python numpy adding rows to 2d empty array

Question

i want to add rows to an empty 2d numpy array through a loop :

yi_list_for_M =np.array([])
M =[]
for x in range(6) :
    #some code 
    yi_m = np.array([y1_m,y2_m])
    yi_list_for_M = np.append(yi_list_for_M,yi_m)

the result is :

[0.         0.         2.7015625  2.5328125  4.63125    4.29375
 5.7890625  5.2828125  6.05452935 5.47381073 6.175      5.5       ]

but i want it to be :

[ [0.         0.]  ,       [2.7015625  2.5328125,] [ 4.63125    4.29375],
 [5.7890625  5.2828125 ], [6.05452935 5.47381073],[ 6.175      5.5 ]      ]

and i dont want to use lists i want to use a 1d numpy array inside a 2d numpy array

I'm having trouble understanding what you mean, but I think yi_list_for_M = np.append([yi_list_for_M,yi_m]) might be what you want? — owo
– owo, Commented Jul 11, 2021 at 17:43
Do not use np.append to dynamically build numpy arrays. Use a list instead. yi_list_for_M = [] and then yi_list_for_M.append(yi_m) — akuiper
– akuiper, Commented Jul 11, 2021 at 17:48
You can convert the yi_list_for_M to numpy array at the end. Use numpy.append will be much slower than using list. — akuiper
– akuiper, Commented Jul 11, 2021 at 17:54
np.append is a poorly named frontend to np.concatenate. It makes a new array. List append operates in-place. You omitted the axis parameter, so you got a 1d array. Read the docs. — hpaulj
– hpaulj, Commented Jul 11, 2021 at 17:57

hp77 · Accepted Answer · 2021-07-11 17:55:40Z

1

I am assuming that you will know what will be the size of each row. If yes then you can do the following:

y = np.empty((0, 3), int) #(0,n), n is the size of your row
for x in range(6):
    y.append(y, np.array([[1, 2]]), axis=0)

answered Jul 11, 2021 at 17:55

hp77

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zswqa · Accepted Answer · 2021-07-11 17:51:54Z

1

yi_list_for_M = np.empty((0,2), int)
for x in range(6):
    y1_m = x**2
    y2_m = x**3
    yi_list_for_M = np.append(yi_list_for_M, np.array([[y1_m, y2_m]]), axis=0)

answered Jul 11, 2021 at 17:51

zswqa

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