[Numpy-discussion] how to tally the values seen
Peter Shinners
pete@shinners....
Wed Apr 14 02:05:09 CDT 2010
On 04/13/2010 11:44 PM, Gökhan Sever wrote:
>
>
> On Wed, Apr 14, 2010 at 1:34 AM, Warren Weckesser
> <warren.weckesser@enthought.com
> <mailto:warren.weckesser@enthought.com>> wrote:
>
> Gökhan Sever wrote:
> >
> >
> > On Wed, Apr 14, 2010 at 1:10 AM, Peter Shinners
> <pete@shinners.org <mailto:pete@shinners.org>
> > <mailto:pete@shinners.org <mailto:pete@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 <http://numpy.int>
> <http://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.
> >
>
>
> numpy.bincount():
>
>
> In [1]: import numpy as np
>
> In [2]: x = np.array([2,8,1,2,7,7,2,7,0,2])
>
> In [3]: np.bincount(x)
> Out[3]: array([1, 1, 4, 0, 0, 0, 0, 3, 1])
>
>
>
> I knew a function exists in numpy for this case too :)
>
> This is also safer way to handle the given situation to prevent index
> out of bounds errors.
Thanks guys. Numpy always surprises me. It's like you guys have borrowed
Guido's time machine.
This is running far faster than I ever hoped.
My next problem involves a bit of indexing and reordering. But I'm gonna
spend a night of my own time to see where I can get with it.
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