Path: utzoo!mnetor!uunet!husc6!cmcl2!beta!unm-la!unmvax!nmtsun!john
From: john@nmtsun.nmt.edu (John Shipman)
Newsgroups: comp.lang.fortran
Subject: Re: Modifying iteration variables in DO loops
Message-ID: <1331@nmtsun.nmt.edu>
Date: 3 Feb 88 14:05:30 GMT
References: <1309@nmtsun.nmt.edu> <4026@aw.sei.cmu.edu>
Organization: Zoological Data Processing
Lines: 35
Keywords: X3 standard FORTRAN 77
Summary: Oops

Apologies for my erroneous earlier posting.  I have this problem
trying to stay awake while reading X3 standards.

So, given that you can't modify the iteration variable, what's the
right way to implement a DO loop?  The standard seems to imply that
you use a hidden counter to control the loop, but your benchmarks won't
shine if you have to maintain this counter as well as the visible
iteration variable.

My guess would be that you dispense with the hidden counter and use the
visible iteration variable to control the loop, just making sure that
the number of iterations will be as specified in the standard---right?

Next question: what if you pass the iteration variable to a subroutine
and that subroutine clobbers it?  It would be hard for the compiler
to know that, especially if the subroutine were linked in later.

      program main
        do 83 loop = 1, 2
          call trouble ( loop )
   83   continue
        write ( 6, * )  loop
      end
      subroutine trouble ( arg )
        arg = arg + 1
        return
      end

Does this program conform to the standard?  If not, is there any
reasonable way for the compiler to catch the violation?

-- 
John Shipman/Zoological Data Processing/Socorro, New Mexico
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  ``If you can't take it, get stronger.'' --Falline Danforth