I have a list of lists that looks something like this:
arr = [[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]]
I want to create a 2 by 2 array with the first 2 elements at the top and the last two at the bottom, something like:
[1, 2, 3], [4, 5, 6]
[7, 8, 9], [10, 11, 12]
What I tried so far was reshape using:
np.array(arr).reshape(2,2)
However whenever I do that I get a ValueError: total size of new array must be unchanged What exactly am I missing or how can I get this to work?
Try using a list comprehension:
nparr = np.array([arr[i:i + 2] for i in range(0, len(arr), 2)]
print(nparr)
Output:
[[[1 2 3] [4 5 6]]
[[7 8 9] [10 11 12]]]
I think what you want is not exactly possible like this but this could satisfy your needs:
arr2 = [arr[:2], arr[2:]]
np.array(arr2)
This returns something like this:
[[[ 1 2 3]
[ 4 5 6]]
[[ 7 8 9]
[10 11 12]]]
You could go for something like this, which reshapes the numpy array.
import numpy as np
arr = [[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]]
out = np.reshape(sum([], arr), (-1, 3))
print(out)
Output:
[[ 1 2 3]
[ 4 5 6]
[ 7 8 9]
[10 11 12]]
Note that arr has a total of 12 elements. You can not arrange 12 elements in a 2,2 manner using reshape.
This is a possible solution for you
np.array(arr).reshape(2, 6)
Output
array([[ 1, 2, 3, 4, 5, 6],
[ 7, 8, 9, 10, 11, 12]])
reshape(2,2,3)?
There seems to be some confusion as to what you want:
In [93]: alist = [[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]]
Making an array from the list:
In [94]: np.array(alist)
Out[94]:
array([[ 1, 2, 3],
[ 4, 5, 6],
[ 7, 8, 9],
[10, 11, 12]])
In [95]: _.shape
Out[95]: (4, 3)
That's a total of 12 elements. reshape has to match that:
In [96]: np.array(alist).reshape(2,2,3)
Out[96]:
array([[[ 1, 2, 3],
[ 4, 5, 6]],
[[ 7, 8, 9],
[10, 11, 12]]])
In [97]: _.shape
Out[97]: (2, 2, 3)
The accepted answer does the same thing:
In [99]: np.array([alist[i:i + 2] for i in range(0, len(alist), 2)])
Out[99]:
array([[[ 1, 2, 3],
[ 4, 5, 6]],
[[ 7, 8, 9],
[10, 11, 12]]])
In nested list form:
In [100]: [alist[i:i + 2] for i in range(0, len(alist), 2)]
Out[100]: [[[1, 2, 3], [4, 5, 6]], [[7, 8, 9], [10, 11, 12]]]
When defining/displaying 2 and 3d arrays, the [] are more important than the line breaks and spacing.
np.array(arr).shape?