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From: eggert@twinsun.com (Paul Eggert)
Newsgroups: comp.lang.c
Subject: towards a faster isdigit()
Message-ID: <1991May8.030515.7004@twinsun.com>
Date: 8 May 91 03:05:15 GMT
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Organization: Twin Sun, Inc
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The traditional implementation of isdigit() in <ctype.h> is typically
something like this:

	#define isdigit(c) ((_ctype_+1)[c] & 4)

which requires indexing through a global array followed by a masking
operation.  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.