Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!ucla-cs!twinsun!eggert
From: eggert@twinsun.com (Paul Eggert)
Newsgroups: comp.lang.c
Subject: Re: towards a faster isdigit()
Message-ID: <1991May9.002529.12044@twinsun.com>
Date: 9 May 91 00:25:29 GMT
References: <1991May8.030515.7004@twinsun.com> <20325@alice.att.com>
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ark@alice.att.com (Andrew Koenig) writes:

>> 	#define isdigit(c) ((unsigned)((c)-'0') < 10)
>... 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: any host where char is the same as int, and where '0' != 0.
The subtraction might overflow.

The following slightly revised isdigit() works around this problem
because unsigned arithmetic cannot overflow:

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

but frankly I worry that some old buggy compilers will generate the
wrong code for this ``more portable'' version.