NotImplementedError

[jack chou]

hello everyone,

When I establish a simple model as follows, the feedback show me the erro (NotImplementedError).I would appreciate it if someone could help me to figure out this problem.Your answer will help me a lot.

from gurobipy import *

m = Model ()

x1 = m.addVar(vtype=GRB.BINARY,name="MT_qt_1")

x2 = m.addVar(vtype=GRB.BINARY,name="MT_qt_2")

m.update()

m.setObjective(1+0.9*max(0,x1-x2),GRB.MINIMIZE)

m.optimize()

for v in m.getVars():

    print('%s %g' % (v.varName, v.x))

print('Obj: %g' % m.objVal) 

Thank you!

[Silke Horn]

Hi,

The reason for the error is that the max function is called ‘max_’. (You are missing an underscore.)

But even with the correct function name, you cannot use it the way you are trying to: The arguments of max_ can only be variables or a constant (not an expression like x1-x2) and you can only use max_ in a constraint of this form: m.addConstr(z == max_(x, y, 3))
Here is the documentation: https://www.gurobi.com/documentation/8.1/refman/py_max_.html

A quick way to fix your code would be to introduce two auxiliary variables, say x3 and x4, and then do the following:
m.addConstr( x3 == x1 - x2 )
m.addConstr( x4 == max_(0, x3) )
m.setObjective( 1 + 0.9 * x4 )

Also note that ‘m.update()’ is not necessary.

- Silke

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[jack chou]

Dear horn, 

Thank you! I have solved my problem under your guidance.

Best regards

在 2019年3月30日星期六 UTC+8下午3:37:49，horn写道：

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