Jerry Hill <[EMAIL PROTECTED]> wrote:
> On Thu, May 22, 2008 at 12:32 PM, Dutton, Sam <[EMAIL PROTECTED]> wrote:
> > I've noticed that the value of math.pi -- just entering it at the
> > interactive prompt -- is returned as 3.1415926535897931, whereas (as every
> > pi-obsessive knows) the value is 3.1415926535897932... (Note the 2 at the
> > end.)
> >
> > Is this a precision issue, or from the underlying C, or something else? How
> > is math.pi calculated?
>
> I believe this is an issue with the precision of IEEE floating point
> numbers[1]. Those are the types of numbers used by the hardware
> floating point processor on your CPU. I'm not an expert, by any
> stretch of the imagination, but I believe you have two choices to
> represent pi as an IEEE double precision float:
>
> 3.1415926535897931 (stored as 0x400921FB54442D18) or
> 3.1415926535897936 (stored as 0x400921FB54442D19)
Just in case you were wondering how you get those hex numbers (like I
was!)
>>> import math
>>> import struct
>>> struct.pack(">d", math.pi).encode("hex")
'400921fb54442d18'
>>> struct.unpack(">d", "400921FB54442D18".decode("hex"))
(3.1415926535897931,)
>>> struct.unpack(">d", "400921FB54442D19".decode("hex"))
(3.1415926535897936,)
>>>
--
Nick Craig-Wood <[EMAIL PROTECTED]> -- http://www.craig-wood.com/nick
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