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From: ark@alice.att.com (Andrew Koenig)
Newsgroups: comp.lang.c
Subject: Re: towards a faster isdigit()
Message-ID: <20325@alice.att.com>
Date: 8 May 91 16:18:49 GMT
References: <1991May8.030515.7004@twinsun.com>
Reply-To: ark@alice.UUCP ()
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 18

In article <1991May8.030515.7004@twinsun.com> 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.

This is surely an implementation issue.

However, it does suggest an interesting puzzle: can you find a
character set that meets the ANSI C constraints (in particular,
the one that says that all digits are contiguous in the character
set) for which this macro will not work?  (Yes, I think there is one)
-- 
				--Andrew Koenig
				  ark@europa.att.com