Path: utzoo!attcan!uunet!ncrlnk!ncrcae!hubcap!gatech!uflorida!haven!adm!xadmx!art%BUENGC.BU.EDU@cunyvm.cuny.edu
From: art%BUENGC.BU.EDU@cunyvm.cuny.edu (A. R. Thompson)
Newsgroups: comp.lang.pascal
Subject: Re: Control variables in FOR loops
Message-ID: <17854@adm.BRL.MIL>
Date: 16 Dec 88 21:08:05 GMT
Sender: news@adm.BRL.MIL
Lines: 83

In article <1439@csuna.UUCP> abcscnuk@csun.UUCP (Naoto Kimura) writes:
>
>    I have gotten into some pretty heated discussions regarding the
>control variable in a for loop.  I was arguing that it was a bad idea to
>use a global variable, in fact it is an error to do so.  I'm arguing
>from the ISO standard, while the people I was arguing against were going
>by specific implementations that allowed it.

Which demonstrates why you should never allow a particular implementation
to define a language.  Only the standard, or lacking a standard, the
official "report" of the creator of the language.

>
>    Should the use of a relatively global variable be allowed as a
>control variable for a for loop ?  Should the use of a passed parameter
>be allowed as a control variable ?  I say no to both of these.

Absolutely not.  The standard is quite clear on this.  In fact, when the
standard was being written a desire was expressed to force the control
variable to be local to the for statement itself.  Try defining that.

>
>    One person that I got into an argument with said that the behavior
>should be that you can change a control variable, but should not have
>any effect on the loop itself.  I don't think that is reasonable.
>Consider the confusion over what the variable 'i' should contain when
>you use it in several procedures as control variables for a for loop.

How can you change the control variable without having any effect on the
loop itself?  If they want to fiddle with the control variable, I suggest
they use the while and repeat statements.  That's what they're for.

>
>    I'd like to know how you feel about the behavior of the following
>program.  What should the output be (if the compiler even lets this
>code through) ?
>
The following listing shows what the Encore Pascal-2 compiler "thinks" of
this program.

You're right and your friends are wrong.

Pascal-2 3.01  Encore UMAX 14-Dec-88   3:47 PM              Page 1-1
nonsense.p

    1
    2      program nonsense(input,output);
    3          var
    4              i : integer;
    5
    6          procedure barf;
    7          begin
    8              for i := 1 to 10 do
                       ^108
*** 108: FOR-loop control variable must be declared at this level

    9                  write(i:3);
   10              write('   i=',i)
   11          end;
   12
   13          procedure ack;
   14          begin
   15              for i := 1 to 10 do begin
                       ^108
*** 108: FOR-loop control variable must be declared at this level

   16                  write('i=',i:2,' ');
   17                  barf;
   18                  writeln
   19                end
   20          end;
   21
   22      begin
   23          for i := 1 to 10 do
   24              ack
   25      end.
   26


Invocation line:
-list nonsense.p

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