Combined_resistance_graph.py

from sympy import *
def lattice_graph(H,W):
    N=H*W
    G=[[] for i in range(N)]
    for i in range(H-1):
        for j in range(W):
            x=i*W+j
            y=(i+1)*W+j
            G[x].append(y)
            G[y].append(x)
    for i in range(H):
        for j in range(W-1):
            x=i*W+j
            y=i*W+j+1
            G[x].append(y)
            G[y].append(x)
    return G
    
def comb_resist(G,in_term=0,out_term=-1):
    N=len(G)
    A=[[0 for j in range(N+1)]for i in range(N)]
    for i in range(N):
        for j in G[i]:
            A[i][i]+=1
            A[i][j]-=1
    A[in_term][N]+=1
    A[out_term][N]-=1
    A_mat=Matrix(A)
    A_rref=A_mat.rref()
    return A_rref[0][in_term,-1]-A_rref[0][out_term,-1]

#sample code

#cube
G=[[] for i in range(8)]
for i in range(8):
    for x in [1,2,4]:
        G[i].append(i^x)

print(comb_resist(G))
#5/6


print(comb_resist(G,in_term=0,out_term=3))
#3/4


G=[[1,1],[0,0,2],[1]]
print(comb_resist(G))
#3/2


G=lattice_graph(2,3)
print(comb_resist(G))
#7/5