Sum array by number in numpy

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The numpy function bincount was made exactly for this purpose and I’m sure it will be much faster than the other methods for all sizes of inputs:

data = [1,2,3,4,5,6]
ids  = [0,0,1,2,2,1]

np.bincount(ids, weights=data) #returns [3,9,9] as a float64 array

The i-th element of the output is the sum of all the data elements corresponding to “id” i.

Hope that helps.

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