Adding constraints Gurobi Python

Question

city=["A","B"] week=[0,1,2,3] S={"A":[5,15,25,35], "B":[80,11,31,30]} model=gp.Model() I = model.addVars(city, week, name="I") model.setObjective(...) # try 1: model.addConstrs(S[c][w] <= I[c][w] for c in city for w in week) # try 2: for c in city: for w in week: model.addConstr(S[c][w] <= I[c][w])

Hello, I am trying to solve a problem and I need to add a constraint for each city and for each week. However, I get a "SyntaxError: invalid syntax" for the for loop when trying to add the constraint. Could you help me I am not sure how to correctly add them?

joni · Accepted Answer · 2022-11-14 15:10:51Z

Note that model.addVars() returns a gurobi tupledict where each key is stored as tuplelist:

>>> print(I) In [64]: I Out[64]: {('A', 0): <gurobi.Var I[A,0]>, ('A', 1): <gurobi.Var I[A,1]>, ('A', 2): <gurobi.Var I[A,2]>, ('A', 3): <gurobi.Var I[A,3]>, ('B', 0): <gurobi.Var I[B,0]>, ('B', 1): <gurobi.Var I[B,1]>, ('B', 2): <gurobi.Var I[B,2]>, ('B', 3): <gurobi.Var I[B,3]>}

Hence, the expression I[c][w] yields a KeyError. Use I[c,w] or I[(c, w)] instead.

That said, your 'constraints' are just lower bounds for the variables; thus, there's no real need for constraints. You can just set the lower bounds for the variables:

for c in city: for w in week: I[c, w].lb = S[c][w]

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