[SCIP] Conversion of CIP.sol to NL.sol

Sun Mar 22 00:00:02 CET 2020

Hello, Stefan.
My colleague, Sergey Smirnov, had applied your patch to the latest SCIP
release 6.0.2 and we tested some problems to check it.
In a few words - patched SCIPAMPL does not return right solution as
SOL-file.

After a number of tests I can suggest the following:
if NL-file contains initial values of the problem's variables then the
SOL-file returned contains these initial values, rather than optimal
solution;
else - values of all variables in SOL-file are equal to zero.

It seems that it is so even for the test problem you mentioned
[scipoptsuite-6.0.2/scip/interfaces/ampl/check/instances/MINLP/ex1266.nl].
See in attachment:
[ex1266.nl] the NL-file we have;
[ex1266.cip.sol] CIP-solution returned by SCIP from ex1266.cip "created
from"  ex1266.nl (it has initial values, see line #317 and below ;
[ex1266.sol] SOL-file returned by patched SCIPAMPL after "normal"
optimization;
[ex1266.cip.2.sol] SOL-file returned by patched SCIPAMPL as a result of the
"trick" you suggested (it .

When we took a look at your patch a suggestion arose: may be it is
something wrong with the array index of
SCIPgetSols(scip)[SCIPgetNSols(scip)-1] ?
The implementation of the function SCIPgetBestSol
<https://scip.zib.de/doc-6.0.2/html/scip__sol_8c_source.php#l02254> returns
scip->primal->sols[0] (line #2337), i.e. THE FIRST element of "sols" array,
not THE LAST!

Best regards,
Vladimir.

On Mon, Feb 10, 2020 at 3:01 PM Stefan Vigerske <svigerske at gams.com> wrote:

> Hi,
>
> there might be a way to get around step 3 and 4, i.e., let SCIPAMPL
> write out the solution that you just read without going through a solve.
> I applied the patch below and ran, from scip/interfaces/ampl:
>
> echo "write prob ex1266.cip q" | ./bin/scipampl
> check/instances/MINLP/ex1266.nl -i
>   ../../bin/scip -c "read ex1266.cip opt write sol ex1266.cip.sol q"
> echo "read ex1266.cip.sol write amplsol q" | ./bin/scipampl
> check/instances/MINLP/ex1266.nl -i && cat check/instances/MINLP/ex1266.sol
>
> The first line converts an .nl file to .cip.
> The second line solves the .cip and saves the .sol.
> The third line reads the .nl and the .sol and writes the AMPL solution.
>
> If you do that, does this bring you back the right solution into AMPL or
> Pyomo?
>
> Stefan
>
>
> --- a/interfaces/ampl/src/reader_nl.c
> +++ b/interfaces/ampl/src/reader_nl.c
> @@ -1850,6 +1850,13 @@ SCIP_RETCODE SCIPwriteAmplSolReaderNl(
>
>      /* get best primal solution */
>      x = NULL;
> +   if( SCIPgetStatus(scip) == SCIP_STATUS_UNKNOWN && SCIPgetNSols(scip)
>  > 0 )
> +   {
> +      SCIP_CALL( SCIPallocBufferArray(scip, &x, probdata->nvars) );
> +      SCIP_CALL( SCIPgetSolVals(scip,
> SCIPgetSols(scip)[SCIPgetNSols(scip)-1], probdata->nvars,
> probdata->vars, x) );
> +      msg = "lastsol";
> +      solve_result_num = 0;
> +   }
>      if( SCIPgetBestSol(scip) != NULL )
>      {
>         SCIP_CALL( SCIPallocBufferArray(scip, &x, probdata->nvars) );
>
>
> On 2/10/20 12:05 AM, Vladimir V. Voloshinov wrote:
> > Dear SCIP developers,
> > can you recommend some way to convert solution in CIP format to AMPL
> > SOL-file (in the presence of corresponding NL-file in AMPL format)?
> >
> > I found that the following trick helps, but may be there is another way?
> > Suppose we have: problem.nl, appropriate problem.nl.cip and
> problem.cip.sol
> > (returned, e.g. by FiberSCIP, for problem.nl.cip as an input model).
> > 1. Start SCIPAMPL console and load problem.nl as model
> > $ scipampl problem.nl -i
> > 2. Read problem.cip.sol from SCIPAMPL console (outputs presented below)
> > SCIP> read problem.cip.sol
> > primal solution from solution file <problem.cip.sol> was accepted as
> > candidate, will be checked when solving starts
> > 3. Load some limits to stop optimization process just after beginning,
> e.g.
> > SCIP> set limits nodes 2
> > 4. Start optimization and wait a few seconds
> > SCIP> optimize
> > 5. Write AMPL SOL-file
> > SCIP> write amplsol
> > Now we get problem.sol containing the solution in AMPL format.
> > The only drawback is the first string with solver status "node limit
> > reached" in problem.sol, but it may be "manually" replaced with "optimal
> > solution found" to get correct SOL-file with appropriate status.
> >
> > See some additional notes below.
> > We use Pyomo Python package to prepare optimization model and write it to
> > NL-file (AMPL format). Usually we use SCIPAMPL to read NL-file and to get
> > solution as SOL-file (AMPL format), which may be loaded "back" to the
> model
> > by Pyomo tools to analyse  results.
> > But for hard problems we need to use ParaSCIP or FiberSCIP. For that we
> use
> > SCIPAMPL console to convert NL-file to CIP-file,
> > https://scip.zib.de/doc-6.0.2/html/reader__cip_8h.php, because ParaSCIP
> and
> > FiberSCIP does not accept AMPL-formats. So, we get solutions also in
> > CIP-format, but Pyomo does not provide tool to load solutions from
> CIP.sol !
> > Up to now,  for special cases, homebred "readers" have been used, but we
> > need more universal approach.
> >
> > Sincerely yours,
> > Vladimir.
> >
> >
> > _______________________________________________
> > Scip mailing list
> > Scip at zib.de
> > https://listserv.zib.de/mailman/listinfo/scip
> >
>
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