Path: utzoo!attcan!telly!druid!darcy
From: darcy@druid.uucp (D'Arcy J.M. Cain)
Newsgroups: comp.lang.c
Subject: Re: Order of evalution of expressions.
Message-ID: <1990Sep20.015810.29674@druid.uucp>
Date: 20 Sep 90 01:58:10 GMT
References: <6398@castle.ed.ac.uk>
Organization: D'Arcy Cain Consulting, West Hill, Ontario
Lines: 31

In article <6398@castle.ed.ac.uk> elee24@castle.ed.ac.uk (H Bruce) writes:
>I have lost my FAQ files but I'm certain this is not on it....
>Is the line
>value = *ptr - *ptr++;
>sensible C ?
> ...
>If not what is the fastest way of computing this type of expression ?
>Would the following lines be optimized by a compiler (so that value is
>not loaded twice) ?
>
>value = *ptr;
>value -= *ptr++;
Why not:
    value = 0;
	ptr++;

I suppose you meant:
	value = *ptr;
	value -= *(++ptr);
or
	value = *ptr++;
	value -= *ptr;

In any case, the order of evaluation is undefined (K&R2 pp 53) so you
you need two statements.

-- 
D'Arcy J.M. Cain (darcy@druid)     |
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