MATLAB: Function returns single value instead of vector

Question

I tried to create a function in MATLAB, which returns a vector of the distances between neighboring points in a list of points (x1,y1),...,(xn,yn).

That is ["distance (x1,y1) to (x2,y2)", ... ,"distance (xn-1,yn-1) to (xn,yn)","distance (xn,yn) to (x1,y1)"].

To do this, my idea was to work with two vectors: The original one [(x1,y1),...,(xn,yn)] and [(xn,yn),(x1,y1),...,(xn-1,yn-1)].

Now I built this function:

function erg = distVec(xy1)

n = length(xy1);
xy2 = cat(1,xy1(2:end,:),xy1(1,:));
 % define snd vector
erg = [];
    for j=n
       erg = cat(2,erg,norm((xy1(j,:)-xy2(j,:))));
        % norm(...) equals distance between two neighboring points
    end
end

But instead of an vector with the distances, this function returns only the last distance evaluated.

What is wrong?

Thanks!

MATLAB doesn't allow you to use tuples like that. In your code, what type is xy1 supposed to have? — jub0bs
It's a matrix with the x-values in the fst column and the y-values in the snd. — sabs

Luis Mendo Luis Mendo · Accepted Answer · 2015-09-02T12:38:18

It appears xy1 is a two-column matrix, in which each point is a row. Then you can obtain the result without loops as follows:

result = sqrt(sum((xy1([2:end 1],:) - xy1).^2, 2));

That is: cyclically shift one row (xy1([2:end 1],:)), subtract original matrix (- xy1), square element-wise ((...).^2), sum along each row (sum(... ,2)), and take square root (sqrt(...)).

MATLAB: Function returns single value instead of vector

2 Answers