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From: seymour@milton.u.washington.edu (Richard Seymour)
Newsgroups: comp.lang.fortran
Subject: Re: Questions about IF
Message-ID: <1991Jun24.213601.11329@milton.u.washington.edu>
Date: 24 Jun 91 21:36:01 GMT
Article-I.D.: milton.1991Jun24.213601.11329
References: <scNB_TG00WBMQ0ykwM@andrew.cmu.edu>
Organization: University of Washington, Seattle
Lines: 21

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
>My questions are about the STANDARD for executing such a command:
>1) Are the expressions evaluated from "left to right"?
>2)...   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?

DEC VAX/VMS Fortran manual babbles on about this at lenght.  if you want
ALL of the subexpressions evaluated, and you want the order (left-right)
which you entered, use the compiler switch /NOOPTimize
otherwise, all bets are off... the compiler reserves the right to cease
 chewing thru them if one proves .false., to re-order them for speed,
to migrate some of them outside for pre-evaluation outside of a DO loop,
and to completely ignore the line if the compiler knows the outcome
(the if(foo.and.boo.and..FALSE.)  case.)
and at times i've seen the /noopt case still shuffle the order (common
subexpression evaluation)
(the   if((a+1.gt.24).and.(a+1.lt.30)) case.. the "a+1" may happen only once.)
--dick