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Date: Tue, 8 Jul 2014 12:49:08 -0500
Subject: Re: NaN comparisons - Call For Anecdotes
From: Skip Montanaro <skip@pobox.com>
To: Marko Rauhamaa <marko@pacujo.net>
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>>> import numpy
>>> a1 = numpy.ones(5)
>>> a1
array([ 1.,  1.,  1.,  1.,  1.])
>>> a0 = numpy.zeros(5)
>>> a0
array([ 0.,  0.,  0.,  0.,  0.])
>>> a1 / a0
__main__:1: RuntimeWarning: divide by zero encountered in true_divide
array([ inf,  inf,  inf,  inf,  inf])
>>> nans = numpy.array([float("nan")] * 5)
>>> nans
array([ nan,  nan,  nan,  nan,  nan])
>>> a1 / nans
array([ nan,  nan,  nan,  nan,  nan])
>>> a1 / a0 * nans
array([ nan,  nan,  nan,  nan,  nan])

You get a runtime warning (this is in Python 2.7), but the division
returns the appropriate value, in this case, infinity. So, yes, they
forge on, and NaN taints things just about the way you'd expect.

Skip

On Tue, Jul 8, 2014 at 12:36 PM, Marko Rauhamaa <marko@pacujo.net> wrote:
> Skip Montanaro <skip@pobox.com>:
>
>> In addition to what others have written, I will add one thing. There
>> are certainly situations where raising an exception is bad. Consider
>> all the people in the scientific computing community doing fancy
>> linear algebra sorts of things, often with missing data. They
>> generally want NaN propagated and not have some long running
>> calculation crash in the middle.
>
> Do the scientific computers mind:
>
>     >>> 1 / 0
>     Traceback (most recent call last):
>       File "<stdin>", line 1, in <module>
>     ZeroDivisionError: division by zero
>
> or would they prefer their fancy linear-algebraic computation to just
> forge on?
>
>
> Marko
> --
> https://mail.python.org/mailman/listinfo/python-list