In Python, How can I convert a List of numeric array into a simple 1D numpy array?

Question

In my codes, I created a list with n np.arrays with various length.

Sample of how to create the list:

MyArr = [None] * n
for l in range(n):
    MyArr[l] = np.array([1, 2, 3])   # Example 1
    # MyArr[l] = np.array([-1, -10])   # Example 2

What I eventually want to do is make MyArr an 1D numeric array, like the following:

MyArr = [np.array([1, 2, 3]), np.array([1, 2]), np.array([10]), np.array([-1, -2, -3])]

Into:

np.array([1, 2, 3, 1, 2, 10, -1, -2, -3])

Because n is pretty large, I think using a for loop is not a good idea. What do you guys think?

@bicarlsen flatten is a method, so you can apply it on MyArr but on nd-array. So its not whats needed in this case. — David
– David, Commented Feb 22, 2021 at 7:25

Tobias Windisch · Accepted Answer · 2021-02-22 07:15:37Z

1

numpy's concatenate function may help:

np.concatenate([arr_1, arr_2, arr_3])

answered Feb 22, 2021 at 7:15

Tobias Windisch

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Comments

David · Accepted Answer · 2021-02-22 07:23:41Z

0

First, there is an issue with the for loop that build MyArr. Notice that you override the lth element because you set it twice in 1 iteration.

Second, to flatten the list of numpy arrays you could use np.hstack in the following way:

l = [np.array([1,2,3]), np.array([-1,10])]
np.hstack(l)

answered Feb 22, 2021 at 7:23

David

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