GATE EXAM PREPARATION IN "C" - DATA TYPE

Predict the output of following program. Assume that the numbers are stored in 2’s complement form.

#include<stdio.h> 
int  main() 
{ 
   unsigned int x = -1; 
   int y = ~0; 
   if (x == y) 
      printf("same"); 
   else
      printf("not same"); 
   return 0; 
}

(A) same
(B) not same


Answer: (A) 

Explanation: -1 and ~0 essentially have same bit pattern, hence x and y must be same. In the comparison, y is promoted to unsigned and compared against x (See this for promotion rules). The result is “same”. However, when interpreted as signed and unsigned their numerical values will differ. x is MAXUNIT and y is -1. Since we have %u for y also, the output will be MAXUNIT and MAXUNIT.