Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!nrl-cmf!cmcl2!phri!marob!djs!samperi
From: samperi@djs.UUCP (Dominick Samperi)
Newsgroups: comp.lang.fortran
Subject: Re: Fortran versus C for numerical analysis
Message-ID: <180@djs.UUCP>
Date: 3 Sep 88 09:28:44 GMT
References: <3535@s.cc.purdue.edu> <3183@lanl.gov>
Reply-To: samperi@djs.UUCP (Dominick Samperi)
Organization: Village Software
Lines: 29

In article <3183@lanl.gov> jlg@lanl.gov (Jim Giles) writes:
|The expression A+B+C may be evaluated as (A+B)+C, A+(B+C), or (A+C)+B.
|Fortran is free to do the calculation in any of those orders.  
|J. Giles
|Los Alamos

I don't have a copy of the official FORTRAN standard handy, but there appears
to be an inconsistency in the specifications. Specifically, most references on
FORTRAN state that the primitive operators associate left to right, except
for **, which associates right to left. This would imply that A+B+C will
be evaluated as (A+B)+C.

On the other hand, I have verified that the FORTRAN definition explicitly 
allows any FORTRAN implementation to rearrange the order of evaluation of 
an expression for optimization in any manner, provided that operator 
precedence rules are obeyed, and parentheses are respected. Does this not 
contradict the left-to-right/right-to-left rules mentioned above?

I also checked into the aliasing example that was mentioned recently in
this group, and the compilers I used could not detect that I was modifying
the same array by two different aliases. Since rules like "functions may not
have side effects that affect the result of expression evaluation" usually
cannot be enforced by the compiler, they might more properly be called
programming guidelines.
-- 
Dominick Samperi
    samperi@acf8.nyu.edu             uunet!hombre!samperi
    cmcl2!acf8!samperi               rutgers!acf8.nyu.edu!samperi
      (^ ell)