[Numpy-discussion] numpy vs algebra Was: Integers to negative integer powers...

[Yaroslav Halchenko]

On Tue, 03 Jan 2017, Stephan Hoyer wrote:
> >> testing on stable debian box with elderly numpy, where it does behave
> >> sensibly:

> >> $> python -c "import numpy; print('numpy version: ', numpy.__version__);
> >> a=2; b=-2;  print(pow(a,b)); print(pow(numpy.array(a), b))"
> >> ('numpy version: ', '1.8.2')
> >> 0.25
> >> 0

> >> what am I missing?

> 2 ** -2 should be 0.25.

> On old versions of NumPy, you see the the incorrect answer 0. We are now
> preferring to give an error rather than the wrong answer.

it is correct up to casting/truncating to an int for the desire to
maintain the int data type -- the same as

>>> int(0.25)
0
>>> 1/4
0

or even

>>> np.arange(5)/4
array([0, 0, 0, 0, 1])

so it is IMHO more of a documented feature and I don't see why pow needs
to get all so special.  Sure thing, in the bring future, unless
in-place operation is demanded I would have voted for consistent float
output.

-- 
Yaroslav O. Halchenko
Center for Open Neuroscience     http://centerforopenneuroscience.org
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