Path: utzoo!mnetor!uunet!husc6!hao!ames!sdcsvax!ucsdhub!hp-sdd!hplabs!hpcea!hpfcdc!boemker
From: boemker@hpfcdc.HP.COM (Tim Boemker)
Newsgroups: comp.lang.c
Subject: Re: Simple question about:  ~
Message-ID: <5080019@hpfcdc.HP.COM>
Date: 26 Jan 88 19:07:25 GMT
References: <1620006@hpcilzb.HP.COM>
Organization: HP Ft. Collins, Co.
Lines: 20

> Could someone please explain why the following statements both give the
> same answer?

	x= -y -1;
vs.
	x = ~y;	

On most computers, integers are stored in two's complement form.  For a
positive integer, that's the normal representation.  For a negative
integer, the two's complement is the one's complement plus one.  (Adding
one may seem curious, but it has an important advantage: there's only one
representation for zero.  In a one'c complement scheme, both a word of 1's
and a word of 0's represent zero.)

Let y be a positive integer.

-y = ~y + 1	by definition of two's complement
-y - 1 = ~y

QED.