how to add two numpy.array with indices or mask

Question

I have two arrays, like as following.

 a=numpy.array((1,2,3,4))
 b=numpy.array((1,2,3,4))

I just want to add the first two elements. How can i do it with a mask [True, True, False, False] , or indice [0,1] After addition b=(2, 4,3,4)

Answer 1

It's simply:

b[:2] += a[:2]

Or:

mask = numpy.array((True, True, False, False))
b[mask] += a[mask]