[Numpy-discussion] strange behavior of numpy.random.multivariate_normal, ticket:1842

Nathaniel Smith njs@pobox....
Thu Feb 16 14:39:41 CST 2012


On Thu, Feb 16, 2012 at 5:20 PM, Pauli Virtanen <pav@iki.fi> wrote:
> Hi,
>
> 16.02.2012 18:00, Nathaniel Smith kirjoitti:
> [clip]
>> I agree, but the behavior is still surprising -- people reasonably
>> expect something like svd to be deterministic. So there's probably a
>> doc bug for alerting people that their reasonable expectation is, in
>> fact, wrong :-).
>
> The problem here is that these warnings should in principle appear in
> the documentation of every numerical algorithm that contains branches
> chosen on the basis of floating point data. For example, optimization
> algorithms --- they terminate after a tolerance is satisfied, and so the
> results can contain similar quasi-random error much larger than the
> rounding error, tol > |err| >> eps.
>
> Floating point sucks, it's full of gotchas for all ages :(

Yes, and maybe I'm just projecting my own particular naivete... I'm
very familiar with numerical stability and rounding as issues, and of
course optimization-based algorithms have the issue you raise. I'm
still surprised to learn that on a single machine, with bit-identical
inputs, using a mature low-level routine like svd, you can get
*qualitatively* different results depending on memory alignment. (I
wouldn't expect dense SVD to use a fixed tolerance optimization
routine either!)

-- Nathaniel


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