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From: schoebel@bs3.informatik.uni-stuttgart.de (Thomas Schoebel)
Newsgroups: comp.lang.modula2
Subject: Re: Need Help With 'BY'
Message-ID: <6892@ifi.informatik.uni-stuttgart.de>
Date: 14 Jan 91 11:48:18 GMT
References: <91003.102704BJ020000@NDSUVM1.BITNET> <josef.663236830@ugum01>
Sender: news@ifistg.uucp
Organization: Informatik, Uni Stuttgart, W. Germany
Lines: 49

In article <josef.663236830@ugum01> josef@nixpbe.nixdorf.de (josef Moellers) writes:
>
>	replace
>
>  FOR Loop := Begin TO End BY By DO
>
>	by
>
>  Loop := Begin;
>  WHILE Loop <= End DO
>     <statements>
>     INC(Loop, By);
>  END;

Danger!

This will not work! If By is negative, the loop is intended to
count backward and normally Begin > End. The above code
will never execute the loop in this case!

So the WHILE condition should read Loop>=End if By is negative,
and Loop<=End otherwise. There are once again two possibilities:

1) Use two different WHILE loops with different conditions. But
   then you have to repeat the <statements> inside the WHILE body twice.
   Also, there is no advantage against using the FOR version.

2) Replace the While condition by Loop <> End. To get correct results,
   the End variable has to be modified one step before the WHILE loop
   begins. Also, the special case of never executing the FOR loop
   has to be treated separately. This code is:

  IF ((By>0) AND (Begin<=End)) OR ((By<0) AND (Begin>=End)) THEN
    INC (End, By);
    Loop:=Begin;
    WHILE Loop <> End DO
      <statements>
      INC (Loop, By);
    END;
  END;

This version avoids to copy the <statements> twice and will give
relative good performance.

Thomas

Thomas Sch"obel, Institut f"ur Informatik, Universit"at Stuttgart
Internet: schoebel@ifi.informatik.uni-stuttgart.de
Phone: +49 711 121-1409