Count the number of all the shortest paths for any two vertices in a graph - algorithm

Question

For any pair of different vertices in a given undirected graph G= , I want to find the number of all the shortest paths ("SP", in abbreviation) (it is not required or necessary to find/print the exact vertices on a certain path). For example, for the following graph given in edge-list format, there are two SPs: (1,3,2) and (1,4,2).
vertex =
1 3
2 4
1 4
2 3
1 8
4 7
3 6
5 2
I want to implement this algorithm based on Floyd-Warshall algorithm , which is a famous algorithm based on the idea of dynamic programming to find the value of the shortest path for each pair of vertices in O(N^3), say the result is an 2D array a[n][n]. n is the number of vertices. For the above graph, it is:
0 2 1 1 3 2 2 1
2 0 1 1 1 2 2 3
1 1 0 2 2 1 3 2
1 1 2 0 2 3 1 2
3 1 2 2 0 3 3 4
2 2 1 3 3 0 4 3
2 2 3 1 3 4 0 3
1 3 2 2 4 3 3 0
The code for constructing graph matrix G and solving for matrix a is as follows:
v = vertex(:,1);
t = vertex(:,2);
G = zeros( max(max(v),max(t)));
% Build the matrix for graph:
for i = 1:length(v)
G(v(i), t(i)) = G(v(i), t(i)) + 1;
G(t(i), v(i)) = G(v(i), t(i)); % comment here is input is bi-directional
end
a = G;
n = length(a);
a(a==0) = Inf;
a(1:n+1:n^2)=0; % diagonal element to be zero
for k = 1:n
for i= 1:n
for j= 1:n % for j=i+1:n
if a(i,j) > a(i,k) + a(k,j)
a(i,j) = a(i,k) + a(k,j);
% a(j,i) = a(i,j);
end
end
end
end
Now, let's defined a 2D array b[n][n] as the number of ALL the SPs for each pair of vertices. For example, we expect b[1][2] = 2.
I wrote the following code in MATLAB (if you are not familiar whit MATLAB, just treat it as a pseudo-code). It gives almost correct values for all the pairs except several wrong values for certain pairs. For example. after running the cod,e b[5][8] = 0, which is wrong (correct answer should be 2)
%%
% find the number of ALL SP paths for ALL pairs based on the "a" array:
% b is a two-dim array, b(i,j) is the total number of SP for pair( i,j)
b = G;
for k=1:n
for i=1:n
for j= i+1:n
if(i==j)
continue; % b(i,i)=0
end
if (k==j) % the same as : G(k,j)==0
continue;
end
if(k==i && G(k,j)~=0)
b(i,j) = 1;
continue;
end
if(a(i,j) ~= a(i,k)+G(k,j)) % w(u,v)=G(u,v) in unweighted graph)
continue;
end
% sigma(s,v) = sigma(s,v) + sigma(s,u);
b(i,j) = b(i,j) + b(k,i);
end
end
end