~ operator

[Pawanpreet Singh]

How to find ~.

int x = ~4.
The answer is x=-5. How to find it.

[Timmy Marks]

Multiply by -1 and subtract one. When calculating the negative of an int, internally you have a 2's complement representation of the positive number - you flip all the bits and then add 1. So, when you flip all the bits, you basically make a negative (flip all bits, add 1) and then subtract the extra 1!

[Pawanpreet Singh]

Yes, i could understand up to some extent, can you please interpret this last line,

suppose ~4

0100

1's complement : 1011

2's complement : 1011
0001
----------------
1100 == but it is 12 (not -5)...
how could i get - 5 from 1100. mean how to subtract 1

[Timmy Marks]

Since the MSB is 1, it will not be interpreted as 12, but as -4 (since you made the 2's complement of 4). If we follow your reasoning that it is 12, it is still wrong because the int 4 is represented not as 0100, but as 00000000000000000000000000000100, so the 2's complement would be 11111111111111111111111111111100. when you subtract 1, (or add -1 which is 11111111111111111111111111111111) you get -5, or 11111111111111111111111111111011. (which is basically just all the bits of 4 flipped)

I thought you were just looking for a way to determine what ~x will be, given x. It is simply -x - 1.

[Pawanpreet Singh]

Thanks man. Great