How to accumulate an array by index in numpy? [duplicate]

Question

I have an array:

a = np.array([0,0,0,0,0,0])

I want to add some other array into each index of a, while the index can appear more than one times. I want to get the some of each index. I write:

a[np.array([1,2,2,1,3])] += np.array([1,1,1,1,1])

but get a to be:

array([0, 1, 1, 1, 0, 0])

But what I want is to get:

array([0, 2, 2, 1, 0, 0])

How to implement this in numpy without for loop?

Answer 1

Using pure numpy, AND avoiding a for loop:

np.add.at(a, np.array([1,2,2,1,3]), np.array([1,1,1,1,1]))

Output:

>>> a = np.array([0,0,0,0,0,0])
>>> np.add.at(a, np.array([1,2,2,1,3]), np.array([1,1,1,1,1]))
>>> a
array([0, 2, 2, 1, 0, 0])

Please note, this does in-place substitution. This is what is desired by you, but it may not be desired by future viewers. Hence the note :)

Answer 2

You could always just iterate yourself. Something like:

for i in [1,2,2,1,3]:
    a[i] += 1

Answer 3

I don't know of a clever numpy vectorized way to do this... the best I can come up with is:

>>> indices = np.array([1,2,2,1,3])
>>> values = np.array([1,1,1,1,1])
>>> a = np.array([0,0,0,0,0,0])
>>> for i, ix in enumerate(indices):
...   a[ix] += values[i]
... 
>>> a
array([0, 2, 2, 1, 0, 0])