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Unable to convert argument to an expression

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3 comments

  • Jonasz Staszek
    Community Moderator Community Moderator
    Gurobi-versary
    Thought Leader
    First Question

    Hi Zahra,

    first of all, I would suggest you post a minimal, reproducible example of the problem you are struggling with.

    Further, take a look at those two links (here and here). Perhaps they will guide you a little bit in formulating your objective.

    As a general remark: as stated in the documentation, model.setObjective() only accepts LinExpr or QuadExpr (or arguments which can be converted to these). Your obj object:

    obj=trace_Jinv.subs({(x * x.conjugate().T)[0,0]: R1,
        (x * x.conjugate().T)[1,1]: R2})

    does not meet these criteria. You may be better-off rewriting the objective directly, the way you wrote it down at the end of your question.

    Also, you may want to store your variables as R1 and R2:

    R1 = m.addVar(lb=0.0, ub=GRB.INFINITY, name="R1")
    R2 = m.addVar(lb=0.0, ub=GRB.INFINITY, name="R2")

    as otherwise Gurobi will see them as sp.Symbol objects and you'll get an error.

    Hope this helps.

    Best regards
    Jonasz

    0
  • Zahra Hoobakht
    Gurobi-versary
    First Question
    First Comment

    Hi Jonasz

    thanks for your response. I narrow down my objective function to the numerator of trace which is 29*R1+17*R2 in which R1 and R2 are symbols coming from sympy library. I want to solve a minimization problem like this but still receiving the same error:

    Min 29*R1+17*R2

    constraints: R1+R2=5

    R2>=3*R1-3

     

    R1,R2>0

    my code for the objective function is :

    from gurobipy import Model, GRB
    import gurobipy as gp
    import sympy as sp
    R1,R2 =sp.symbols('R1 R2')
    objective=R1+R2
    # Create a model
    m = Model("Minimize expression")

    r1 = m.addVar(lb=0.0, ub=gp.GRB.INFINITY, name="r1")
    r2 = m.addVar(lb=0.0, ub=gp.GRB.INFINITY, name="r2")
    obj=objective.subs({ R1:r1,
        R2: r2})


    m.setObjective(obj, gp.GRB.MINIMIZE)

    ------------------------------------------------------------------------------------------

    GurobiError                               Traceback (most recent call last)
    Cell In[2], line 15
         10 r2 = m.addVar(lb=0.0, ub=gp.GRB.INFINITY, name="r2")
         11 obj=objective.subs({ R1:r1,
         12     R2: r2})
    ---> 15 m.setObjective(obj, gp.GRB.MINIMIZE)

    File src\\gurobipy\\model.pxi:1488, in gurobipy.Model.setObjective()

    File src\\gurobipy\\util.pxi:44, in gurobipy._simpleexpr()

    GurobiError: Unable to convert argument to an expression

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  • Jonasz Staszek
    Community Moderator Community Moderator
    Gurobi-versary
    Thought Leader
    First Question

    Hi Zahra,

    as I wrote earlier, you cannot use Sympy objects as arguments to your objective function.

    Perhaps this:

    m.setObjective(r1 + r2, gp.GRB.MINIMIZE) 

    is what you are looking for?

    Best regards
    Jonasz

    0

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