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From: ok@cs.mu.oz.au (Richard O'Keefe)
Newsgroups: comp.lang.c
Subject: Re: ambiguous ?
Message-ID: <2520@munnari.oz.au>
Date: 25 Oct 89 05:20:31 GMT
References: <6637@ficc.uu.net> <14111@lanl.gov> <6658@ficc.uu.net>
Sender: news@cs.mu.oz.au
Lines: 22

In article <6658@ficc.uu.net>, peter@ficc.uu.net (Peter da Silva) writes:
>     DATA I /0/
>     J = I * GETCH(5)
>     J = 0 * GETCH(6)

> Is this legal?
Yes (CF=0.9)
> Is it guaranteed that GETCH(5) will be evaluated?
If I is not modified elsewhere in the subprogram, no it is not.
A Fortran compiler is allowed to exploit identities such as 0*X = 0
> Is it guaranteed that GETCH(6) will be evaluated?
No.

(Giles)
> >       if (getch(5) && getch(6)) {...}

In the Fortran equivalent
	IF (GETCH(5) .AND. GETCH(6)) ...
either operand may be evaluated first, and the other may or may not
be evaluated depending on the outcome and the compiler-writer's choice.
This is what it _means_ to exploit the mathematical properties of "and"
as Giles recommends.