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From: steve@taumet.com (Stephen Clamage)
Newsgroups: comp.lang.c
Subject: Re: towards a faster isdigit()
Message-ID: <719@taumet.com>
Date: 9 May 91 00:02:42 GMT
Article-I.D.: taumet.719
References: <1991May8.030515.7004@twinsun.com> <716@taumet.com>
Organization: Taumetric Corporation, San Diego
Lines: 25

I wrote:

|eggert@twinsun.com (Paul Eggert) writes:

|>Why not use the following implementation instead?

|>	#define isdigit(c) ((unsigned)((c)-'0') < 10)

|>This needs just a subtraction followed by a comparison.  It's faster on
|>all the systems I've tried it on, and is strictly conforming ANSI C.

|Maybe you didn't try it on enough systems :-)

|The macro you suggest requires a test and jump, and on some modern RISC
|machines the penalty is very high.

Bruce Evans pointed out to me that since isdigit() is used in tests,
the test-and-jump is already paid for.  I checked my test code to 
see why I got the results I did, and found they were the opposite
of what I reported.  I'm sorry for wasting everyone's time with this.

Paul's macro is indeed faster.
-- 

Steve Clamage, TauMetric Corp, steve@taumet.com