building a matrix starting from another matrix

I want to build a square matrix. Let's suppose We have this matrix called nodes

1 4.3434  3.4565
2 6.2234  5.1234
3 10.4332 2.3243
4 7.36543 1.1434

where the column 2 and 3 represents position x and y of nodes n

and a matrix called heads where its elements are some elements of nodes matrix

2 6.2234 5.1234
3 10.4332 2.3243

I created this function to build the matrix of the distance of every nodes from the heads

function [distances] = net_dist(nodes,heads)

nnodes = length(nodes(:,1));
distances = zeros(nnodes);

for i = 1 : nnodes
    for j = 1 : nnodes
        if nodes(i,1) == nodes(j,1) && ismember(nodes(j,1),heads(:,1))
            distances(i,j) = sqrt((nodes(i,2) - nodes(j,2))^2 + (nodes(i,3) - nodes(j,3))^2);
        elseif (nodes(i,1) == nodes(j,1) || nodes(i,1) ~= nodes(j,1)) && ismember(nodes(j,1),heads(:,1))
            distances(i,j) = sqrt((nodes(i,2) - nodes(j,2))^2 + (nodes(i,3) - nodes(j,3))^2); 
        elseif (nodes(i,1) == nodes(j,1) || nodes(i,1) ~= nodes(j,1)) && ~ismember(nodes(j,1),heads(:,1))
            distances(i,j) = 1E9;
        end
    end
end

return;

This function should return the distance of every nodes from a heads. The positions of nodes that aren't heads are filled with number 1E9. I don't understand why when I execute this function instead to receive sqrt values I receive all 0.

Definitely I would obtain such similar thing

   1   2   3   4 
 1 1E9 d   d   1E9

 2 1E9 0   d   1E9

 3 1E9 d   0   1E9

 4 1E9 d   0   1E9

Answer 1

[Jan]

At first I'd simplify the code. E.g. this is useless:

if (nodes(i,1) == nodes(j,1) <OR> nodes(i,1) ~= nodes(j,1))

Of course two number are either equal or not equal, as long as they are not NaNs.

function distances = net_dist(nodes,heads)

nnodes = size(nodes, 1);  % Better than "length(nodes(:,1))"
distances = zeros(nnodes);

for i = 1 : nnodes
    for j = 1 : nnodes
        match = ismember(nodes(j,1),heads(:,1));
        if nodes(i,1) == nodes(j,1) && match
            distances(i,j) = sqrt((nodes(i,2) - nodes(j,2))^2 + ...
                                  (nodes(i,3) - nodes(j,3))^2);
        elseif match
            distances(i,j) = sqrt((nodes(i,2) - nodes(j,2))^2 + ...
                                  (nodes(i,3) - nodes(j,3))^2); 
        else  % Now "elseif ~match" *must* be true!
            distances(i,j) = 1E9;
        end
    end
end

Now we see, that the first two IF-branches are performed whenever match is true. So a further simplification:

distances = repmat(1E9, nnodex, nnodes);
matchList = ismember(nodes(:,1), heads(:,1));
for i = 1 : nnodes
    for j = 1 : nnodes
        if matchList(j)
            distances(i,j) = sqrt((nodes(i,2) - nodes(j,2))^2 + ...
                                  (nodes(i,3) - nodes(j,3))^2);
        end
    end
end

Finally I do not understand also, why you get zeros in the output.

[Salvatore Mazzarino]

This is my output

http://imageshack.us/a/img62/738/outputhb.png

as you can see I get zero

[Salvatore Mazzarino]

No Ok I understood because I got zeros. really the values in that position are there.