Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!mips!apple!snorkelwacker.mit.edu!ai-lab!life!burley
From: burley@albert.gnu.ai.mit.edu (Craig Burley)
Newsgroups: comp.lang.fortran
Subject: Re: Questions about IF
Message-ID: <BURLEY.91Jun23220051@albert.gnu.ai.mit.edu>
Date: 24 Jun 91 02:00:51 GMT
References: <scNB_TG00WBMQ0ykwM@andrew.cmu.edu>
Sender: news@ai.mit.edu
Organization: Free Software Foundation 545 Tech Square Cambridge, MA 02139
Lines: 43
In-reply-to: br0w+@andrew.cmu.edu's message of 23 Jun 91 16:55:59 GMT

In article <scNB_TG00WBMQ0ykwM@andrew.cmu.edu> br0w+@andrew.cmu.edu (Bruno W. Repetto) writes:

   Suppose there is this IF command:

     IF(<expression-1>.AND.<expression-2>.AND. ... .AND.<expression-n>)statement

   1) Are the expressions evaluated from "left to right"?

The standard doesn't mandate any particular order, even if the expressions
have side effects -- if there are order dependencies in the side effects, then
the program (probably) is not standard-conforming.

   2) Supposing 1) above is true, if expressions 1 to i evaluate to .TRUE., and
      expression i+1 evaluates to .FALSE., are the subsequent expressions
      evaluated (if any remain), or is the execution of the IF command concluded
      right then?

(First of all, 1) above is NOT true, but the answer might explain things
further.)

Again, it's entirely up to the compiler.  A program that depends on expressions
i+2 through the rest being evaluated even if i+1 evaluates .FALSE. is not
standard-conforming -- for that matter, so is a program that depends on any
of the expressions 1 through i being evaluated if i+1 evaluates .FALSE. (i.e.
they evaluate to .TRUE.).  In other words, if the compiler can somehow "divine"
that testing expression i+1 first is a good idea (i.e. it has no side effects
and is fairly simple), it can do it and skip the rest, so in

    IF (LFUNC1(X).AND.LFUNC2(Y).AND..FALSE.) STOP

a standard-conforming compiler may make the entire statement effectively
disappear even though LFUNC1 and LFUNC2 might perform important side effects;
if they do (including modifying their arguments), the program is
(potentially) not standard-conforming.

The upshot of all this is that a great deal of flexibility is extended to
the Fortran compiler writer by the standard, at the "expense" of the
Fortran programmer, as compared to, say, the C language.  This is one of the
many reasons for preferring Fortran over C or vice versa, depending on the
task at hand.
--

James Craig Burley, Software Craftsperson    burley@gnu.ai.mit.edu