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

[David L Goldsmith]

I agree w/ Chuck - I'd consider what Tim describes is happening a "bug".

DG

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