Re: Proposed solution to bug #6503. -- August 09, 2006 - 17:18

On 8/9/06, Chris MacRaild wrote:
> On Wed, 2006-08-09 at 12:42 +0100, Gary S. Thompson wrote:
> > Chris MacRaild wrote:
> >
> > >On Wed, 2006-08-09 at 19:48 +1000, Edward d'Auvergne wrote:
> > >
> > >
> > >>>>As the function 'self.grid_search()' executes 'self.minimise()' this
> > >>>>single change will cover both the grid search and minimisation.  The
> > >>>>function 'isnan()' currently non-existent but could be implemented as
> > >>>>Gary previously mentioned by checking the hex bit pattern of NaN.
> > >>>>
> > >>>>
> > >>>If isnan() is to be implimented in this way, then a similar isinf() is
> > >>>by far the best way to go. float('inf') is platform dependent, and
> > >>>certainly will not work under Windows (I think I'm right in saying
> > >>>windows has no valid string representation of inf or nan that would work
> > >>>in that context).
> > >>>
> > >>>
> > >>That one I didn't know about.  I thought Python handled inf correctly
> > >>but checking the bit value should be easy.  What about checking that
> > >>the value is less than 1e300 instead, as that is the arbitrary
> > >>starting point for the grid search?
> > >>
> > >>
> > >
> > >In practise I'm fairly sure that will work. The purist might say that
> > >the starting value should be INF, and the test should be for INF, but I
> > >can't imagine anyone wanting to accept a grid search with Chi2 > 1e300
> > >
> > >
> >
> > does comparison against inf work on all architectures/compilers....
> >
>
> The official line is that all Python behaviour with respect to special
> float values is an accident arising from the specific
> architecture/compiler comination. This applies to NaN, +INF, -INF, and
> signed zero. That said, I dont recall INF behaviour being raised as an
> issue in the various email list discussions I've read. I've just done a
> quick test on my machine and see none of the craziness I saw with NaN
> comparisons.

Maybe we could add to the test-suite two tests, one to see if the
isnan() functions properly and the other to test if isinf() functions.
The NaN and Inf values can be generated and the functions tested to
see if they recognise them.  These two functions need to be added to
the 'nan_catch_test' branch.  An idea may be to implement both of
these within a file called something like 'float.py' in the base relax
directory.  Both functions could be accessible in the structure
'self.relax.float'.  Gary, would you like to use the information in
your post at https://mail.gna.org/public/relax-devel/2006-08/msg00013.html
and try to implement the NaN bit pattern matching function?  Do you
think signalling NaNs are an issue or would the generated NaN be a
quite NaN?

Edward