On Mon, 07 Sep 2009 08:23:58 -0700, Mark Dickinson wrote:
> There's sys.float_info.min:
>
>>>> import sys
>>>> sys.float_info
> sys.float_info(max=1.7976931348623157e+308, max_exp=1024,
> max_10_exp=308, min=2.2250738585072014e-308, min_exp=-1021,
> min_10_exp=-307, dig=15, mant_dig=53, epsilon=2.2204460492503131e-16,
> radix=2, rounds=1)
>>>> sys.float_info.min
> 2.2250738585072014e-308
>
> But that only gives you the smallest *normal* positive float, which is
> 2**-1022 on most systems. The smallest positive subnormal value is
> usually 2**-1074. If you want something that would still work if Python
> ever switched to using IEEE 754 binary128 format (or some other IEEE 754
> format), then
>
> sys.float_info.min * 2**(1-sys.float_info.mant_dig)
>
> will work.
Here's another implementation to give the next float. It should work for
"normal" binary implementations:
from struct import pack, unpack
def next(x, y=None):
"""Returns the next float from x in the direction of y.
If y is not given, the direction is towards positive infinity.
"""
if x == y:
return x
if y is None or y > x:
delta = cmp(x, 0.0) or 1
elif y < x:
delta = -(cmp(x, 0.0) or 1)
else:
raise ValueError("unordered x, y: %f, %f" % (x, y))
return unpack('d', pack('q', unpack('q', pack('d', x))[0]+delta))[0]
>>> next(0.0)
4.9406564584124654e-324
>>> next(0.0, -1)
nan
You can also get signed zero:
>>> x = -next(0.0)
>>> next(x) == 0
True
>>> next(x)
-0.0
--
Steven
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