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Asked: 2026-06-11T18:53:39+00:00

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

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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 between 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
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1 Answer

  1. Editorial Team

    You do not get zeros, you get correct distances. You probably think you get zeros, because 1e9 is a large value and when you try to print your matrix you get

    distances =

1.0e+09 *

1.0000    0.0000    0.0000    1.0000
1.0000         0    0.0000    1.0000
1.0000    0.0000         0    1.0000
1.0000    0.0000    0.0000    1.0000

    You can see that the two 0-entries are true zeros, while the others are approximately 0 down to 4 digits after the coma. Try to print one of the matrix elements and see they are not zeros

    distances(1,2)

ans =

2.5126

    You can also use nnz function to quickly know how many non-zeros you have

    nnz(distances)

ans =

14
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