Question

I can’t get the 2-complement calculation to work.

I know C compiles ~b that would invert all bits to -6 if b=5. But why?

int b=101, inverting all bits is 010 then for 2 complement’s notation I just add 1 but that becomes 011 i.e. 3 which is wrong answer.

How should I calculate with bit inversion operator ~?

Answer 1

~b is not a 2-complement operation. It is a bitwise NOT operation. It just inverts every bit in a number, therefore ~b is unequal to -b.

Examples:

b = 5
binary representation of b:  0000 0000 0000 0101
binary representation of ~b: 1111 1111 1111 1010
~b = -6

b = 17
binary representation of b:     0000 0000 0001 0001
binary representation of ~b:    1111 1111 1110 1110
~b = -18
binary representation of ~(~b): 0000 0000 0001 0001
~(~b) = 17