Using np.multiply(out=array[mask]) does not work

Question

I tested the behavior of in-place operations with numpy. I observed the following:

test = np.arange(0, 10).reshape(2, 5)
mask = test > 5

This works

np.multiply(test, 3, out=test)

# Output: 
[[ 0  3  6  9 12]
 [15 18 21 24 27]]

but this

np.multiply(test[mask], 3, out=test[mask])

print(test)
# Output:
[[0 1 2 3 4]
 [5 6 7 8 9]]

doesn’t. Why?

Asked By: LionCereals

||

Source

Answer 1

Here are some tests for comparison sake:

With few Truthy values in the mask:

import numpy as np

test = np.arange(100000)
mask = test < 100

%timeit np.multiply(test, 3, out=test, where=mask)
# 6.98 µs ± 90.2 ns per loop (mean ± std. dev. of 7 runs, 100,000 loops each)

%timeit test[mask] *= 3
# 21.3 µs ± 159 ns per loop (mean ± std. dev. of 7 runs, 100,000 loops each)

%timeit test[mask] = test[mask] * 3
# 21.9 µs ± 214 ns per loop (mean ± std. dev. of 7 runs, 10,000 loops each)

with a lot of Truthy values:

test = np.arange(100000)
mask = test < 90000

%timeit np.multiply(test, 3, out=test, where=mask)
# 60.9 µs ± 554 ns per loop (mean ± std. dev. of 7 runs, 10,000 loops each)

%timeit test[mask] *= 3
# 100 µs ± 474 ns per loop (mean ± std. dev. of 7 runs, 10,000 loops each)

%timeit test[mask] = test[mask] * 3
# 120 µs ± 1.09 µs per loop (mean ± std. dev. of 7 runs, 10,000 loops each)

with random mask:

test = np.arange(100000)
mask = np.random.choice([True, False], size=100000)

%timeit np.multiply(test, 3, out=test, where=mask)
# 411 µs ± 4.25 µs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)

%timeit test[mask] *= 3
# 712 µs ± 7.93 µs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)

%timeit test[mask] = test[mask] * 3
# 714 µs ± 5.72 µs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)

for completeness, without a mask:

%timeit np.multiply(test, 3, out=test)
# 22.4 µs ± 126 ns per loop (mean ± std. dev. of 7 runs, 10,000 loops each)

so I would recommend np.multiply with out=arr and where=mask.

Answered By: Chrysophylaxs

Using np.multiply(out=array[mask]) does not work

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