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werenike opened this issue Nov 24, 2016 · 2 comments
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Allow random.choice from empty array when size=0 #8311

werenike opened this issue Nov 24, 2016 · 2 comments

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@werenike
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werenike commented Nov 24, 2016 •
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This piece of code:

np.random.choice([],size=0)

Generates this error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "mtrand.pyx", line 1397, in mtrand.RandomState.choice (numpy/random/mtrand/mtrand.c:15499)
ValueError: a must be non-empty

This error makes sense when size>0, but when size=0, random.choice could simply return []. This would be useful when the a and size parameters are variable. Right now this situation requires an additional check:

if size > 0:
  result = np.random.choice(a,size)
else:
  result = np.array([])

Would it be a good idea to simply return an empty array when size=0? This is my first issue so I hope this is the right way to address this.

I created a simple patch that implements this: 57ab0728d4cf73c1f4b3af2450fe603bf8670b75

I'm using Python 3.5.2 with numpy 1.11.2.

Edit: My implementation returns an empty slice from the original array. My thinking was that this would assign the correct base to the resulting array, but on second thought this is not what happens with random.choice normally, so I think returning np.array([]) would be better.
@eric-wieser
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You should probably promote this to a PR, and add a test with it

@anntzer
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anntzer commented Dec 20, 2016

You probably want to return an array of the same dtype of the input choices (if it is already an ndarray itself), too.

@werenike werenike mentioned this issue Feb 28, 2017
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@girving girving mentioned this issue Feb 16, 2018
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@mattip mattip mentioned this issue Jun 19, 2018
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@Walleclipse Walleclipse mentioned this issue Sep 17, 2019
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@eric-wieser @anntzer @werenike
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