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

Charles R Harris charlesr.harris@gmail....
Wed Feb 21 17:43:21 CST 2007


On 2/21/07, Timothy Hochberg <tim.hochberg@ieee.org> wrote:
>
>
>
> On 2/21/07, Charles R Harris <charlesr.harris@gmail.com> wrote:
> >
> >
> >
> > On 2/21/07, Robert Kern < robert.kern@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.
>

True. But there is missing functionality here.

In [4]: pi = numpy.float128('3.1415926535897932384626433832795028841971')
---------------------------------------------------------------------------
exceptions.TypeError                                 Traceback (most recent
call last)

/home/charris/workspace/microsat/daemon/<ipython console>

TypeError: a float is required

It's somewhat pointless to have a data type that you can't properly
initialize. I think the string value should work, it works for python types.

Chuck
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