[Numpy-discussion] Array accumulation in numpy

[Tony Ladd]

Thanks to all for a very quick response. np.bincount does what I need.

Tony

On 02/19/2013 10:04 AM, Benjamin Root wrote:
>
>
> On Tue, Feb 19, 2013 at 10:00 AM, Tony Ladd <tladd at che.ufl.edu 
> <mailto:tladd at che.ufl.edu>> wrote:
>
>     I want to accumulate elements of a vector (x) to an array (f) based on
>     an index list (ind).
>
>     For example:
>
>     x=[1,2,3,4,5,6]
>     ind=[1,3,9,3,4,1]
>     f=np.zeros(10)
>
>     What I want would be produced by the loop
>
>     for i=range(6):
>          f[ind[i]]=f[ind[i]]+x[i]
>
>     The answer is f=array([ 0.,  7.,  0.,  6.,  5.,  0.,  0.,  0.,
>      0., 3.])
>
>     When I try to use implicit arguments
>
>     f[ind]=f[ind]+x
>
>     I get f=array([ 0.,  6.,  0.,  4.,  5.,  0.,  0.,  0.,  0.,  3.])
>
>
>     So it takes the last value of x that is pointed to by ind and adds
>     it to
>     f, but its the wrong answer when there are repeats of the same
>     entry in
>     ind (e.g. 3 or 1)
>
>     I realize my code is incorrect, but is there a way to make numpy
>     accumulate without using loops? I would have thought so but I cannot
>     find anything in the documentation.
>
>     Would much appreciate any help - probably a really simple question.
>
>     Thanks
>
>     Tony
>
>
> I believe you are looking for the equivalent of accumarray in Matlab?
>
> Try this:
>
> http://www.scipy.org/Cookbook/AccumarrayLike
>
> It is a bit touchy about lists and 1-D numpy arrays, but it does the job.
> Also, I think somebody posted an optimized version for simple sums 
> recently to this list.
>
> Cheers!
> Ben Root
>
>
>
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-- 
Tony Ladd

Chemical Engineering Department
University of Florida
Gainesville, Florida 32611-6005
USA

Email: tladd-"(AT)"-che.ufl.edu
Web    http://ladd.che.ufl.edu

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