[Numpy-discussion] how to tally the values seen

[Gökhan Sever]

On Wed, Apr 14, 2010 at 1:10 AM, Peter Shinners <pete at shinners.org> wrote:

> I have an array that represents the number of times a value has been
> given. I'm trying to find a direct numpy way to add into these sums
> without requiring a Python loop.
>
> For example, say there are 10 possible values. I start with an array of
> zeros.
>
>  >>> counts = numpy.zeros(10, numpy.int)
>
> Now I get an array with several values in them, I want to add into
> counts. All I can think of is a for loop that will give my the results I
> want.
>
>
>  >>> values = numpy.array((2, 8, 1))
>  >>> for v in values:
> ...    counts[v] += 1
>  >>> print counts
> [0 1 1 0 0 0 0 0 1 0]
>

This is easy:

I[3]: a
O[3]: array([ 0.,  1.,  1.,  0.,  0.,  0.,  0.,  0.,  1.,  0.])

I[4]: a = np.zeros(10)

I[5]: b = np.array((2,8,1))

I[6]: a[b] = 1

I[7]: a
O[7]: array([ 0.,  1.,  1.,  0.,  0.,  0.,  0.,  0.,  1.,  0.])

Let me think about the other case :)


> I also need to handle the case where a value is listed more than once.
> So if values is (2, 8, 1, 2) then count[2] would equal 2.
>
> What is the most efficient way to do this?
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-- 
Gökhan
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