Interview Question (Programming)

What does the following function check?

bool f(unsigned int v)
{
  return ((v!=0)&&!(v & v-1));
}

(A) v is an odd number (B) v is a multiple of 2 (C) v is a power of 2 (D) v has a 0 bit

Answer :
(C) v is a power of 2

Explanation :
1) If v is a power of 2,

• v is never 0 (even 2^0 = 1) and (v != 0) will always be 1(True).
• (v & v-1) will always be 0(False) and hence their negation will always be 1(True).

2) If v is not a power of 2, either

• v = 0, or
• (v & v-1) is something other than 0 and their negation is 0(False)

Written by Munia Balayil