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Re: NaN comparisons - Call For Anecdotes

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  Re: NaN comparisons - Call For Anecdotes Skip Montanaro <skip@pobox.com> - 2014-07-08 12:23 -0500
    Re: NaN comparisons - Call For Anecdotes Marko Rauhamaa <marko@pacujo.net> - 2014-07-08 20:36 +0300
      Re: NaN comparisons - Call For Anecdotes Skip Montanaro <skip@pobox.com> - 2014-07-08 12:49 -0500

#74188 — Re: NaN comparisons - Call For Anecdotes

On Tue, Jul 8, 2014 at 9:53 AM, Anders J. Munch <2014@jmunch.dk> wrote:
> Most people don't need to deal with NaN's in Python at all,
> fortunately. They just don't appear in normal computation, because the
> interpreter raises an exception instead.

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.

Skip

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#74190

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

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#74191

>>> 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

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