Subtraction without minus sign in C

Question

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

Answer 1

Assembly (accumulator) style:

int result = a;
result -= b;

Answer 2

For subtracting in C two integers you only need:

int subtract(int a, int b)
{
    return a + (~b) + 1;
}

I don't believe that there is a simple an elegant solution for float or double numbers like for integers. So you can transform your float numbers in arrays and apply an algorithm similar with one simulated here

Answer 3

I suppose this

b - a = ~( a + ~b)

Answer 4

Given that encoding integers to support two's complement is not mandated in C, iterate until done. If they want you to jump through flaming hoops, no need to be efficient about it!

int subtract(int a, int b)
{
  if ( b < 0 )
    return a+abs(b);
  while (b-- > 0)
    --a;
  return a;
}

Silly question... probably silly interview!

Answer 5

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).