Why ~0 is -1 ?

Question

Why ~0 is -1 and not 1 ?

Answer 1

binary representation of 0 in 8 bits:
00000000

after bitwise NOT ~ operation all bits are inverted :
11111111

11111111 in binary represents signed -1 of decimal system. (the most significant bit is 1 ,therefore number is negative)

I'm using 8 bit example but it is 2 byte for 16bit system ,4byte for 32 bit system.

This article can help you understand better :
https://medium.com/@LeeJulija/how-integers-are-stored-in-memory-using-twos-complement-5ba04d61a56c

Answer 2

bahha🐧 ,

your point about treating integer literal as unsigned is correct.

but unfortunately what you stated about getting 1 as output is wrong.

when you compliment all bits of 0 you'll get largest possible unsigned int represented by
UINT_MAX macro , 0xffffffff in hex ,4294967295 in decimal for 32 bit system.

and it is obviously not 1.

Answer 3

in addition to the explanation above if you need 1 you have to treat it as unsigned value ~0U to get 1

Answer 4

🇮🇳Omkar🕉 you are right, thanks.
you would have to use absolute value to get one.
#include <stdio.h>
#include<stdlib.h>
unsigned n = ~0;
int main() {
printf("%d",abs(n));
return 0;
}