10000 [Long arithmetics] Element-wise involution fails · Issue #7405 · numpy/numpy · GitHub
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[Long arithmetics] Element-wise involution fails #7405

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ivanychev opened this issue Mar 10, 2016 · 2 comments
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[Long arithmetics] Element-wise involution fails #7405

ivanychev opened this issue Mar 10, 2016 · 2 comments

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@ivanychev
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Hey there. I've noticed a bug related to long arithmetic operations. Attached a screenshot below.

NumPy v.1.10.4
Python 3.5
OS X 10.11.4

2016-03-10 16 47 47

Custom involution and the operation that is performed to numpy.int64 types do well. Now look at the last two cells with elementwise involution. It appears to be that NumPy fails to compute the last value in the resulting array correctly.
@pv
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pv commented Mar 10, 2016

The computation appears to be performed in floating point for some reason np.int64(15.**15.) == 437893890380859392.

@ewmoore
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ewmoore commented Mar 10, 2016

For arrays, numpy calls the C pow function for integral types (ref) for scalars, it calls a power routine that uses integer multiplication (ref). Looks to me as though this is a long latent bug, as the technique used for scalar power (added in 2006) should probably be used for arrays as well.

@ewmoore ewmoore mentioned this issue Apr 1, 2016
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ewmoore added a commit to ewmoore/numpy that referenced this issue Apr 1, 2016
@ewmoore
BUG: don't use pow for integer power ufunc loops.
e7ddb39 
Fixes numpygh-7405.
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@pv @ewmoore @ivanychev
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