Loading multiple 2d arrays with different shapes into a new array on a third dimension

I’m currently struggling with a probably rather simple question but I can’t get my head around it.

Assuming I have the follow two 2d arrays with different shapes, I can combine them into a new array using:

a = np.zeros((2, 3))
b = np.zeros((4, 5))
c = np.array([a, b])

print(c.shape)
# Output 
# (2,)

for elements in c:
    print(elements.shape)
    # Output:
    # (2, 3)
    # (4, 5)

So far so good! But how would I do this if I have a large list where I’d have to iterate over? Here is a simple example with just 4 different 2d arrays:

This works as expected:

a = np.zeros((2,3))
b = np.zeros((4,5))
c = np.zeros((6,7))
d = np.zeros((8,9))
e = np.array([a, b, c, d])
print(e.shape)

# Output
# (4,)

for elements in e:
    print(elements.shape)

# Output
# (2, 3)
# (4, 5)
# (6, 7)
# (8, 9)

This doesn’t work as expected and my question would be how to do this in an iterative way:

a = np.zeros((2,3))
b = np.zeros((4,5))
c = np.zeros((6,7))
d = np.zeros((8,9))
e = None
for elements in [a, b, c, d]:
    e = np.array([e, elements])
    
print(e.shape)
# Output
# (2,) <--- This should be (4,) as in the upper example, but I don't know how to achieve that :-/
for elements in e:
    print(elements.shape)
    # (2,)
    # (8, 9)

I understand that in each iteration I’m just combining two arrays why it always stays at shape of (2,), but I wonder how this can be done in an elegant way. So basically I want to have a third dimension which states the count or amount of arrays that are stored. E.g. if I iterate of 1000 different 2d arrays I’d expect to have a shape of (1000,)

Hope my question is understandable – if not let me know! Thanks a lot!

Answer

If I understood your issue correctly, you can achieve what you want in a list comprehension. This will yield the exact same solution as your code above that you described as working.

a = np.zeros((2,3))
b = np.zeros((4,5))
c = np.zeros((6,7))
d = np.zeros((8,9))

e = np.array([element for element in [a, b, c, d]])

print(e.shape)

for elements in e:
    print(elements.shape)

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Answer