[Scip] User constraints checking

[Abdelkader Ouali]

Dear Gerald, 
Thanks, that was really helpful.I didn't pay attention to those flags. Of course, I copied and modified the code from constraint handler used for cuts. 
Best regards---A. Ouali 

Date: Tue, 11 Aug 2015 00:04:21 +0200
From: gamrath at zib.de
To: oualiaek at hotmail.fr
CC: scip at zib.de
Subject: Re: [Scip] User constraints checking


  
    
  
  
    Dear Abdelkader,

    

    the first line of the log shows that SCIP found a feasible solution
    with value -3. After that, SCIP is only looking for solutions with a
    strictly better objective value. The infeasibility reported is for
    this updated problem, which means that no improving solution does
    exist. This is clear when looking at your problem, because all
    (binary) variables have an objective coefficient of -1 and are set
    to 1 in the solution. By the way: your linear constraint allows only
    that solution.

    

    So why is this solution accepted? This is because the flags of the
    created constraint are wrong: If you have a look at the last lines
    of usercons.cpp, you see that you set only the propagate flag to
    TRUE when creating your constraint, but the default is to have the
    first five bools set to TRUE. For example, number 3 and 4 are for
    enforcement and check of solutions. If those are set to FALSE, SCIP
    won't call the callbacks on this constraint (which might be useful
    for cuts, for example).

    

    Best,

    Gerald

    

    Am 10.08.2015 um 23:35 schrieb
      Abdelkader Ouali:

    
    
      
      Dear
          Gerald,
        

          
        In order to clarify my situation, I made a quick example
          containing a user constraint handler which stays always
          infeasible, I set both scip_check and scip_enfolp to return SCIP_INFEASIBLE.
        

          
        When checking scip logs
            during solving process, i found a message claims
            infeasibility :
        feasible solution found by trivial heuristic after 0.0
            seconds, objective value -3.000000e+00
        presolving:
        propagator <pseudoobj> detected infeasibility  
        

            
        but after that, I get :
        
          SCIP Status        : problem is solved [optimal
              solution found]
          Solving Time (sec) : 0.00
          Solving Nodes      : 0
          Primal Bound       : -3.00000000000000e+00 (1
              solutions)
          Dual Bound         : -3.00000000000000e+00
          Gap                : 0.00 %
        
        

        
        SCIP should print [infeasible] rather then [optimal
          solution found]
        Please find the source code and the file containing SCIP
          logs for the example in the attachment.
        

        
        Best regards
        ---
        A. Ouali
        

          

          
            Date: Mon, 10 Aug 2015 18:55:41 +0200

            From: gamrath at zib.de

            To: oualiaek at hotmail.fr; scip at zib.de

            Subject: Re: [Scip] User constraints checking

            

            Dear Abdelkader,

              

              can you send some more information and perhaps the output
              of SCIP? I cannot imagine that SCIP states that the
              problem is infeasible but prints a solution afterwards.

              

              The scip_check method is mainly used to check feasibility
              of solutions generated by heuristics. Checking whether the
              node LP solution is feasible is done by the scip_enfolp
              method, which should normally do something to enforce
              feasibility, e.g., add a constraint or perform branching,
              but can also just return infeasible. If you did not modify
              this method to always return infeasible, it might happen
              that LP solutions are accepted to be feasible.

              

              Best,

              Gerald

              

              On 10.08.2015 14:30, Abdelkader Ouali wrote:

            
            
              
              Dear
                  SCIP users,
                

                
                
                  I am implementing my own constraints using the
                    constraint handler as explained in the TSP example
                    of SCIP,
                  The problem I am facing is the feasibility
                    checking with my own constraints, which they don't
                    include problem variables in their scope, but use
                    them to check the feasibility of other given
                    information, 
                  In order to see if SCIP respond to my
                    constraints, I set the callback function scip_check
                    returns always infeasible. In solving stage, SCIP
                    detects the infeasibility, but after that it prints
                    solutions that ignore my constraints.
                
                

                
                I want to know what is going on inside SCIP ! it
                  detects infeasibility but rather prints infeasible
                  solutions.
                

                
                Thanks in advance for any explanations.
                ---
                A. Ouali
              
              

              
              

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