Question

I have been performing bitwise operation on a variable.

int p=3,q=5;
int a=~p,b=~q; //complement a and b
printf("%d %d\t%d %d",p,a,q,b);

The theoretical output for ‘b’ is 10 and in case if it’s signed, it has to be -2.
But the output is -6.

Can someone explain me the working of it?

Answer 1

~ is the bitwise complement operator in c (or python) which essentially calculates -x - 1.

So a table would look like:

0  -1
1  -2
2  -3
3  -4 
4  -5 
5  -6

In two’s complement representation, if a number x’s most significant bit is 1, then the actual value would be −(~x + 1).

For instance,

0b11110000 = -(~0b1111 + 1) = -(15 + 1) = -16.

This is a natural representation of negative numbers, because

0000001 =  1
0000000 =  0
1111111 = -1  (wrap around)
1111110 = -2
1111101 = -3 etc.

See http://en.wikipedia.org/wiki/Two%27s_complement for detail.