Adjacency matrix from edge list (preferrably in Matlab)

Question

I have a list of triads (vertex1, vertex2, weight) representing the edges of a weighted directed graph. Since prototype implementation is going on in Matlab, these are imported as a Nx3 matrix, where N is the number of edges. So the naive implementation of this is

id1 = L(:,1);
id2 = L(:,2);
weight = L(:,3);
m = max(max(id1, id2)) % to find the necessary size
V = zeros(m,m)
for i=1:m
   V(id1(i),id2(i)) = weight(i)
end

The trouble with tribbles is that "id1" and "id2" are nonconsecutive; they're codes. This gives me three problems. (1) Huge matrices with way too many "phantom", spurious vertices, which distorts the results of algorithms to be used with that matrix and (2) I need to recover the codes in the results of said algorithms (suffice to say this would be trivial if id codes where consecutive 1:m).

Answers in Matlab are preferrable, but I think I can hack back from answers in other languages (as long as they're not pre-packaged solutions of the kind "R has a library that does this").

I'm new to StackOverflow, and I hope to be contributing meaningfully to the community soon. For the time being, thanks in advance!

Edit: This would be a solution, if we didn't have vertices at the origin of multiple vertices. (This implies a 1:1 match between the list of edge origins and the list of identities)

for i=1:n
   for j=1:n
   if id1(i) >0 & i2(j) > 0
       V(i,j) = weight(i);
   end
   end
   end

Answer 1

Here is another solution:

First put together all your vertex ids since there might a sink vertex in your graph:

v_id_from = edge_list(:,1);
v_id_to = edge_list(:,2);
v_id_all = [v_id_from; v_id_to];

Then find the unique vertex ids:

v_id_unique = unique(v_id_all);

Now you can use the ismember function to get the mapping between your vertex ids and their consecutive index mappings:

[~,from] = ismember(v_id_from, v_id_unique);
[~,to] = ismember(v_id_to, v_id_unique);

Now you can use sub2ind to populate your adjacency matrix:

adjacency_matrix = zeros(length(from), length(to));
linear_ind = sub2ind(size(adjacency_matrix), from, to);
adjacency_matrix(linear_ind) = edge_list(:,3);

You can always go back from the mapped consecutive id to the original vertex id:

original_vertex_id = v_id_unique(mapped_consecutive_id);

Hope this helps.