How can I transform blocks into a blockdiagonal matrix (NumPy)

Question

I have three same-size square matrices in NumPy. I would like to combine these to a block-diagonal matrix.

Example:

a1 = np.array([[1,1,1],[1,1,1],[1         


        

Answer 1

If you want this particular array r, perhaps the easiest way would be:

r=np.kron(np.diag([1,2,3]),np.ones((3,3),dtype='int'))

If a1, a2, a3 can be arbitrary 2-dimensional arrays, then perhaps the easiest way is:

a1=np.asmatrix(a1)
a2=np.asmatrix(a2)
a3=np.asmatrix(a3)
z=np.asmatrix(np.zeros((3,3)))
r=np.asarray(np.bmat('a1, z, z; z, a2, z; z, z, a3'))

An alternative slower method is:

r=(np.kron([[1,0,0],[0,0,0],[0,0,0]],a1)   
   +np.kron([[0,0,0],[0,1,0],[0,0,0]],a2)
   +np.kron([[0,0,0],[0,0,0],[0,0,1]],a3))