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Re: reading NaNs

Aubrey Jaffer wrote:
Having a universal read/write representation for arbitrary bit
patterms prevents including information like the procedure causing the
NaN in its printed representation.

I don't see this. Can you elaborate?

BTW, I neither favor nor oppose reading or writing of arbitrary bit patterns. I am favor of being able to specify the precision of the NaN, though.

 | However, I still think we need a read syntax.  Suppose program A
 | calculates a value and writes it to a file and program B reads the
 | value from the file and uses it.  Is is not useful for program A to
 | be able to communicate to program B that it got a NaN?


If program A writes out its state, it would be useful to see that NaNs
were computed.  It gives operators a chance to capture the use case
which provoked the error.  If the program state is very valuable, then
it can be repaired manually.

But if program B reads its initial state from the file, its reading of
NaNs puts errors into its state which can propagate and corrupt it.

Um, how about program B doing:

  (let ((x (read)))
    (if (nan? x)
      (display "Help! Help! Program A is feeding me NaNs!")))

Sure, if you don't check your input, you can be screwed, but what's new there?

Another example. Astronomical detectors tend to have a fair number of bad pixels. We normally deal with these by associating a binary "bad pixel map" with each image. The images are initially fixnums, but after initial processing they are converted to flonums. It is a lot simpler to simply replace the bad pixels in the processed images with NaNs rather than continuing to work with the bad pixel maps. I want to be able to archive processed images, so I need to be able to read and write NaNs. (Okay, this is a bit of a cheat, because the data are written in a binary format, but my point is the same: reading a NaN is not necessarily an error.)


Dr Alan Watson
Centro de Radioastronomía y Astrofísica
Universidad Astronómico Nacional de México