C | Data Types | Question 1

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.

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