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

• David Robinson: 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?
• user1893354: oh sorry. Numpy array.

Answer 1

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]])

Answer 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]]

Answer 3

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...

Answer 4

>>> A = [1,2,3,4]
>>> B = [[x] for x in A]
>>> print B
[[1], [2], [3], [4]]