[Python-Dev] Float --> Integer Ratio (original) (raw)
Raymond Hettinger python at rcn.com
Fri Jan 25 00:10:52 CET 2008
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rational.py contains code for turning a float into an exact integer ratio. I've needed something like this in other situations as well. The output is more convenient than the mantissa/exponent pair returned by math.frexp().
I propose C-coding this function and either putting it in the math module or making it a method for floats. That would simplify and speed-up rational.py while making the tool available for other applications. Also, the tool is currently in the wrong place (while the output is clearly useful for rational.py, the internals of the function are entirely about floats and ints and has no internal references to the rational module).
Raymond
def _binary_float_to_ratio(x): """x -> (top, bot), a pair of ints s.t. x = top/bot.
The conversion is done exactly, without rounding.
bot > 0 guaranteed.
Some form of binary fp is assumed.
Pass NaNs or infinities at your own risk.
>>> _binary_float_to_ratio(10.0)
(10, 1)
>>> _binary_float_to_ratio(0.0)
(0, 1)
>>> _binary_float_to_ratio(-.25)
(-1, 4)
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