~ operator in C

Question

The output of this program is -13. I have never fully understood ~ operator in C. Why does it give -13 as output? How to limit ~ operator to just 4 bits of a number?

<         


        

Answer 1

(12)10 in binary is (1100)2

The tilde is the bitwise complement operator which makes 1100 --> 0011. However if you working on a 32 bit platform actually what we get is:

0000 0000 0000 0000 0000 0000 0000 1100

Whose bitwise complement is:

1111 1111 1111 1111 1111 1111 1111 0011
|

Now since the left most bit is for sign the number becomes negative. If you use unsigned int you will be able to understand better what is happening:

unsigned int a = 12;
a = ~a;

Will give:

1111 1111 1111 1111 1111 1111 1111 0011

Which is 4294967283