How to concatenate a vector into rows of a numpy matrix?

Question

Suppose numpy vector a and matrix b as below:

import numpy as np
a = np.array([1,2])
b = np.array([[3,4],[5,6]])

I want to concatenate vector a into each row of matrix b. The expected output is as below:

output=np.array([[1,2,3,4],[1,2,5,6]])

I have a working code as below:

output=np.array([np.concatenate((a,row)) for row in b] )

Is there any faster numpy function to perform such a task? Any suggestion is appreciated!

Answer 1

output = np.zeros((2,4), int)
output[:, :2] = a    # broadcasts (2,) to (1,2) to (2,2)
output[:, 2:] = b

Answer 2

You can broadcast a to the shape of b with np.broadcast_to and then stack them horizontally with np.hstack:

np.hstack([np.broadcast_to(a, b.shape), b])
array([[1, 2, 3, 4],
       [1, 2, 5, 6]])