Can't change the code to list comprehension

Question

My question is kind of simple yet I don't seem to find the answer.

I have the following line in my code that works fine:

Uniquesample = np.matrix([[   1.   ,    0.   ],
                   [   2.   , -106.965],
                   [   3.   ,  -83.835],
                   [   4.   ,   12.5  ],
                   [   5.   , -141.6  ],
                   [   6.   ,  -17.25 ],
                   [   7.   ,  -94.785],
                   [   8.   ,  -26.785],
                   [   9.   , -125.835],
                   [  10.   ,    6.865],
                   [  11.   ,   16.23 ],
                   [  12.   ,   61.45 ],
                   [  13.   ,   42.625],
                   [  14.   , -163.655],
                   [  15.   , -116.3  ],
                   [  16.   ,   15.82 ],
                   [  17.   , -166.055],
                   [  18.   ,   90.025],
                   [  19.   ,   14.215],
                   [  20.   ,   82.465]])
L, W = Uniquesample.shape
ModelNumber = 8
Members = np.zeros((L,ModelNumber*W))

seq=[4, 7, 9, 2, 15, 16, 19]
i=0
Count = [7]
for j in seq:
    Members[:Count[i]:,W*i:W*(i+1)] = Uniquesample[j]

Ps: Uniquesample is a 800 x 2 matrix, so i only put the first 20 rows here for simplicity. I tried to change the for loop to another format using "list comprehension":

Members[:Count[i]:,W*i:W*(i+1)] = [Uniquesample[j] for j in seq]

But it does not work. I get the following error: "ValueError: setting an array element with a sequence. The requested array would exceed the maximum number of dimension of 2."

The thing is. Did I "translate" the code correctly to the list comprehension format?

Thank you very much

user_12370687 · Answer

I used seq=[4, 7, 9, 2, 15, 16, 19] in below example. The initial Members array looks like this:

In the for-loop, Members[:Count[i]:,W*i:W*(i+1)] = Uniquesample[j] will replace those 7x2 elements (in red) with the final value in for-loop.

However the list comprehension Members[:Count[i]:,W*i:W*(i+1)] = [Uniquesample[j] for j in seq] will assign each element differently.

FYI, I'm using numpy version '1.18.5', so it may not run properly on www.onlinegdb.com because of old numpy version.

user_15453142 · Answer

The point is, in first code you are assigning one item of Uniquesample[j] each time in Members[:Count[i]:,Wi:W(i+1)]. First Uniquesample[0] to Members[:Count[i]:,Wi:W(i+1)], then Uniquesample[1] to same Members[:Count[i]:,Wi:W(i+1)] and so on.

In your second code you are trying to put a list of Uniquesample items into a location, which can only hold one item. That's why the error.

Can't change the code to list comprehension

2 Answers