Path: utzoo!utgpu!water!watmath!clyde!rutgers!husc6!hao!oddjob!gargoyle!ihnp4!ihlpg!tainter
From: tainter@ihlpg.ATT.COM (Tainter)
Newsgroups: comp.lang.c
Subject: Re: Simple question about:  ~
Message-ID: <4719@ihlpg.ATT.COM>
Date: 29 Jan 88 16:38:56 GMT
References: <1620006@hpcilzb.HP.COM> <2550041@hpisod2.HP.COM>
Organization: AT&T Bell Laboratories - Naperville, Illinois
Lines: 21

In article <2550041@hpisod2.HP.COM>, decot@hpisod2.HP.COM (Dave Decot) writes:
> > Could someone please explain why the following statements both give the
> > same answer?

> > 	short int x, y = 12;
> > 		x = -y -1;
> > 		x = ~y;	
> The one's complement means flip all bits.  (~ x)

> The two's complement means flip all bits and add one.  (- x)

In fact, given the definitions it should be obvious that
		(~x) == (-x - 1)
A derivation for those having long since seen their intro algebra classes :-)
(-x - 1) :=: ((~x+1) - 1)
(-x - 1) :=: (~x + (1 - 1))
(-x - 1) :=: (~x)

> Dave Decot

--j.a.tainter