[Numpy-discussion] what goes wrong with cos(), sin()

[Timothy Hochberg]

On 2/21/07, Charles R Harris <charlesr.harris at gmail.com> wrote:
>
>
>
> On 2/21/07, Robert Kern <robert.kern at gmail.com> wrote:
> >
> > Christopher Barker wrote:
> > > Robert Kern wrote:
> > >> Christopher Barker wrote:
> > >>> I wonder if there are any C math libs that do a better job than
> > you'd
> > >>> expect from standard FP? (short of unlimited precision ones)
> > >> With respect to π and the zeros of sin() and cos()? Not really.
>
>
> <snip>
>
> Well, you can always use long double if it is implemented on your
> > platform. You
> > will have to construct a value for π yourself, though. I'm afraid that
> > we don't
> > really make that easy.
> >
> > --
>
>
> pi = 3. 1415926535 8979323846 2643383279 5028841971 6939937510 5820974944
> 5923078164 0628620899 8628034825 3421170679 8214808651 *...
>
> *
> I dont know what that looks like when converted to long double. Lessee,
>
> In [1]: import numpy
>
> In [2]: pi = numpy.float128(3.1415926535897932384626433832795028841971)
>

I think this is where you go wrong. Your string of digits is first a python
float and *then* is converted to a long double. In the intermediate stage it
gets truncated and you don't get the precision back.


In [3]: numpy.pi - pi
> Out[3]: 0.0
>
> In [7]: '%25.20f'%numpy.pi
> Out[7]: '   3.14159265358979311600 '
>
> In [8]: '%25.20f'%pi
> Out[8]: '   3.14159265358979311600'
>
> I think we have a bug. Or else extended arithmetic isn't supported on this
> machine.
>
> Chuck
>
>
>
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>


-- 

//=][=\\

tim.hochberg at ieee.org
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