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Concatenating 1D NumPy arrays

schedule Aug 11, 2023
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We can either concatenate 1D arrays horizontally or vertically.

Concatenating horizontally

To concatenate horizontally, we can use either use the np.concatenate(~) or np.hstack(~) method.

Here's a quick example where we concatenate two 1D Numpy arrays:

x = np.array([1,2])
y = np.array([3,4,5])
z = np.concatenate([x,y]) # np.hstack([x,y]) works as well
z
array([1, 2, 3, 4, 5])

Since the concatenate(~) method takes in an array, you can easily concatenate numerous 1D arrays at once.

WARNING

Do not use + to concatenate Numpy arrays

For standard Python lists, the + operator can be used to concatenate lists: [1,2] + [3,4] = [1,2,3,4]. The + operator for Numpy arrays works entirely differently:

np.array([1,2]) + np.array([3,4])
array([4, 6])

Notice how instead of a concatenation, we got an element-wise addition.

Concatenating vertically

To concatenate 1D Numpy arrays vertically, we can use the np.vstack(~) method:

x = np.array([1,2])
y = np.array([3,4])
z = np.vstack([x,y])
z
array([[1, 2],
[3, 4]])

Notice how we end up with a 2D Numpy array.

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Published by Isshin Inada
Edited by 0 others
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