# Numpy list of 1D Arrays to 2D Array

## Question:

I have a large list files that contain 2D numpy arrays pickled through numpy.save. I am trying to read the first column of each file and create a new 2D array.

I currently read each column using numpy.load with a mmap. The 1D arrays are now in a list.

col_list = []
for f in file_list:
col_list.append(Temp[:,0])

How can I convert this into a 2D array?

You can just call np.array on the list of 1D arrays.

>>> import numpy as np
>>> arrs = [np.array([1,2,3]), np.array([4,5,6]), np.array([7,8,9])]
>>> arrs
[array([1, 2, 3]), array([4, 5, 6]), array([7, 8, 9])]
>>> arr2d = np.array(arrs)
>>> arr2d.shape
(3, 3)
>>> arr2d
array([[1, 2, 3],
[4, 5, 6],
[7, 8, 9]])

The array may be recreated:

a = np.array(a.tolist())

You can use

numpy.stack(arrays, axis=0)

if you have an array of arrays. You can specify the axis in case you want to stack columns and not rows.

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