[Coin-discuss] Solving ILP within B&B framework

[Wiebe Kirsten (Stud. FdEWB)]

Thanks John,
it seems to work without the cut generators :-)
Greetz
Kirsten

________________________________

From: coin-discuss-bounces at list.coin-or.org on behalf of John J Forrest
Sent: Fri 7/14/2006 18:52
To: Discussions about open source software for Operations Research
Subject: Re: [Coin-discuss] Solving ILP within B&B framework



Kirsten, 

It is fairly obvious that you would get the first "access violation reading location".  Without knowing more about your driver I can't say much about the second one. 

Fixing many variables could easily allow CglProbing to find that the problem is infeasible and then this can happen.  I have modified code in branches/devel to trap this before error message. 

Try with no cut generators at all. 

John Forrest 



"Wiebe Kirsten (Stud. FdEWB)" <K.Wiebe at Student.Unimaas.NL> 
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07/14/2006 08:04 AM 
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[Coin-discuss] Solving ILP within B&B framework	

		




Hello everyone,
I'm currently writing my master thesis in operations research about rankings in tournaments using the linear ordering polytope. My idea is to find all Slater solutions for that tournament with the help of a branch and bound, where, in each node of the b&b tree, the ILP of the linear ordering polytope is solved to optimality. For that I start with solving the ILP once to optimality and then "going up" in the tree fixing all variables above the currently processed node to what they were before (in the current solution of the ILP) and the variable that is branched on in that node to its complement, i.e. to (1 - (what is was before)). I implemented a function that detects 3-cycles, i.e. infeasibility if the transitivity is not satisfied; but it seems that there is still something going wrong. For a little instance where there are 4 participants for the tournament and hence 6 variables representing the binary relations the algorithm, that until now is simply supposed to go up in the b&b tree once, is working ok. In a larger instance, with 40 participants and hence 780 binary relations, I get that error message about "Access violation reaing location ..." after 33 iterations and my output says:

Clp6002E 192 bad bound pairs or bad objectives were found - first at R9884
Clp0004I Stopped due to errors - objective value 0
Cgl0000I Cut generators found to be infeasible!

The error message occurs where "model" is defined (model is the object with which I can define the branching method and node comparison and add cuts:

solver1=getModelForILP();
// Use Preprocessing of Coin
solver1.initialSolve();
solver2=&solver1;
solver2=PreProcess.preProcess(solver1,true,5);
model = new CbcModel(*solver2); <-- HERE IS THE ERROR

Does that mean that I cannot use the CoinOR cuts here, or is already something wrong with using the preprocessing?

I think it is about the preprocessing, because when I include

if(solver2 == NULL)
{
   cout << "Preprocessing did not work, node skipped\n";
   return 1;
}

before "model = new CbCModel(*solver2);" and hence return to main to continue without processing the node, the algorithm continues until iteration 95, but then there is again the error message about "access violation reading location..." when the program tries to access the column solution of column 423 (the variable that is branched on there). When debugging and looking until which column there are solutions I get that there are only solutions until column 218. In the beginning I had that problem already at iteration 33 and iteration 91, but that is not the case anymore and I just don't get why...

Greetz
Kirsten
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