negation operator

[tue march]

Hi guys,

public class Flip{
public static void main(String argv[]){
System.out.println(~4);
}
}

can anybody explain why the output of this program is -5.

Thanks in advance
Good guy

[David Harkness]

~ is not negation; it's called two's complement. You flip all the bits, so

00000000000000000000000000000100 == 4

becomes

11111111111111111111111111111011 == -5

Can you see how it can be used for negation?

[Ernest Friedman-Hill]

Originally posted by David Harkness:
~ is not negation; it's called two's complement.

<nerd>
Two's complement would be flipping the bits and then adding one. Just flipping the bits is "one's complement".
</nerd>

[Ryan McGuire]

To combine and extend what David and Ernest said...

If we just look at 16 bits 4 = 0000000000000100. The ~ does a one's complement operation, so ~4 = 1111111111111011. The - does a two's complement operation, which is the one's complement plus 1, so -4 = 1111111111111100. Since 5 = 0000000000000101, -5 = 1111111111111011, which is the same things as ~4.

Why use two's complement for negative numbers? Because it makes subtraction work. For instance let's try 5 + (-4): 0000000000000101 + 1111111111111100 = 0000000000000001 with a 1 bit that got carried off the left end. With the one's complement, the answer is off by one.

One other clever thing about two's complment is that -0 = 0, while ~0 != 0.

Ryan
[ April 26, 2005: Message edited by: Ryan McGuire ]

[David Harkness]

Too long outside of the classroom. Thanks guys for clarifying!