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From: karl@haddock.ISC.COM (Karl Heuer)
Newsgroups: comp.lang.c
Subject: Re: Expressions compared to zero in *c*
Message-ID: <1125@haddock.ISC.COM>
Date: Tue, 15-Sep-87 12:40:35 EDT
Article-I.D.: haddock.1125
Posted: Tue Sep 15 12:40:35 1987
Date-Received: Thu, 17-Sep-87 04:40:56 EDT
References: <374@mcdsun.UUCP> <766@maccs.UUCP>
Reply-To: karl@haddock.ima.isc.com (Karl Heuer)
Organization: Interactive Systems, Boston
Lines: 12
Summary: Use unsigned subtract, signed compare

In article <766@maccs.UUCP> gordan@maccs.UUCP (Gordan Palameta) writes:
>However, if the comparison is done as  a - b > 0, everything magically
>works thanks to the properties of two's complement arithmetic under
>overflows.  In fact it works regardless of whether a,b are signed long or
>unsigned long (however the result a - b _must_ be considered as a signed long
>value, necessitating a cast if a,b are unsigned).

A slight quibble here: a and b should be cast into an unsigned type before
doing the subtraction.  Unsigned arithmetic is *guaranteed* to be modulo 2**N,
but signed arithmetic may cause a surprise when overflow occurs.

Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint