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).