python - Accumulate constant value in Numpy Array

Question

I'm trying to sum +1 to some specific cells of a numpy array but I can't find any way without slow loops:

coords = np.array([[1,2],[1,2],[1,2],[0,0]])
X      = np.zeros((3,3))

for i,j in coords:
  X[i,j] +=1 

Resulting in:

X = [[ 1.  0.  0.]
     [ 0.  0.  3.]
     [ 0.  0.  0.]]

X[coords[:,0],coords[:,1] += 1 returns

X = [[ 1.  0.  0.]
     [ 0.  0.  1.]
     [ 0.  0.  0.]]

Any help?

Answer 1

numpy.at is exactly for those situations.

In [1]: np.add.at(X,tuple(coords.T),1)

In [2]: X
Out[2]: 
array([[ 1.,  0.,  0.],
       [ 0.,  0.,  3.],
       [ 0.,  0.,  0.]])