[SciPy-user] how to make a sinc(x) function - "divide by zero" !

Arnd Baecker arnd.baecker at web.de
Mon Dec 12 11:39:29 CST 2005


On Mon, 12 Dec 2005, Sebastian Haase wrote:

> Hi,
> I was trying to implement a "sinc" [sin(x)/x] in numarray. But the
> "where"-semantics makes it choke on the x=0: 0/0 case.
> (This should of course work for x being an array - so "if" is no option)
> The best I could come up with was:
>
> def sinc(r):
>     na.Error.pushMode(all="ignore")
>     a = na.where(r, na.divide(na.sin(r),r), 1)
>     na.Error.popMode()
>     return a
>
> but I still seem to get  a warning...
>
> >>> F.sinc(0)
>
> Warning: Encountered invalid numeric result(s)  in divide
> 1.0
>
> What is a better way of doing this ?

Yuo could try to use `vectorize` (warning: untested code)

def sinc(x):
    if x==0.0:                # presumably better to check for small x
        return 0.0            # here ...
    else:
       return sin(x)/x

sinc_vectorized=scipy.vectorize(sinc)

HTH,

Arnd



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