Unable to convert argument to an expression
AnsweredHi
I am trying to define my objective function but I am getting "Unable to convert argument to an expression" error. I am using the Sympy library because I need to get Hessian of some matrices based on some variables and then optimize them based on the constraints I have
trJinv=sp.trace(Jinv)
trace_Jinv=sp.expand(trJinv)
from gurobipy import Model, GRB
import gurobipy as gp
# Create a model
m = Model("Minimize expression")
m.addVar(lb=0.0, ub=GRB.INFINITY, name="R1")
m.addVar(lb=0.0, ub=GRB.INFINITY, name="R2")
constraints = []
variableNames = ["R1","R2"] # save all variable names
R1 = sp.Symbol("R1")
R2 = sp.Symbol("R2")
obj=trace_Jinv.subs({(x * x.conjugate().T)[0,0]: R1,
(x * x.conjugate().T)[1,1]: R2})
constraints.append(R1+R2==5)
m.setObjective(obj, gp.GRB.MINIMIZE)
my objective is to minimize (29*𝑅1+17*𝑅2)/(300*𝜋**2*𝑅1**2+627*𝜋**2*𝑅1*𝑅2+12*𝜋**2*𝑅2**2)
I would appreciate it if you could help me
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Official comment
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Gurobi Staff
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Community Moderator
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
Jonasz0 -
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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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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
Jonasz0 -
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