CODEWITHSUNDEEP
c++cbitwise-operatorsbit
AustinT
AustinT

Reputation: 2026

What does ~0 do?

Does ~0 mean its flipping 000000000 to 1111111111?

printf("Check: %i", ~0);

The printf results to -1, which is why I am confused.
Does -1 essentially mean the same thing as 11111111111111111 bits?

Upvotes: 4

Views: 764

Answers (4)

0x90
0x90

Reputation: 40982

~0 == 0xFFFFFFFF

where 0xFFFFFFFF= 32 times 1 , which is -1 in 2's compliement representation

since ~ is a bitwise operation and turns zero to one in each bit:

~0b1010 == 0b0101

Upvotes: 3

Edwin Buck
Edwin Buck

Reputation: 70909

Does ~0 mean its flipping 000000000 to 1111111111?

Yes, that's what it means.

printf("Check: %i", ~0);

The printf results to -1, which is why I am confused.

That's because of 2's complement arithmetic, where we have conventionally accepted zero to be

000000000000

and subtracting one from it requires a "borrow" that requires a borrow, and so on, until you "roll" the entire register

111111111111

Logically, if you add "1" to that number, it will carry, and carry, and so on until it "rolls" in the opposite direction, yielding 000000000 again.

Does -1 essentially mean the same thing as 11111111111111111 bits?

Yes, as long as you are using 2's complement signed integers.

---- Edited, to include details from cincutar's now deleted post (I wish he didn't delete it) ---

To see the same memory formatted as a (unsigned) hexadecimal number, use the command 

printf("Check: %x", ~0);

which will print the output

Check ffffffff

which is (for those who don't read hexadecimal well)

11111111111111111111111111111111

Upvotes: 6

SamuelDavis
SamuelDavis

Reputation: 3324

It is due to the mathematical operation "two's complement". A nice video tutorial of two's complement can be found on youtube. Here's one of them :)

youtube link

Upvotes: 3

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726599

Does ~0 mean its flipping 000000000 to 1111111111?

Yes, it does.

Does -1 essentially mean the same thing as 11111111111111111 bits?

In 2s complement representation, it does.

Upvotes: 17

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