For completeness, in numpy >= 1.8 you can also use np.add
‘s at
method:
In [8]: m, n = np.random.rand(2, 10)
In [9]: m_idx, n_idx = np.random.randint(10, size=(2, 20))
In [10]: m0 = m.copy()
In [11]: np.add.at(m, m_idx, n[n_idx])
In [13]: m0 += np.bincount(m_idx, weights=n[n_idx], minlength=len(m))
In [14]: np.allclose(m, m0)
Out[14]: True
In [15]: %timeit np.add.at(m, m_idx, n[n_idx])
100000 loops, best of 3: 9.49 us per loop
In [16]: %timeit np.bincount(m_idx, weights=n[n_idx], minlength=len(m))
1000000 loops, best of 3: 1.54 us per loop
Aside of the obvious performance disadvantage, it has a couple of advantages:
np.bincount
converts its weights to double precision floats,.at
will operate with you array’s native type. This makes it the simplest option for dealing e.g. with complex numbers.np.bincount
only adds weights together, you have anat
method for all ufuncs, so you can repeatedlymultiply
, orlogical_and
, or whatever you feel like.
But for your use case, np.bincount
is probably the way to go.