How to make a 2d numpy array from an empty numpy array by adding 1d numpy arrays?

Question

So I'm trying to start an empty numpy array with a = np.array([]), but when i append other numpy arrays (like [1, 2, 3, 4, 5, 6, 7, 8] and [9, 10, 11, 12, 13, 14, 15, 16] to this array, then the result im basically getting is
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16].

But what i want as result is: [[1, 2, 3, 4, 5, 6, 7, 8], [9, 10, 11, 12, 13, 14, 15, 16]]

Empty and append work for lists. Don't try to copy that model with arrays. Lists have one dimension, you want a 2d array. np.array([]) is not an 'empty' array Pay attention to ita dimension. — hpaulj
– hpaulj, Commented Sep 22, 2022 at 23:11

sander · Accepted Answer · 2022-09-22 16:36:24Z

1

IIUC you want to keep adding lists to your np.array. In that case, you can use something like np.vstack to "append" the new lists to the array.

a = np.array([[1, 2, 3],[4, 5, 6]])
np.vstack([a, [7, 8, 9]])

>>> array([[1, 2, 3],
       [4, 5, 6],
       [7, 8, 9]])

answered Sep 22, 2022 at 16:36

sander

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miitchel Over a year ago

this is what i get when i try this on an empty numpy array,

ValueError: all the input array dimensions for the concatenation axis must match exactly, but along dimension 1, the array at index 0 has size 0 and the array at index 1 has size 8

Pierre D · Accepted Answer · 2022-09-22 16:47:53Z

1

You can also use np.c_[], especially if a and b are already 1D arrays (but it also works with lists):

a = [1, 2, 3, 4, 5, 6, 7, 8]
b = [9, 10, 11, 12, 13, 14, 15, 16]

>>> np.c_[a, b]
array([[ 1,  9],
       [ 2, 10],
       [ 3, 11],
       [ 4, 12],
       [ 5, 13],
       [ 6, 14],
       [ 7, 15],
       [ 8, 16]])

It also works "multiple times":

>>> np.c_[np.c_[a, b], a, b]
array([[ 1,  9,  1,  9],
       [ 2, 10,  2, 10],
       [ 3, 11,  3, 11],
       [ 4, 12,  4, 12],
       [ 5, 13,  5, 13],
       [ 6, 14,  6, 14],
       [ 7, 15,  7, 15],
       [ 8, 16,  8, 16]])

answered Sep 22, 2022 at 16:47

Pierre D

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