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Re: The extended Eaker-Pelc CASE as general control structure

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"HAA" <someone@microsoft.com> writes:
>Anton Ertl wrote:
>> ...
>> On the benefits: Consider a loop with three exit conditions, with a
>> different action necessary on each exit, and afterwards a common
>> action is necessary.  With the extended CASE you can write this as
>>
>> case
>>   condition1 ?of exit-action1 endof
>>   condition2 ?of exit-action2 endof
>>   condition3 ?of exit-action3 endof
>>   ...
>> next-case
>> common-action
>>
>> If you want to do this with EXIT or a multi-WHILE loop, things get
>> cumbersome.  Of course, Forthers often try to rationalize such
>> problems as encouraging factoring or somesuch, but if it really is
>> better factoring, do you really need to restrict your tools to achieve
>> this; and the workarounds for such problems are not always better
>> factoring.
>
>But such examples are the exception - not the rule.  It's cheaper (and
>IMO cleaner) to factor out a large multi-test loop than to create tools
>you'll rarely use to put it in-line.

Actually multi-exit loops are not that rare; every search loop has
multiple exits: one for the search and the other for reaching the end
of the search space.  If we use CASE for every multi-exit loop, it
will not be rare.  And the benefit IMO is that it is more readable
than BEGIN ... WHILE ... WHILE ... REPEAT ... ELSE ... THEN, and also
more readable than unnatural factorings based on using EXIT.

Sure, a common Forth approach is to provide several special, cheaper
solutions instead of an expensive general one, and let the user select
among these special solutions depending on the problem, and try to
bend the problem towards these special solutions if necessary.

And once you have that mindset, you tend to see all general solutions
as expensive, and bending the problem as a virtue.  But in the present
case the general solution is cheap, and using it has the potential
benefit of more readability from always using the same approach for
multi-condition loops.

>WHILE exists and is an everyday word.  This fact alone makes it worth
>learning how to use it to your advantage.
>
>: gcd ( n1 n2 -- n )
>    begin begin
>        2dup > while  tuck - again  then
>        2dup < while  over - again  then
>    drop ;

Cute.  AGAIN THEN is REPEAT, and with more conventional indentation we
get:

: gcd ( n1 n2 -- n )
  begin
    begin
      2dup > while
         tuck -
    repeat
    2dup < while
      over - 
  repeat ;

>: collatz ( u -- )
>    \ print the 3n+1 sequence starting at u until we reach 1
>    begin begin
>        dup .
>        1 over = if  drop exit  then
>        dup 1 and while  3 * 1+ again  then
>        2/
>    again ;

: collatz ( u -- )
  begin
    begin
      dup .
      dup 1 = if
        drop exit then
      dup 1 and while
        3 * 1+
    repeat
    2/
  again ;

But here we already use EXIT (making common code afterwards
cumbersome), because we cannot use WHILE in that spot to jump behind
the AGAIN.

- anton
-- 
M. Anton Ertl  http://www.complang.tuwien.ac.at/anton/home.html
comp.lang.forth FAQs: http://www.complang.tuwien.ac.at/forth/faq/toc.html
     New standard: http://www.forth200x.org/forth200x.html
   EuroForth 2015: http://www.rigwit.co.uk/EuroForth2015/

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The extended Eaker-Pelc CASE as general control structure anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-02-06 18:15 +0000
  Re: The extended Eaker-Pelc CASE as general control structure JennyB <jennybrien@googlemail.com> - 2016-02-07 03:57 -0800
    Re: The extended Eaker-Pelc CASE as general control structure anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-02-08 12:59 +0000
      Re: The extended Eaker-Pelc CASE as general control structure JennyB <jennybrien@googlemail.com> - 2016-02-08 09:48 -0800
        Re: The extended Eaker-Pelc CASE as general control structure anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-02-08 18:10 +0000
          Re: The extended Eaker-Pelc CASE as general control structure JennyB <jennybrien@googlemail.com> - 2016-02-09 14:02 -0800
            Short-circuit evaluation (was: The extended Eaker-Pelc CASE ...) anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-02-10 16:10 +0000
  Re: The extended Eaker-Pelc CASE as general control structure "WJ" <w_a_x_man@yahoo.com> - 2016-02-07 14:32 +0000
  Re: The extended Eaker-Pelc CASE as general control structure "WJ" <w_a_x_man@yahoo.com> - 2016-02-07 14:53 +0000
    Re: The extended Eaker-Pelc CASE as general control structure Albert van der Horst <albert@spenarnc.xs4all.nl> - 2016-02-07 15:56 +0000
      Re: The extended Eaker-Pelc CASE as general control structure stephen@mpeforth.com (Stephen Pelc) - 2016-02-07 18:11 +0000
        Re: The extended Eaker-Pelc CASE as general control structure "HAA" <someone@microsoft.com> - 2016-02-08 12:04 +1100
          Re: The extended Eaker-Pelc CASE as general control structure stephen@mpeforth.com (Stephen Pelc) - 2016-02-08 10:29 +0000
            Re: The extended Eaker-Pelc CASE as general control structure anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-02-08 11:00 +0000
              Re: The extended Eaker-Pelc CASE as general control structure stephen@mpeforth.com (Stephen Pelc) - 2016-02-08 12:28 +0000
                Re: The extended Eaker-Pelc CASE as general control structure anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-02-08 15:26 +0000
                Re: The extended Eaker-Pelc CASE as general control structure stephen@mpeforth.com (Stephen Pelc) - 2016-02-08 16:16 +0000
                Re: The extended Eaker-Pelc CASE as general control structure anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-02-08 16:37 +0000
                Re: The extended Eaker-Pelc CASE as general control structure anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-02-08 16:21 +0000
            Re: The extended Eaker-Pelc CASE as general control structure "HAA" <someone@microsoft.com> - 2016-02-11 21:12 +1100
              Re: The extended Eaker-Pelc CASE as general control structure stephen@mpeforth.com (Stephen Pelc) - 2016-02-11 10:46 +0000
      Re: The extended Eaker-Pelc CASE as general control structure JennyB <jennybrien@googlemail.com> - 2016-02-07 12:33 -0800
        Re: The extended Eaker-Pelc CASE as general control structure stephen@mpeforth.com (Stephen Pelc) - 2016-02-08 10:38 +0000
  Re: The extended Eaker-Pelc CASE as general control structure "HAA" <someone@microsoft.com> - 2016-02-08 12:07 +1100
    Re: The extended Eaker-Pelc CASE as general control structure anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-02-08 11:11 +0000
      Re: The extended Eaker-Pelc CASE as general control structure Albert van der Horst <albert@spenarnc.xs4all.nl> - 2016-02-08 14:49 +0000
      Re: The extended Eaker-Pelc CASE as general control structure "HAA" <someone@microsoft.com> - 2016-02-09 20:31 +1100
        Re: The extended Eaker-Pelc CASE as general control structure hughaguilar96@gmail.com - 2016-02-09 12:38 -0800
      Re: The extended Eaker-Pelc CASE as general control structure "WJ" <w_a_x_man@yahoo.com> - 2016-02-11 14:23 +0000
        Re: The extended Eaker-Pelc CASE as general control structure "WJ" <w_a_x_man@yahoo.com> - 2016-02-11 19:45 +0000

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