Originally posted by: kc78

Hi

I was wondering if i fixed all the binary variables to the values returned after i solved it as a MILP, does CPLEX automatically switch it to solving it as a LP?

Thanks!
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Originally posted by: SystemAdmin

No, the problem remains a MIP even all integer variables are fixed. It is even possible to have a MIP that does not have any integer variables.
This means, that after solving this fixed model you still cannot access LP solution features like a dual solution or reduced costs. If you want this, then you need to explicitly change the problem type to LP.

Tobias
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Originally posted by: SystemAdmin

Hi all,

Tobias said:

> you need to explicitly change the problem type to LP.

How can I do it in OPL? How can I specify (in OPL) that I want a MIP model to be solved as LP problem?

Thanks in advance,
Paolo
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Originally posted by: SystemAdmin

Hello again,

I didn't notice OPL has its own forum:

http://www.ibm.com/developerworks/forums/forum.jspa?forumID=2053

so I suppose my topic resulted to be OT. I'm sorry.

By the way, browsing the OPL forum I found how to solve the LP relaxation of a MIP problem, that is by using convertAllIntVars() method of IloOplModel class:

dvar ...
minimize ...
subject to {...}

main {
thisOplModel.generate();
thisOplModel.convertAllIntVars();
cplex.solve();
}

Thanks anyway to everyone,
Paolo
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#DecisionOptimization