newbie problem with creating constraints using Python

[Betty Love]

I'm having trouble adding a set of constraints.  I'm getting a "IndexError: list index out of range" error on the last piece of code below.

Any suggestions would be appreciated.

 

lines = range(3)

products = range(3)

workers = range(20)

cost_per_worker = [700,1000,1500] # on production lines 1,2,3

products_per_worker = [[50,90,120],[75,110,130],[90,125,150]] # product i/line j

demand = [600,800,1000] #for products 1,2,3

# Production line setup costs

setup = [2000,3000,4000]

m = Model('Rader 3.3')

# Variables

# y[i] = 1 if line i is used; 0 otherwise

y = {}

for line in lines:

    y[line] = m.addVar(obj=setup[line],vtype=GRB.BINARY,name='y%s' % (line))

 

# w[i,j,k] = 1 if worker i produces product j on line k

w={}

for i in workers:

    for j in products:

        for k in lines:

            w[i,j,k] = m.addVar(obj=cost_per_worker[k],vtype=GRB.BINARY,name='w%d%d%d' % (i,j,k))

m.update()

#Meet demand for product j

for j in products:

    m.addConstr(quicksum(products_per_worker[j,k]*quicksum(w[i,j,k] for i in workers)

          for k in lines) >= demand[j], 'demand_%d' % (j))

[Jakob Sch.]

Hi Betty,

an IndexError is usually a sign that you tried to access an index in a list or dictionary, that is not there... I modified your code so that, at least on my computer with gurobi 5.6, it works without errors... Change the last lines to
for j in products:
    m.addConstr(quicksum([products_per_worker[j][k] * quicksum([w[(i, j, k)] for i in workers]) for k
        in lines]) >= demand[j], 'demand_%d' % (j))
I noted that you are mixing some things up in your code. You should pay attention to the difference of lists and dictionaries... A good starting point for some basic python is http://www.greenteapress.com/thinkpython/ or http://www.diveintopython.net/. You could also have a look at the documentary from python itself: http://docs.python.org/2/tutorial/datastructures.html

Hope that helps...
Best regards,
Jakob

[Betty Love]

Figured it out!  

products_per_worker[j,k]

should have been

products_per_worker[j][k]

[Betty Love]

Thanks, Jakob.  I did figure out my error in accessing the list products_per_worker  as well as making the argument to quicksum a list.  I've been working off the netflow.py example which uses the tuplelist() object and have had trouble wrapping my brain around that.  Now it's getting clearer!  Thank you again!