Path: utzoo!attcan!uunet!portal!cup.portal.com!PLS
From: PLS@cup.portal.com
Newsgroups: comp.lang.fortran
Subject: Re: Fortran versus C for numerical analysis
Message-ID: <9181@cup.portal.com>
Date: 17 Sep 88 02:01:18 GMT
References: <893@amelia.nas.nasa.gov> <3064@lanl.gov> <820@cernvax.UUCP>
Organization: The Portal System (TM)
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hjm@cernvax.UUCP (Hubert Matthews) writes

>Consider the following cases:
>
>      I = (5 / 4) * 5		(I gets 5)
>
>      I = 5 / (4 * 5)		(I gets 0)
>
>      I = 5 / 4 * 5		(I gets 5 in F77, I gets something in C)

Now just a minute. Surely you don't mean to say that the first two are both
equivalent to the third except for a choice in order of operations? The
second is a completely different formula. The two correct choices are

       I = (5 / 4) * 5           = 5
  
       I = (5 * 5) / 4           = 6

I suspect there are very few compilers that do this kind of reordering. 

  ++PLS
 onsider the following cases:

      I = (5 / 4) * 5		(I gets 5)

      I = 5 / (4 * 5)		(I gets 0)

      I = 5 / 4 * 5		(I gets 5 in F77, I gets something in C)

Personally, I wouldn't use case 3; I would always enforce an evaluation order
by using parentheses.  If this could be ambiguous in C, I wouldn't use it for
numerical work.  The same problem occurs when using mixed-mode arithmetic.
Ev