Combine 2D arrays with different shape into a 3D array numpy

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I'm trying to combine 2 2D arrays with different size into a 3D array like this:

a1 = np.array([[0,0,0,0,0,0],[1,1,1,1,1,1]])
print(a1.shape) #(2,6)

a2 = np.array([[0,0,0,0],[1,1,1,1]])
print(a2.shape) #(2,4)

combined = np.stack((a1,a2)) #ValueError: all input arrays must have the same shape

I'm trying to get the following:

>>> [[[0,0,0,0,0,0],[1,1,1,1,1,1]],[[0,0,0,0],[1,1,1,1]]]

Could someone help me?

numpy arrays python
2 Answers

You cannot have a non-rectangular shape arrays in numpy. Now you have some options depending on what you are trying to achieve:

  1. Use lists:

    combined = [a1.tolist(), a2.tolist()]
#[[[[0, 0, 0, 0, 0, 0], [1, 1, 1, 1, 1, 1]]], [[[0, 0, 0, 0], [1, 1, 1, 1]]]]

  2. Use a list of arrays:

    combined = [a1, a2]
#[array([[[0, 0, 0, 0, 0, 0],[1, 1, 1, 1, 1, 1]]]), array([[[0, 0, 0, 0],[1, 1, 1, 1]]])]

  3. Use an array of lists:

    combined = np.array([a1.tolist(), a2.tolist()])
#[[[list([0, 0, 0, 0, 0, 0]) list([1, 1, 1, 1, 1, 1])]], [[list([0, 0, 0, 0]) list([1, 1, 1, 1])]]]

I would suggest using lists, since there is not much benefit of using numpy when elements are objects like lists.

user_49759810

As the numpy docs tell, the arrays must be of the same shape. Try this:-

a1 = np.array([[0,0,0,0,0,0],[1,1,1,1,1,1]])
print(a1.shape) #(2,6)

# notice extra 2's and 3's to make it of same shape as a1
a2 = np.array([[0,0,0,0,2,2],[1,1,1,1,3,3]]) 
print(a2.shape) #(2,4)

combined = np.stack((a1,a2))

To get this :-

[[[0,0,0,0,0,0],[1,1,1,1,1,1]],[[0,0,0,0],[1,1,1,1]]]

you can try python lists. And if you want to stick to numpy arrays try padding the a2 array with 0 or something, otherwise shape mismatch of a1 and a2 won't stack them along the third dimension.

Which final array should it make:- 2x4x2 (as per a2 shape) or 2x6x2 (as per a1 shape)? Since both have different shapes.

user_111903890
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