Python integer programming: looking for constraint on set partitioning

Question

I have a set partitioning problem that requires each available period to be used only once.

def solve (slots, reqs, periods):
    # binary variable to id if combination is used
    x = pulp.LpVariable.dicts('slot', slots, lowBound=0, upBound=1, cat=pulp.LpInteger)

    # linear program problem
    sked_model = pulp.LpProblem("NLE Specials Scheduling Model", pulp.LpMaximize)

    # objective function
    sked_model += sum([x[slot] for slot in slots]), "Slot selection" # all combinations considered equal

    # Supply maximum number of slots
    sked_model += sum([x[slot] for slot in slots]) == len(reqs), "Number_of_slots to sked"

ERROR OCCURS HERE

    # A period can only be used once
    for period in periods:
        sked_model += sum([x[slot] for slot in slots if period == slot[0:2]]) <= 1, "Period_used_no_more_than_once" 

    # The problem is solved using PuLP’s choice of Solver
    sked_model.solve()

The error received is "pulp.constants.PulpError: overlapping constraint names:" on the # period can only be used once constraint.

The problem is that you give the same name to different constraints. Try "Period_used_no_more_than_once_{}".format(period) instead of "Period_used_no_more_than_once". Also, see this — Ioannis
– Ioannis, Commented Apr 16, 2018 at 20:56

Rick · Accepted Answer · 2018-04-17 11:29:00Z

0

Solved this.

The error came from the constraint name, "Period_used_no_more_than_once", being used more than once when the constraints were built. By replacing that name with "", the constraint worked as desired.

New constraint:

for period in periods: sked_model += sum([x[slot] for slot in slots if period == slot[0:2]]) <= 1, ""

answered Apr 17, 2018 at 11:29

Rick

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