RE: Adding if/then/else statement to GMPL

[Meketon, Marc]

Since GMPL is a subset of AMPL, I would begin by looking at:
https://ampl.com/BOOK/CHAPTERS/16-script.pdf

As far as examples, the classic 'cutting stock' problem is a good one.  The 
AMPL site has two different iterative approaches:
https://ampl.com/BOOK/EXAMPLES/EXAMPLES2/cut.mod

https://ampl.com/BOOK/EXAMPLES/EXAMPLES2/cut.run

and

https://ampl.com/BOOK/EXAMPLES/EXAMPLES2/cut2.mod

https://ampl.com/BOOK/EXAMPLES/EXAMPLES2/cut2.run

and the data file is at:
https://ampl.com/BOOK/EXAMPLES/EXAMPLES2/cut.dat


-----Original Message-----
From: Domingo Alvarez Duarte <mingodad@gmail.com>
Sent: Monday, August 24, 2020 10:34 AM
To: Meketon, Marc <Marc.Meketon@oliverwyman.com>; Andrew Makhorin 
<mao@gnu.org>; help-glpk@gnu.org
Subject: Re: Adding if/then/else statement to GMPL

Hello Meketon !

Could you share your view of how it would be expressed (an ideal model
sample) ?

If you want to talk about it, maybe I'll be interested in implement it !

Can you share a collection of models data to be used as base for the 
test/implementation ?

Cheers !

On 24/8/20 16:00, Meketon, Marc wrote:
> I've always felt that GMPL needed if-then-else, for-loops, 'let' statements 
> and the ability to re-solve to be a true modeling language.  And Andrew has 
> always disagreed.
>
> Many of the models that I create ultimately are 'iterative' where I need to 
> take the results of one model and use it to setup another model.  To me, that 
> is also modeling.  GMPL doesn't have it.
>
> So often, I use GMPL for an initial model - it is a wonderful language, and I 
> find it faster to code than alternatives.  But then when I 'get it right' I 
> have to re-code it in PYOMO or PULP or write directly to an 'lp' file within 
> a Python or C# or other language script.
>
> Having the ability to run, adjust variables, add/take away constraints, 
> re-run would be extremely useful, and make GMPL more of a one-stop modeling 
> language.
>
> -----Original Message-----
> From: Help-glpk
> <help-glpk-bounces+marc.meketon=oliverwyman.com@gnu.org> On Behalf Of
> Andrew Makhorin
> Sent: Sunday, August 23, 2020 2:56 PM
> To: Domingo Alvarez Duarte <mingodad@gmail.com>; help-glpk@gnu.org
> Subject: Re: Adding if/then/else statement to GMPL
>
> On Sun, 2020-08-23 at 15:36 +0200, Domingo Alvarez Duarte wrote:
>> Hello !
>>
>> Also I've added the break/continue statements here
>> https://github.com/mingodad/GLPK/commit/9d70a37b16bd377722eeb3880fcf8
>> 6
>> bb3b812118
>>
>>
>> Again any comment/suggestion is welcome !
>>
>> Cheers !
>>
>>
>>
> Please note that GNU MathProg is a *modeling* language; it is not a 
> general-purpose programming language. If you need to produce a non-trivial 
> solution report (since all such-like statements are allowed only on the 
> post-solving stage), it would be more practical to write the solution to a 
> temporary file and then process it with a separate program.
>
>
>
>
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