[Numpy-discussion] Floating point question

Gideon Simpson simpson@math.toronto....
Mon Mar 2 14:37:33 CST 2009


I recently discovered that for 8 byte floating point numbers, my  
fortran compilers (gfortran 4.2 and ifort 11.0) on an OS X core 2 duo  
machine believe the  smallest number 2.220507...E-308.  I presume that  
my C compilers have similar results.

I then discovered that the smallest floating point number in python  
2.5 is 4.9065...E-324.  I have been using numpy to generate data,  
saving it with savetxt, and then reading it in as ASCII into my  
fortran code.  Recently, it crapped out on something because it didn't  
like reading it a number that small, though it is apparently perfectly  
acceptable to python.

My two questions are:

1.  What is the best way to handle this?  Is it just to add a filter  
of the form

	u = u * ( np.abs(u) > 2.3 e-308)

2.  What gives?  What's the origin of this (perceived) inconsistency  
in floating points across languages within the same platform?

I recognize that this isn't specific to Scipy/Numpy, but thought  
someone here might have the answer.

-gideon



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