Convert Python Numpy array to array of single arrays

Question

I have a feeling that this is very easy but I can't quite figure out how to do it. Say I have a Numpy array

[1,2,3,4]

How do I convert this to

[[1],[2],[3],[4]]

In an easy way?

Thanks

To be precise: you're trying to convert np.array([1, 2, 3, 4]) to np.array([[1], [2], [3], [4]])? Or convert it to a traditional Python list of lists? — David Robinson
– David Robinson, Commented Jul 4, 2013 at 19:28

Akavall · Accepted Answer · 2013-07-04 19:31:43Z

3

You can use np.newaxis:

>>> a = np.array([1,2,3,4] 
array([1, 2, 3, 4])
>>> a[:,np.newaxis]
array([[1],
       [2],
       [3],
       [4]])

answered Jul 4, 2013 at 19:31

Akavall

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Comments

Ashwini Chaudhary · Accepted Answer · 2013-07-04 19:42:16Z

2

You can use numpy.reshape:

>>> import numpy as np
>>> a = np.array([1,2,3,4])
>>> np.reshape(a, (-1, 1))
array([[1],
       [2],
       [3],
       [4]])

If you want normal python list then use list comprehension:

>>> a = np.array([1,2,3,4]) 
>>> [[x] for x in a]
[[1], [2], [3], [4]]

edited Jul 4, 2013 at 19:42

answered Jul 4, 2013 at 19:28

Ashwini Chaudhary

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Pawel Miech · Accepted Answer · 2013-07-04 19:35:17Z

1

The most obvious way that comes to mind is:

>>> new = []
>>> for m in a:
        new.append([m])

but this creates normal Python's list of lists, I'm not sure if this is what you want...

answered Jul 4, 2013 at 19:35

Pawel Miech

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Milan · Accepted Answer · 2013-07-04 20:40:11Z

1

>>> A = [1,2,3,4]

>>> B = [[x] for x in A]

>>> print B
[[1], [2], [3], [4]]

edited Jul 4, 2013 at 20:40

answered Jul 4, 2013 at 19:40

Milan

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