Python 代码因我的优化问题而出错

Question

I am new in using python. I tried to write a code for my optimization research model. Could you help me please? I don't know what's wrong with the code. I am using python 2 by the way. Thank you.

for i in range(len(lift)):

 prob+=lpSum(dec_var[i])<=1

#constraints

col_con=[1,0,0,2,2,3,1,1]

dec_var=np.array(dec_var)

col_data=[]

for j in range(len(brands)):

 col_data.append(list(zip(*dec_var)[j]))

 prob+=lpSum(col_data[j])<=col_con[j]

#problem

prob.writeLP("SO.lp")

#solve the problem

prob.solve()

print("The maximum Total lift obtained is:",value(prob.objective)) # print the output

#print the decision variable output matrix

Matrix=[[0 for X in range(len(lift[0]))] for y in range(len(lift))]

for v in prob.variables():

 Matrix[int(v.name.split("_")[2])][int(v.name.split("_")[3])]=v.varValue

 matrix=np.int_(Matrix)

print ("The decision variable matrix is:")

print(matrix)

the error was:

TypeError Traceback (most recent call last) in 13 for j in range(len(brands)): 14 ---> 15 col_data.append(list(zip(*dec_var)[j])) 16 17 prob+=lpSum(col_data[j])<=col_con[j]

TypeError: 'zip' object is not subscriptable

Answer 1

Your code breaks in this line:

col_data.append(list(zip(*dec_var)[j]))

Lets go through it step by step: dec_var is an array, probably with multiple dimensions. Something like this:

dec_var=np.array([[1,2,3,4],[5,6,7,8]])
dec_var
#array([[1, 2, 3, 4],
#       [5, 6, 7, 8]])

The star operator (*) breaks the array into 'variables'. Something more or less like this: a = [1,2,3,4], b = [5,6,7,8]. ('a' and 'b' don't really exist, just trying to paint a picture).

Next, you apply zip(), which allows you to iterate two iterables together. You would usually use it like this:

for i,j in zip([1,2],[3,4]):
    print(i,j)
#1,3
#2,4

However, zip itself is not subscriptable, that is the error you are getting.

To make it subscriptable, you can apply list on it.

list(zip([1,2],[3,4]))[0]
#(1,3)

In other words.. The solution to your problem most likely is changing the position of the [j] subscript:

From:

col_data.append(list(zip(*dec_var)[j]))

To:

col_data.append(list(zip(*dec_var))[j])