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Internet Message Format | 1996-08-05 | 1.9 KB

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From: [e-mail] (Ray Dunn)
Newsgroups: comp.lang.c
Subject: Re: Floating point calculation order
Date: 25 Jan 1996 17:09:21 GMT
Organization: Ultimate Technographics Inc.
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In referenced article, Lawrence Kirby says...
>Parentheses in C allow you to change the operator/operand grouping
>within expressions. They don't enforce any order of evaluation beyond
>that implied by the grouping. So:
>
> x = p * q / r;
>
>and
>
> x = (p * q) / r;
>
>mean *exactly* the same thing in C since the parentheses don't change
>the grouping.

You're right, of course, but they only mean the same thing because *
and / have the same precedence and associate left to right.

Remember that the original posting was from someone complaining that
the division was done first (presumably on a non-conforming compiler).
In this case parenthesizing the multiplication would indeed change the
order of evaluation and solve his problem.

I do think it's a good idea to parenthesize expressions to make it
clear what you expect to happen.

It's probably helpful to use this opportunity to remind beginners that
although the * is guaranteed to be done before the /, there is no
definition of which order the *operands* will be evaluated, so for
example, in:

  x = f() * g() / h();

the calls of f, g, and h may be made in any order the compiler chooses,
even if terms are parenthesised and/or operators of different
precedence are used.
--
Ray Dunn (opinions are my own) | Phone: [phone]
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