difference between python gurobi and AMPL gurobi
回答済み
I have a minimization model as follow :
\ LP format - for model browsing. Use MPS format to capture full model detail.
Minimize
Z + 0 y6 + 0 y17 + 0 y25 + 0 y32 + 0 y37
Subject To
cut1: - Z - y1 - 3.5 y2 - 2.5 y3 - 4 y4 - y5 - 2 y7 - 1.5 y8 - 2 y9
- 0.5 y10 - 4.5 y11 - 2 y12 - 3.5 y13 - 4 y14 - 2 y15 - 2.5 y16 - 2 y18
- 5 y19 - 5.5 y20 - 4 y21 - 1.5 y22 - y23 - 6.5 y24 - 2 y26 - 2 y27
- 0.5 y28 - 8 y29 - 2.5 y30 - y31 - 4.5 y33 - 4 y34 - 4 y35 - 0.5 y36
<= 0
Bounds
y1 <= 1
y2 <= 1
y3 <= 1
y4 <= 1
y5 <= 1
y6 <= 1
y7 <= 1
y8 <= 1
y9 <= 1
y10 <= 1
y11 <= 1
y12 <= 1
y13 <= 1
y14 <= 1
y15 <= 1
y16 <= 1
y17 <= 1
y18 <= 1
y19 <= 1
y20 <= 1
y21 <= 1
y22 <= 1
y23 <= 1
y24 <= 1
y25 <= 1
y26 <= 1
y27 <= 1
y28 <= 1
y29 <= 1
y30 <= 1
y31 <= 1
y32 <= 1
y33 <= 1
y34 <= 1
y35 <= 1
y36 <= 1
y37 <= 1
End
the Z is unrestricted variable , while 0<= y <=1
if I optimize this model on python by using gurobipy package :
MP_model.optimize ()
the objective is 0, while the same model on AMPL is giving -91 as objective ??!!
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When reading LP files, Gurobi assumes variables have a lower bound of zero unless otherwise specified. If \( \texttt{Z} \) were unbounded, the LP file would contain the line
Z free
in the \( \texttt{Bounds} \) section. The LP file contains no such line, so Gurobi interprets the lower bound of \( \texttt{Z} \) to be \( 0 \):
>>> import gurobipy as gp
>>> m = gp.read('model.lp')
Using license file /Users/towle/gurobi.lic
Read LP format model from file model.lp
Reading time = 0.00 seconds
: 1 rows, 38 columns, 33 nonzeros
>>> m.getVarByName('Z').LB
0.0If \( \texttt{Z} \) were unbounded, then \( -91 \) is the correct optimal objective value:
>>> m.getVarByName('Z').LB = -gp.GRB.INFINITY
>>> m.optimize()
Gurobi Optimizer version 9.1.2 build v9.1.2rc0 (mac64)
Thread count: 2 physical cores, 4 logical processors, using up to 4 threads
Optimize a model with 1 rows, 38 columns and 33 nonzeros
Model fingerprint: 0xddb989e0
Coefficient statistics:
Matrix range [5e-01, 8e+00]
Objective range [1e+00, 1e+00]
Bounds range [1e+00, 1e+00]
RHS range [0e+00, 0e+00]
Presolve removed 1 rows and 38 columns
Presolve time: 0.02s
Presolve: All rows and columns removed
Iteration Objective Primal Inf. Dual Inf. Time
0 -9.1000000e+01 0.000000e+00 0.000000e+00 0s
Solved in 0 iterations and 0.03 seconds
Optimal objective -9.100000000e+01If you built this model using the gurobipy package, you did not correctly define \( \texttt{Z} \) to be an unbounded variable. By default, variables added to a model using Model.addVar() or Model.addVars() have a lower bound of \( 0 \). To add an unbounded variable, set its lower bound to \( \texttt{-GRB.INFINITY} \):
z = MP_model.addVar(lb=-gp.GRB.INFINITY, name='Z')
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