[Numpy-discussion] accumulation operation

Julian Taylor jtaylor.debian at googlemail.com
Wed Jan 22 12:32:35 EST 2014


On 22.01.2014 18:23, Ralf Juengling wrote:
> Executing the following code,
> 
>  
> 
>>>> import numpy as np
> 
>>>> a = np.zeros((3,))
> 
>>>> w = np.array([0, 1, 0, 1, 2])
> 
>>>> v = np.array([10.0, 1, 10.0, 2, 9])
> 
>>>> a[w] += v
> 
>  
> 
> I was expecting ‘a’ to be array([20., 3., 9.]. Instead I get
> 
>  
> 
>>>> a
> 
> array([ 10.,   2.,   9.])
> 
>  
> 
> This with numpy version 1.6.1.
> 
> Is there another way to do the accumulation I want?
> 

you want:

np.add.at(a, w, v)

which is available in numpy 1.8




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