Error in the objective functionsyn

回答済み

Bushido Pora

2023年04月10日 11:53

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Im trying to add a quadratic expression to my objective function but fail.

Here's the code

x = model.addMVar((a, b), vtype=GRB.BINARY, name = "x_{i,j}")

qexpr = gp.QuadExpr(0)

for s in range(s):

    for i in(a):

        for j in range(b):

            for k in range(b):

                if j != k:

                    qexpr.add(x[i,j]*x[i,k]*data[k,j])

But I get the following error 

"File src\gurobipy\quadexpr.pxi:176, in gurobipy.QuadExpr.add() GurobiError: Invalid argument to QuadExpr addition"

I tried with a linear expression lexpr.add(x[i,j]) but it led to

"GurobiError: Unsupported type (<class 'gurobipy.mquadexpr'="">) for LinExpr addition argument"

I read this may be due to the fact that x is not a Var but an Mvar which are not handled by expression but I'm not sure thus I'm writing this post.

Hope you can help me

Bushido

関連

• Matrix API
• Syntax Error

0

• 正式なコメント

  

  Simranjit Kaur

  • Gurobi Staff

  2026年11月13日 01:45

  This post is more than two years old. Some information may not be up to date. For current information, please check the Gurobi Documentation or Knowledge Base. If you need more help, please create a new post in the community forum, or try Gurobot, our chatbot interface offering instant, expert-level support.
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  Mario Ruthmair

  • Gurobi Staff

  2023年04月13日 09:52

  Hi Bushido,

  QuadExpr does not support MVars. In particular, if x is an MVar, then x[i,j] is a 0-dimensional MVar, not a Var object. You could fix this in several ways:

  • You keep the MVar but use the item() method to get a Var object, i.e.:

  qexpr.add(x[i,j].item() * x[i,k].item(), data[k,j])

  • You use classical Vars which makes sense if you do not exploit matrix operations on variables as in your code piece, i.e.

  x = model.addVars(a, b, vtype=GRB.BINARY, name = "x")

  Independent from that, note that QuadExpr.add() sets the coefficient in the second parameter.

  Best regards,
  Mario

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  Bushido Pora

  2023年04月16日 18:30

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  Hello, 

  Thank you for your answer.

  Indeed using Vars directly was a solution however I now have a scalar expressed as a vector Matrix vector product in the objective and cant manage to make it work.

  a  = model.addMVar((3,5), vtype=GRB.BINARY)
  b = np.ones((5,5))
  qexpr = gp.QuadExpr(0)
  qexpr.add(a[0,:] @ b @ a[1,:].transpose())

  leads to :

  GurobiError: Invalid argument to QuadExpr addition
  
  

   

  0
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  Mario Ruthmair

  • Gurobi Staff

  2023年04月18日 08:42

  This is similar to before: You are working with MVars but want to store an expression in a QuadExpr. You need to use either an MQuadExpr or work with Vars instead of MVars.

  MQuadExpr are temporary objects, there is no constructor. But you can create an expression by assignment:

  qexpr = a[0, :] @ b @ a[1, :].transpose()

  and then extend it via operators "+=".

  Depending on what you want to express in your objective function, there might be a one-liner based on matrix notation instead of summing up terms.

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  Bushido Pora

  2023年04月20日 11:08

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  Thanks a lot.

  0

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