Re: Adding if/then/else statement to GMPL

[Domingo Alvarez Duarte]

Hello !

With this commit 
https://github.com/mingodad/GLPK/commit/2d445283a68c96fe64f0d7e5b26d9cd7e5cf6acf 
it now solve all original problems and also is able to solve 
examples/cut-fix-size.mod that shows what new GMPL extensions I've done 
(still as a prof of concept that need more work) and is usable with caution.

The 'let' update propagation is not done properly, but if we preallocate 
parameter/set dimensions we can work by changing the predicate filters 
({j in PATTERNS : j <= nPAT_top}).

Also to overwrite the type of solver to be used the syntax of solve is 
"solve[.lp/mip/ip] [problem name];".

Would be nice if anyone could provide examples of problems that they 
want to be able to express in GMPL to test/improve/extend the 
implementation.

Again any comment/suggestion/help is welcome !

====


problem Cutting_Opt;
# ----------------------------------------

param nPAT integer >= 0, default 0;
param nPAT_top integer >= 0, default 0;
param roll_width;

set PATTERNS := 1..nPAT;
set WIDTHS;

param orders {WIDTHS} > 0;
param nbr {WIDTHS,PATTERNS} integer >= 0;

let nPAT := 2 * card(WIDTHS);
let nPAT_top := 0;
for {i in WIDTHS} {
   let nPAT_top := nPAT_top + 1;
   let nbr[i,nPAT_top] := floor (roll_width/i);
   let {i2 in WIDTHS: i2 <> i} nbr[i2,nPAT_top] := 0;
}
#display nPAT, nPAT_top;
#display nbr;

check {j in PATTERNS : j <= nPAT_top}: sum {i in WIDTHS} i * nbr[i,j] <= 
roll_width;

var Cut {PATTERNS} integer >= 0;

minimize Number: sum {j in PATTERNS : j <= nPAT_top} Cut[j];

subject to Fill {i in WIDTHS}:
   sum {j in PATTERNS : j <= nPAT_top} nbr[i,j] * Cut[j] >= orders[i];


problem Pattern_Gen;
# ----------------------------------------

param price {WIDTHS} default 0;

var Use {WIDTHS} integer >= 0;

minimize Reduced_Cost:
   1 - sum {i in WIDTHS} price[i] * Use[i];

subject to Width_Limit:
   sum {i in WIDTHS} i * Use[i] <= roll_width;

problem Mix : Cut, Reduced_Cost;

problem Pattern_Gen;

if nPAT > 0 then {
    problem Mix;
    #problem Mix2; #problem creation not allowed here
}

display Cutting_Opt, Pattern_Gen, Mix;

repeat {
#for {1..4} {
   solve.lp Cutting_Opt;
   let {i in WIDTHS} price[i] := Fill[i].dual;
   #display price;

   solve/*.mip*/ Pattern_Gen;
   display Reduced_Cost, Use;
   if Reduced_Cost < -0.00001 and nPAT_top <= nPAT then
   {
      let nPAT_top := nPAT_top + 1;
      let {i in WIDTHS} nbr[i,nPAT_top] := Use[i];
      #display nPAT;
   }
   else break;
}

check {j in PATTERNS : j <= nPAT_top}: sum {i in WIDTHS} i * nbr[i,j] <= 
roll_width;

solve/*.mip*/ Cutting_Opt;
display Cut;
#solve Pattern_Gen;

data;

param roll_width := 110;
set WIDTHS :=
     20
     45
     50
     55
     75;
param orders :=
     [20] 48
     [45] 35
     [50] 24
     [55] 10
     [75] 8;
end;

====

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 <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.
> >
> >
> >
> >
> > ________________________________
> > This e-mail and any attachments may be confidential or legally privileged. If 
> > you received this message in error or are not the intended recipient, you 
> > should destroy the e-mail message and any attachments or copies, and you are 
> > prohibited from retaining, distributing, disclosing or using any information 
> > contained herein. Please inform us of the erroneous delivery by return e-mail. 
> > Thank you for your cooperation.
> ________________________________
> This e-mail and any attachments may be confidential or legally privileged. If 
> you received this message in error or are not the intended recipient, you 
> should destroy the e-mail message and any attachments or copies, and you are 
> prohibited from retaining, distributing, disclosing or using any information 
> contained herein. Please inform us of the erroneous delivery by return e-mail. 
> Thank you for your cooperation.