Stack columns in numpy 2-dim array to get 1-dim array

Question

How to stack all columns in a 2-dimensional Numpy array into a 1-dimensional array.

I.e. I have:

x = np.array([[1, 3, 5],[2, 4, 6]])

And I want to get:

np.array([1, 2, 3, 4, 5, 6])

Is there a way to achieve this without a loop or list comprehension?

A simpler way to do your concatenate: np.concatenate([x[:,i] for i in range(3)]) — hpaulj
– hpaulj, Commented Sep 3, 2018 at 21:02
thanks - updated (both list comprehension and float dtype) - though obviously key element of the question is unchanged and the accepted answer is superior in my view — PvK
– PvK, Commented Jan 7, 2019 at 14:56

jpp · Accepted Answer · 2018-09-03 17:36:47Z

4

You can use ravel:

x = np.array([[1, 3, 5],[2, 4, 6]])

res = x.ravel('F')  # or x.T.ravel()

# array([1, 2, 3, 4, 5, 6])

answered Sep 3, 2018 at 17:18

jpp

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Comments

BENY · Accepted Answer · 2018-09-03 17:27:18Z

4

Using flatten with 'F'

x.flatten('F')
Out[114]: array([1, 2, 3, 4, 5, 6])

answered Sep 3, 2018 at 17:27

BENY

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Nice! Never knew about 'F'. Have stolen it, it works just as well with ravel.

@jpp yep :-) it is :-)