how to fix 'gurobipy.LinExpr' object is not iterable

Question

I am having some issues with my Linear Program Model in gurobi. I am quite new to programming, and don't really understand what the error is. It comes up as 'gurobipy.LinExpr' object is not iterable with my Objective Function. This is my code currently

from gurobipy import * 

IDtoLVC = [
[40.4,61.2,69.3,60.9,86.8,13.1,70.5,69.7],
[59.5,5.6,45.8,85.6,62.6,46.9,77.8,29.7],
[27.2,62.7,26.9,30.1,30.3,62.6,11.7,45.0]
]

CCDPop = [2180,4887,2174,4244,2444,2156,2433,2096,4148,3636,4935,3160,4153,2058,4614,3862,4588,4264,2559,2770,3964,2333,3681,4786,3256]

CCDtoLVC = [
[48.8,0,0,0,0,19.2,0,0],
[46.3,0,0,0,0,8.3,0,0],
[0,37.2,0,0,0,17.7,0,0],
[0,14.0,0,0,0,0,0,36.1],
[0,18.4,0,0,0,0,0,42.2],
[35.5,0,0,0,0,20.1,0,0],
[33.3,0,0,0,0,6.8,0,0],
[24.9,0,0,0,0,18.7,0,0],
[0,17.4,0,0,0,30.8,0,0],
[0,12.4,0,0,0,0,0,20.1],
[23.5,0,0,31.6,0,0,0,0],
[6.9,0,0,28.3,0,0,0,0],
[17.9,0,27.7,0,0,0,0,0],
[0,24.4,20.3,0,0,0,0,16.4],
[0,28.9,26.1,0,0,0,0,11.5],
[28.3,0,0,12.0,0,0,0,0],
[13.0,0,0,14.4,0,0,24.9,0],
[13.1,0,26.7,31.0,0,0,24.0,0],
[0,0,6.5,0,17.1,0,0,25.4],
[0,0,20.3,0,16.4,0,0,23.3],
[0,0,0,15.1,0,0,35.3,0],
[27.2,0,0,14.1,0,0,7.8,0],
[0,0,25.6,0,25.0,0,14.7,0],
[0,0,26.7,0,12.0,0,0,0],
[0,0,30.2,0,15.5,0,0,0]
]

LVCCost = [194, 105, 184]


# Sets
I = range(3)    # no. of IDs
J = range(8)    # no. of LVCs
K = range(25)   # no. of CCDs
T = range(6)    # length of time

# Model
m = Model('Vaccine Distribution')

X = { (i,j,t): m.addVar() for i in I for j in J for t in T }
Y = { (j,k,t): m.addVar() for j in J for k in K for t in T }


m.setObjective((0.20 * quicksum(quicksum(quicksum(X[i,j,t] * IDtoLVC[i][j] for i in I for j in J for t in T)))) +
               (quicksum(quicksum(quicksum(Y[j,k,t] * CCDtoLVC[k][j] for j in J for k in K for t in T)))) +
               (quicksum(quicksum(quicksum(X[i,j,t] * LVCCost[i] for i in I for j in J for t in T)))),
               GRB.MINIMIZE)

m.optimize()

Answer 1

You don't need three nested calls to quicksum in order to express a triple sum. Instead, you just call quicksum once:

sum1 = quicksum(X[i,j,t] * IDtoLVC[i][j] for i in I for j in J for t in T)
sum2 = quicksum(Y[j,k,t] * CCDtoLVC[k][j] for j in J for k in K for t in T)
sum3 = quicksum(X[i,j,t] * LVCCost[i] for i in I for j in J for t in T)
m.setObjective(0.20 * sum1 + sum2 + sum3, GRB.MINIMIZE)