Numpy: Function to take arrays a and b and return array c with elements 0:b[0] with value a[0], values b[0]:b[1] with value a[1], and so on

Say I have two arrays:

a = np.asarray([0,1,2])
b = np.asarray([3,7,10])

Is there a fast way to create:

c = np.asarray([0,0,0,1,1,1,1,2,2,2])
#  index             3       7     10

This can be done using a for loop but I wonder if there is a fast internal numpy function that achieves the same thing.

You can use diff to get the successive differences, r_ to add the first b value and repeat to duplicate the values:

a = np.asarray([0, 1, 2])
b = np.asarray([3, 7, 10])

c = np.repeat(a, np.r_[b[0], np.diff(b)])

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