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!

Answer 1

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(...) ).

Answer 2

Well, length(xy1) returns, of course, the length of the vector, eg 7.

You for loop does for j=n , thus for j=7 . It does not return the last distance evaluated, it returns the only distance evaluated.

Change the for loop to for j=1:n