numpy fill an array with arrays

Question

I want to combine an unspecified (finite) number of matrices under a Kroneckerproduct. In order to do this I want to save the matrices in an array but I don't know how to do this. At the moment I have:

for i in range(LNew-2):
    for j in range(LNew-2):
        Bulk = np.empty(shape=(LNew-1,LNew-1))
        if i == j:
            Bulk[i,j]=H2
        else:
            Bulk[i,j]=idm

Here the H2 and idm are both matrices, which I want to combine under a Kronecker product. But since Bulk is an ndarray object I suppose it wont accept arraylike objects inside it.

edit:

This is the function in which I want to use this idea. I am using it to build a Hamiltonian matrix for a quantum spin chain. So H2 is the Hamiltonian for a two particle chain, H2 is a 4x4 matrix and idm is the 2x2 identity matrix.

and now the three particle chain is np.kron(H2,idm)+np.kron(idm,H2)

and for four particles np.kron(np.kron(H2,idm),idm)+np.kron(idm,np.kron(H2,idm))+np.kron(idm,np.kron(idm,H2)) and so on.

def ExpandHN(LNew):
idm = np.identity(2)
H2 = GetH(2,'N')
HNew = H2
for i in range(LNew-2):
    for j in range(LNew-2):
        Bulk = np.empty(shape=(LNew-1,LNew-1))
        if i == j:
            Bulk[i,j]=H2
        else:
            Bulk[i,j]=idm
i = 0
for i in range(LNew-2):
    for j in range(LNew-3):
        HNew += np.kron(Bulk[i,j],Bulk[i,j+1]) #something like this

return HNew

As you can see the second set of for loops hasn't been worked out.

That being said, if someone has a totaly different but working solution I would be happy with that too.

Answer 1

If I understand correctly, the your question boils down to how to create arrays of arrays with numpy. I would suggest to use the standard python object dict :

Bulk = dict()
for i in range(LNew-2):
    for j in range(LNew-2):
        if i == j:
            Bulk[(i,j)]=H2
        else:
            Bulk[(i,j)]=idm

The usage of tuples as keys allows you to maintain an array-like indexing of the matrices. Also note, that you should define Bulk outside of the two for loops (in any case).

HTH