Bit twiddling floating point numbers

[Bryan Olson]

Mark Dickinson wrote:
> Jeff Goldfin wrote:
>> I can pack and unpack a float into a long
>> e.g.
>> struct.unpack('I',struct.pack('f',0.123))[0]
>> but then I'm not sure how to work with the resulting long.
>>
>> Any suggestions?
> 
> One alternative to using struct is to use math.ldexp and math.frexp:
> 
>>>> m, e = frexp(pi)
>>>> m
> 0.78539816339744828
>>>> e
> 2
>>>> int(m*2**53)
> 7074237752028440L
> 
> Then you can do your bit twiddling on int(m*2**53), before using
> ldexp to 'repack' the float.

Ah, those are handy. Jeff described his problem: "In particular,
I would like to round my float to the n most significant bits."
I think this works:

   from math import frexp, ldexp, floor

   def round_mantissa(x, nbits):
       shifter = 1 << nbits
       (m, e) = frexp(x)
       m = floor(m * shifter + 0.5) / shifter
       return ldexp(m, e)


-- 
--Bryan