Subtracting two numbers without using '-' operator

Question

i tried with the following code , but i can't understand why it's giving me wrong answer. i am computing the 2's complement and adding with another no.

#include <stdio.h>

int add(int a, int b) {
    while (a) {
        a = (a & b) << 1;
        b = a^b;
    }
    return b;
}

int sub(int a, int b) // add a with b's 2's complement.
{
    return (add(a, add(~b, 1)));
}

int main() {
    int a, b, res;
    a = 3, b = 1;
    res = sub(a, b);
    printf("%d\n", res);
    return 0;
}

`sub()` is giving you the wrong result because `add()` is wrong. The logic in `sub()` is fine. — NullUserException, Aug 07 '10 at 13:49
This brings back memories too. Our professor wouldn't let us use loops, or `if` statements for that matter. — NullUserException, Aug 07 '10 at 13:51
@NullUserException: So what if it's homework; it's also tagged as C. `+` and `-` are always available in C. This feels very much like "not a real question" to me. — CB Bailey, Aug 07 '10 at 14:04
@Charles Bailey: it's not a homework question. I am just learning how to use the bitwise operators — pranay, Aug 07 '10 at 14:08
I didn't say that it was a homework question, that was NullUserException. You should use bitwise operators when you need to do bitwise operations, typically when you are using values as a mask. If you want to subtract numbers then you should use a subtraction operator, bitwise operations are not the most appropriate solution. — CB Bailey, Aug 07 '10 at 14:17

score 6 · Accepted Answer · edited Apr 17 '16 at 01:45

6

i used a different add() function as suggested by NullUserException, it works now:

int add(int a,int b)
{
  int x;
  x = a^b;

  while(a&b)
  {
    b = ((a&b)<<1);
    a = x;
    x = a^b;
    //b=(a^b);
  }

  return x;
}

edited Apr 17 '16 at 01:45

Dorian

1,079
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answered Aug 07 '10 at 14:15

pranay

2,339
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This will through an error on while(a&b) statement as a and b is not boolean – Chirag Tayal Feb 24 '13 at 04:47
7

@ChiragTayal in C almost anything can be a boolean. – Flexo Mar 10 '13 at 10:03
1

@ChiragTayal 0 is false and all other numbers are true in C – REALFREE Jul 20 '14 at 02:19
The "while" part could be replaced by "while ((a&b) <> 0)" for more clarity, altought not required – umlcat Jun 05 '23 at 20:31

ShinTakezou · Answer 2 · 2010-08-07T17:46:21.763

2

Considering how negative numbers are represented, the following will compute a - b:

int a, b, c;
// assign to a and b
c = a + (~b + 1); // () not needed, just to show the point

as the OP already noted:) This moves the attention to your add implementation, that is of course wrong. The following is an odd way to do it (just since other better ways are already given)

int add1(int a, int b, int *c)
{
  int r = *c & 1;
  a &= 1; b &= 1;
  *c = a&b | a&r | b&r;
  return a^b^r;
}
int inv(int a)
{
  int i, r = 0;
  for(i = 0; i < sizeof(int)*8; i++)
  {
    r = r<<1 | (a&1);
    a >>= 1;
  }
  return r<<1;
}
int add(int a, int b)
{
  int r = 0, i;
  int c = 0;
  for(i=0; i < sizeof(int)*8; i++)
  {
    r |= add1(a>>i, b>>i, &c);
    r <<= 1;
  }
  return inv(r);
}

int sub(int a, int b)
{
  return add(a, add(~b, 1));
}

(keeping the same idea the code can be made better, just too tired to do it finer)

edited Aug 07 '10 at 17:46

answered Aug 07 '10 at 16:18

ShinTakezou

9,432
1
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1

the title being "Subtracting two numbers without using ‘-’ operator" which does not mean implement add/sub bitwise like your shown code, that is just the way you thought it could be done, but there's no clue about why teacher asked you that. Maybe he wants to be sure you understood 2's complement, and ~ being a bitwise operator, makes the whole code bitwise - so this too is taken into account. Moreover, here there's not this simple insight, so the answer can't be "unuseful" (as the down button say) – ShinTakezou Aug 07 '10 at 16:24
ops - all that code just hides that! - and it is obscure the connection between title and code. This shows the solution is right, provided that add is implemented correctly :) - titles should be more "catch it" – ShinTakezou Aug 07 '10 at 16:25

Max Kapsecker · Answer 3 · 2018-04-01T00:44:23.863

You also can implement this recursively. In C this might look like:

#include <stdio.h>

int add(int a, int b){
    if(b == 0) return a;
    int sum = a ^ b;
    int carry = (a & b) << 1;
    return add(sum, carry);
}

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

int main(){

    int a = 3;
    int b = 1;

    int sum = add(a, b);
    printf("%i + %i = %i \n", a, b, sum);

    int difference = subtract(a, b);
    printf("%i - %i = %i \n", a, b, difference);

    return 0;
}

score 1 · Answer 4 · edited Jun 05 '23 at 20:43

add method implementation is incorrect. do like this -> A java way of this.

public int add(int a, int b){
  if (b > 0) {
    do {
      a = a & b; //carry
      b = a ^ b;  //addition
      a = a << 1; //carry shift to one bit left
    }while(a != 0);  //exit 
   } else {
     b = a;
   }
   return b;     //addition result
 }

  public int sub(int a, int b){
    return add(a, add(~b, 1)); 
 
  }

score -4 · Answer 5 · answered Feb 05 '17 at 16:32

import java.util.List;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.Hashtable;
import java.util.LinkedList;
import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

import javax.xml.soap.Node;

public class mainone {





 public static void main(String args[]){


 int a=12;
 int b=4;
 Integer  c=new Integer(b);
 String  d=Integer.toString(c);
 String e="-";
 String f=e.concat(d);
 Integer g=Integer.parseInt(f);
 System.out.println(a+g);





 }



 }

Subtracting two numbers without using '-' operator

5 Answers5

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