a wrong result

[jack chou]

hi everyone 

When I run this code in python :

from gurobipy import *

m = Model ("operation")

x3 = m.addVar(vtype=GRB.CONTINUOUS,name="MT_qt_3")

x4 = m.addVar(vtype=GRB.CONTINUOUS,name="MT_qt_4")

m.setObjective( x3+x4 ,GRB.MINIMIZE) 

m.addConstr( 2 <= x3 <= 4 ) 

m.addConstr( 2 <= x4 <= 4 ) 

m.optimize()

for v in m.getVars():

    print('%s %g' % (v.varName, v.x))

print('Obj: %g' % m.objVal)

The feedback is "0", but obviously the answer is wrong.The answer should be "4". I wonder whether there is something wrong in my code.I would appreciate if someone could help me to solve this problem.

Thank you!

[Robert Luce]

Hello,

the problem is that the synax

m.addConstr( 2 <= x3 <= 4 )

doesn't do what you want to do. Such chained expressions are not supported by gurobipy, and in this case the 2<=x3 part will be silently discarded (hence 0 is a feasible solution).  Instead you chould do

m.addConstr( 2 <= x3 )
m.addConstr( x3 <= 4 )

or simply pass these bounds as data for the keyword arguments 'lb'/'ub' to the addVar() function, see

http://www.gurobi.com/documentation/8.1/refman/py_model_addvar.html

Best,

Robert

[jack chou]

Dear Robert 

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

Best regards

Jack

在 2019年3月30日星期六 UTC+8下午11:29:13，jack chou写道：