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Same nodes but different solution time (VRP Time Windows)

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3 comments

  • David Torres Sanchez
    Gurobi Staff Gurobi Staff

    Hi Rihot,

    Sounds like there is an error in your formulation.
    You can try writing it to a LP file and make sure that the constraints match the formulation in the book: \(\texttt{model.write("vrptw.lp")}\).

    Cheers, 
    David

    1
  • Rihot Gusron
    First Comment
    First Question

    Hi Mr. Sanchez,

    Thanks for the response. I did double check the model and seems can't find the error. Actually the time windows constraint seems suspicious, as when i comment that out, the solver was found the solution in a matter of second. I just extracted the lp model and hosted on google drive. Do you mind to have a look and maybe give any insight/advice about this?

    The link to .lp : click here

    Best Regard.

    0
  • David Torres Sanchez
    Gurobi Staff Gurobi Staff

    Hi Rihot,

    You are right, as far as I can tell the time window constraints are fine. Looks like these:

    $$u_{ik} +s_i + t_{ij} - u_{jk} \leq M(1-x_{ijk})~~\forall (i,j)$$

    With \(M=1000\).

    Maybe try constraining the time windows a little bit more, e.g. provide a lower window and maybe a different upper window for each node.
    If you are struggling with data creation, you can try using some real instances available and just grab fewer nodes: http://web.cba.neu.edu/~msolomon/c101.htm

    Cheers, 
    David

    0

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