Broadcasting, precisely in Python

Shapes align from the right; a dimension of one stretches.

import numpy as np

rows = np.arange(3).reshape(3, 1)
cols = np.arange(4).reshape(1, 4)

print((rows + cols).shape)
print(rows + cols)

values = np.arange(3)
print((values[:, np.newaxis] * values).shape)
print(np.broadcast_shapes((3, 1), (1, 4), (3, 4)))

How it works

1. Trailing dimensions must match or be 1.
2. newaxis inserts the 1 you need to make that true.
3. broadcast_shapes answers the question without allocating.

Keywords and builtins used here

• as
• print

The run, in numbers

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Step 4 of 4 in Types & shapes, step 12 of 22 in NumPy.

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