[Numpy-discussion] Array accumulation in numpy

[Sebastian Berg]

On Tue, 2013-02-19 at 10:00 -0500, Tony Ladd 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.
> 

You might be interested in this:
https://github.com/numpy/numpy/pull/2821

But anyway, you should however be able to do what you want to do using
np.bincount with the weights keyword argument.

Regards,

Sebastian

> Would much appreciate any help - probably a really simple question.
> 
> Thanks
> 
> Tony
>