Graeme O'Keefe wrote:
> Hi,
>
> I know this really belongs on the sourceforge.net:numarray list, but it
> is a very inactive list.
It's still the right place to ask. Your question would get answered in a
timely manner. We're quiet; we're not gone. :-)
Followup-to has been reset.
> I have noted the following with numarray:
>
> >>> x = numarray.zeros([3,3], numarray.Int16)
> >>> i = [0,1,2,0,1,2]
> >>> j = [0,1,2,0,1,2]
> >>> x[i, j] += 1
> >>> print x
> [[1, 0, 0]
> [0, 1, 0]
> [0, 0, 1]]
> I was expecting (hoping)
> [[2, 0, 0]
> [0, 2, 0]
> [0, 0, 2]]
>
> which is what you obviously get if you:
>
> for n in range(len(i)) : x[i[n], j[n]] += 1
>
> This is the sort of operation one does all the time when histogramming
> data-streams. Is there a way to achieve this without looping over i/j.
This has been discussed recently on the numpy-discussion list (Subject:
vectorizing histogram-like computation). The semantics of array indexing
aren't anything like that for loop. I doubt that the two kinds of
semantics could be merged consistently.
x[i,j] += 1
is closer to
x[i,j] = x[i,j] + 1
which amounts, in this case, to
x[i,j] = array([1, 1, 1, 1, 1, 1])
There's nothing in this last operation to suggest that the numbers ought
to be added together (and if the initial values weren't zero, a more
complicated operation would have to considered).
In short, I consider it cleaner to keep specialized histogram code
tucked away in a function which you can eventually optimize to C as
required.
--
Robert Kern
rk...@uc...
"In the fields of hell where the grass grows high
Are the graves of dreams allowed to die."
-- Richard Harter
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