Re: Adding if/then/else statement to GMPL

Hello !

With this commit I'm close for a proof of concept of GMPL with multi 
problem, multi solve, repeat and let statements.

The let needs work for the dependence hell when update a parameter.

The multi solve statements is implemented as calling a user provided 
callaback (see in examples/glpsol.c for a crude example).

https://github.com/mingodad/GLPK/commit/65655c05fc8e315b3e43b98a1f232466614a0b46

It compiles and run examples/cut2.mod on an infinite loop due to 
problems with propagation of updates with 'let' statement.

Ideally it should be compatible with existing models (not yet fully 
implemented/checked).

Any comment/suggestion/help is welcome !

Cheers !

On 24/8/20 17:02, Meketon, Marc wrote:
> 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 <[email protected]>
> Sent: Monday, August 24, 2020 10:34 AM
> To: Meketon, Marc <[email protected]>; Andrew Makhorin <[email protected]>; [email protected]
> 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
>> <[email protected]> On Behalf Of
>> Andrew Makhorin
>> Sent: Sunday, August 23, 2020 2:56 PM
>> To: Domingo Alvarez Duarte <[email protected]>; [email protected]
>> 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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