On Dec 7, 12:16 am, David Cournapeau <courn...@gmail.com> wrote:
> If you can depend on IEEE 754 semantics, one relatively robust method
> is to use the number of representable floats between two numbers. The
> main advantage compared to the proposed methods is that it somewhat
> automatically takes into account the amplitude of input numbers:
FWIW, there's a function that can be used for this in Lib/test/
test_math.py in Python svn; it's used to check that math.gamma isn't
out by more than 20 ulps (for a selection of test values).
def to_ulps(x):
"""Convert a non-NaN float x to an integer, in such a way that
adjacent floats are converted to adjacent integers. Then
abs(ulps(x) - ulps(y)) gives the difference in ulps between two
floats.
The results from this function will only make sense on platforms
where C doubles are represented in IEEE 754 binary64 format.
"""
n = struct.unpack('<q', struct.pack('<d', x))[0]
if n < 0:
n = ~(n+2**63)
return n
--
Mark
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