integer linear program in pulp package generate error

Question

I'm trying to run sample code for transportation problem in Jupyter book, but generate error
TypeError: list indices must be integers or slices, not str. What's problem here? How to solve it? Thanks!

    from pulp import *

# Creates a list of all the supply nodes
Warehouses = ["A","B"]

# Creates a dictionary for the number of units of supply for each supply node
supply = {"A": 1000,
        "B": 4000}

# Creates a list of all demand nodes
Bars = ["1", "2", "3", "4", "5"]

# Creates a dictionary for the number of units of demand for each demand node
demand = {"1": 500,
        "2": 900,
        "3": 1800,
        "4": 200,
        "5": 700}

costs = [   #Bars
        #1 2 3 4 5
         [2,4,5,2,1],#A  Warehouses
        [3,1,3,2,3] #B
         ]



# Creates the prob variable to contain the problem data
prob = LpProblem("Beer Distribution Problem", LpMinimize)

# Creates a list of tuples containing all the possible routes for transport
Routes = [(w,b) for w in Warehouses for b in Bars]

# A dictionary called route_vars is created to contain the referenced variables (the routes)
route_vars = LpVariable.dicts("Route",(Warehouses,Bars),0,None,LpInteger)


# The objective function is added to prob first
prob += lpSum([route_vars[w][b]*costs[w][b] for (w,b) in Routes]), "Sum of Transporting Costs"

# The supply maximum constraints are added to prob for each supply node (warehouse)
for w in Warehouses:
    prob += lpSum([route_vars[w][b] for b in Bars]) <= supply[w], "Sum of Products out of Warehouse %s"%w

# The demand minimum constraints are added to prob for each demand node (bar)
for b in Bars:
    prob += lpSum([route_vars[w][b] for w in Warehouses]) >= demand[b], "Sum of Products into Bars %s"%b

Erwin Kalvelagen · Accepted Answer · 2020-04-13 09:13:41Z

1

This is more of a basic Python question than a PuLP question.

w,b are strings. So in your code, you are evaluating costs['A']['1']. If you type this in, you see the same error message. To be able to use string indices, you need to use a dict instead of a list (array).

Solution: make costs a dict

One way to do this is:

 costs = {'A': {'1':2,'2':4,'3':5,'4':2,'5':1},
          'B': {'1':3,'2':1,'3':3,'4':2,'5':3}}

answered Apr 13, 2020 at 9:13

Erwin Kalvelagen

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1 Comment

J Lin Over a year ago

This is old code which were written in pulp package on 2007. I tried to study optimization, and now it finally works, Thanks a lot!

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