How to preallocate 2D array before for loop?

Hi!

I have a 2D array with Points in every row (called occWorld). Now I want to calculate the distances between every point and write the points in a new array, when the distance between them is below 0.24. Matlab tells me to preallocate my array, but I cant figure out how to do that.

I want to find all the points in a range of 0.24 meter and cluster them.

This is what I have got:

Trees = [];

for f = 1:length(occWorld(:,1))

    for e = f+1:length(occWorld(:,1))
        
        Points = [occWorld(f,:);occWorld(e,:)];
        Dist = pdist(Points);

        if Dist < 0.24             
            Trees = [Trees;Points];
            Trees =  unique(Trees,'rows');
        end          

    end
    % delete before new set of points ist calculated
    Trees = [];
    
end

Thank you!

[Walter Roberson]

Note that because of that Trees = []; just before the end of the for loop, the only output from this code will be Trees = [] and Dist being a scalar numeric value.

This is an important point because it affects our preallocation strategy.

[Jan]

length(occWorld(:,1)) is less efficient than size(occWorld,1).

Answer 1

[Jan]

Do not collect the points, but a list of their indices. If you use logical indexing, you can omit the expensive unique also:

n = size(occWorld, 1);
M = false(n, 1);
Dist2 = 0.24^2;
for f = 1:n
    P1 = occWorld(f, :);
    for e = f+1:n
        v = P1 - occWorld(e, :);
        if v * v.' < Dist2
            M(e) = true;
            M(f) = true;
        end          
    end
end
Trees = occWorld(M, :);

Your example replies Trees=[] in every case, as Walter has mentioned. I guess, that you do want to obtain the Trees as result.