multiplying an integer by 5 using bitwise operator

Question

I am trying to multiply an integer by 5 using bitwise operations. But there seems to be a bug in my code that I can't identify. Here's my code

#include <stdio.h>

#define print_integer(exp) printf("%s : %d\n", #exp, (exp))

int multiply(int num) {
    int ans;
    ans = num << 2 + num;
    return ans;
}

int main() {
    int a = 1, b = 2, c = 3;
    print_integer(multiply(a));
    print_integer(multiply(b));
    print_integer(multiply(c));
    return 0;
}

Edit:- The bug is in line ans = num << 2 + num;

`ans = (num<<2) + num;` https://en.wikipedia.org/wiki/Operators_in_C_and_C%2B%2B#Operator_precedence — wildplasser, Dec 04 '16 at 15:14
Do you consider `+` to be a _bitwise operator_? I would not think so. Yet if `+` is a _bitwise operator_, then `*` is certainly one too and code simplifies to `int multiply(int num) { return num*5; }`. — chux - Reinstate Monica, Dec 04 '16 at 15:22
Possible duplicate of [Multiplication of two integers using bitwise operators](http://stackoverflow.com/questions/4456442/multiplication-of-two-integers-using-bitwise-operators) — phuclv, Dec 04 '16 at 15:46

score 3 · Accepted Answer · answered Dec 04 '16 at 15:20

The precedence between << and + is counter intuitive. Use parentheses and compile with -Wall to get useful warnings abut this kind of potential mistake:

#include <stdio.h>

#define print_integer(exp) printf("%s : %d\n", #exp, (exp))

int multiply(int num) {
      return (num << 2) + num;
}

int main(void) {
    int a = 1, b = 2, c = 3;
    print_integer(multiply(a));
    print_integer(multiply(b));
    print_integer(multiply(c));
    return 0;
}

score -1 · Answer 2 · answered Dec 04 '16 at 15:19

-1

You are adding the num after the change ( num<<2 ) , set a temp value for num .

int tmp = num ; 
ans = num << 2 + tmp ;

answered Dec 04 '16 at 15:19

Isaac Razilov

19
3

2

`num << 2` doesn't change the value of `num`. – giusti Dec 04 '16 at 15:57
1

This will work no differently than OP's original code. – Ted Hopp Dec 04 '16 at 16:07

multiplying an integer by 5 using bitwise operator

2 Answers2