NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part1/1597

Internet Message Format | 1996-08-05 | 2.3 KB

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From: Lawrence Kirby <[e-mail]>
Newsgroups: comp.lang.c
Subject: Re: my atoi function, could someone suggest...
Date: Mon, 15 Jan 96 15:30:01 GMT
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References: <4cf7ap$[e-mail]> <4cq937$[e-mail]> <4d6v51$[e-mail]> <[e-mail]> <4dbjsf$[e-mail]>
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In article <4dbjsf$[e-mail]> [e-mail] "Ray Dunn" writes:

>In referenced article, Lawrence Kirby says...
>>        unsigned value = 0;
>>        unsigned digit = *str - '0';
>>
>>        if (digit < 10) {
>>            value = digit;
>
>This is just a matter of personal style, but using the fact that an
>unsigned number minus a larger number results in a large unsigned
>number (so that "digit < 10" is false when *str is less than '0'), is
>rather obtuse IMO.

It is sometimes interesting to present alternatives and discuss their
merits afterwards. I did stress that the code was simply intended to show
alternative approaches. As for being 'obtuse' I'd say that the only
reason that, say,

 if ((ch = getchar()) != EOF)

isn't obtuse is because it is a commonly encountered idiom. If people are
familiar with the unsigned test above it ceases to be obtuse (and to, say,
some programmers with machine code experience it may seem completely
natural). Whether it is sensible code to thrust onto the great unwashed masses
of C programmers is certainly a debatable point! :-)) I think C programmers
should have a good feel for unsigned integer types and examples like this
help with that. As an aside a portable macro implementation of isdigit()
which may be the most efficient on some systems is:

#define isdigit(ch) ((unsigned)+(ch) - '0' < 10)

In this approach ch is only evaluated once so the macr doesn't suffer
fromt side-effect problems. The unary + allows the compiler to diagnose
illegal arguments such as pointers which would be accepted by the cast
alone.

--
-----------------------------------------
Lawrence Kirby | [e-mail]
Wilts, England | [e-mail]
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