MATLAB: Building a matrix starting from another matrix

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

Best Answer

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.
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