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RE: [Help-glpk] OPB output file anomaly

From: Joey Rios
Subject: RE: [Help-glpk] OPB output file anomaly
Date: Fri, 29 Feb 2008 13:26:09 -0800

Thanks again for the response.

> You could write your instance also in cplex format specifying the
> option '--wcpxlp outfile' along with '--wopb' and then check both
> the files. AFAIK cplex lp format is very similar to opb format.

I did as you suggested and it was quite illustrative.  The formats are indeed very similar. For my empty LHS constraints in OPB:

<= 15
<= 15

The cplex format has:

Departure_Rate(LAS,3): 0 w(AC12_11,DUMMY_DST,151) <= 15
Departure_Rate(LAS,4): 0 w(AC12_11,DUMMY_DST,151) <= 15

So it does look like these constraints are trivially satisfied if I am interpreting the output correctly.  So I guess that would be a good little patch in the lpx_write_pb() function.  Line 129 looks like this:

k=glp_get_mat_row(lp, j, ndx, val);

Immediately following this line I added:

if( k == 0 ) continue;

And the empty LHS inequalities went away.  It may have broken something else for all I know, but maybe Oscar could let us know if adding that line hurts anything else.

I realized something else I had to do to the output OPB file for acceptance by minisat+.  For some reason, minisat+ doesn't like brackets or commas.  This is a problem since my binary variables are 3-dimensional and all have brackets and commas in there respective names.  This is clearly a minisat+ problem though and not a GLPK issue.  The reason I bring it up is that maybe I hurt the file when I remove/replace these symbols with sed.  I don't think I hurt the integrity of the file, but will have to verify that.  The only clue I have that I may have damaged the file is the fact that I am still getting a different solution out of minisat+. 

But at least now I am pretty sure the problem isn't with glpk.  If I discover anything more regarding this issue, I'll be sure to update the thread for posterity.  Thanks for the help Andrew and Oscar.


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