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;

Answer 1

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;
}