numpy: Broadcasting

import numpy as np

a = np.arange(9).reshape(3,3)
print(a.shape)
print(a)

b = np.arange(4, 4+9).reshape(3, 3)
print(b.shape)
print(b)

a+b

So this is easy and one-to-one.

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What if the shapes do not match?

a = np.arange(9).reshape(3, 3)
print(a.shape)
print(a)

b = np.arange(3)
print(b.shape)
print(b)

What will this do?

a+b

It has broadcast along the last axis!

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Can we broadcast along the first axis?

a

c = b.reshape(3, 1)
c

print(a.shape)
print(c.shape)

a+c

Rules:

• Shapes are matched axis-by-axis from last to first.
• A length-1 axis can be broadcast if necessary.

Iclicker question

#clear Mark the correct output for print(B + np.dot(A,c)) # A) [[3 5] # [6 8]] # B) [[ 3 11] # [ 3 8]] # C) [[11 10] # [12 9]] # D) [[11 13] # [9 9]]