[Numpy-discussion] Fancier indexing

[Keith Goodman]

On Thu, May 22, 2008 at 8:59 AM, Kevin Jacobs <jacobs at bioinformed.com>
<bioinformed at gmail.com> wrote:
> After poking around for a bit, I was wondering if there was a faster method
> for the following:
>
> # Array of index values 0..n
> items = numpy.array([0,3,2,1,4,2],dtype=int)
>
> # Count the number of occurrences of each index
> counts = numpy.zeros(5, dtype=int)
> for i in items:
>   counts[i] += 1
>
> In my real code, 'items' contain up to a million values and this loop will
> be in a performance critical area of code.  If there is no simple solution,
> I can trivially code this using the C-API.

How big is n? If it is much smaller than a million then loop over that instead.