

-16

This is two's complement sign negation. Processor is doing it when you use '-' operator when you want to negate value or subtrackt it.

Converting float is simpler. Just negate first bit (shoosh gave you example how to do this).

EDIT:

Ok, guys. I give up. Here is my compiler independent version:

#include <stdio.h>

unsigned int adder(unsigned int a, unsigned int b) {
    unsigned int loop = 1;
    unsigned int sum  = 0;
    unsigned int ai, bi, ci;

    while (loop) {
        ai = a & loop;
        bi = b & loop;
        ci = sum & loop;
        sum = sum ^ ai ^ bi;      // add i-th bit of a and b, and add carry bit stored in sum i-th bit
        loop = loop << 1;
        if ((ai&bi)|(ci&ai)|(ci&bi)) sum = sum^loop; // add carry bit
    }

    return sum;
}

unsigned int sub(unsigned int a, unsigned int b) {
    return adder(a, adder(~b, 1));    // add negation + 1 (two's complement here)
}


int main() {
    unsigned int a = 35;
    unsigned int b = 40;

    printf("%u - %u = %d\n", a, b, sub(a, b)); // printf function isn't compiler independent here

    return 0;
}

I'm using unsigned int so that any compiler will treat it the same.

If you want to subtract negative values, then do it that way:

 unsgined int negative15 = adder(~15, 1);

Now we are completly independent of signed values conventions. In my approach result all ints will be stored as two's complement - so you have to be careful with bigger ints (they have to start with 0 bit).