Concatenate two arrays of different dimensions numpy

Question

I have two numpy arrays of the form np.array([1,2,3])) I want to concatenate them so I get:

[[1,2,3],[4,5,6]]

and it also has to work when there is a numpy array of the form [[1,2,3],[4,5,6]] and another one [7,8,9] and I want to get:

[[1,2,3],[4,5,6],[7,8,9]]

I tried np.concatenate but I couldn't get it to work

What error did you get? Describe your array dimensions, input and desired. — hpaulj
– hpaulj, Commented Jan 28, 2021 at 20:31
I kept getting [1,2,3,4,5,6] instead. The array dimensions change, the only constant is the length of the inside arrays (in the examples 3) — jm96
– jm96, Commented Jan 28, 2021 at 20:37

tom10 · Accepted Answer · 2021-01-28 21:35:31Z

2

You can use vstack for this:

import numpy as np

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

d = np.vstack((a,b))

e = np.vstack((d, c))

print(d)
print(e)

Gives:

[[1 2 3]
 [4 5 6]]    

[[1 2 3]
 [4 5 6]
 [7 8 9]]

edited Jan 28, 2021 at 21:35

answered Jan 28, 2021 at 20:37

tom10

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2 Comments

hpaulj Over a year ago

Though to use concatenate the 1d inputs have to first have a new leading axis added.

tom10 Over a year ago

@hpaulj: Thanks for the comment, and yes, it wasn't setting the axis (as I mistakenly said) but adding a leading axis. I just removed the concatenate comment since it didn't help the answer and doing concatenate seems a distraction in this case.

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