Angle between two vectors in a for loop

I need to find angles between vectors starting from 5th. These are the two loops i wrote:

A = [X;Y];

A = [A; zeros(1,50)];

W_wek = [1 0 0];

%W_wek = [0 1 0];

for k=1:length(A)

W_wek = [A(1,:)-B(1,:); A(2,:)-B(2,:); A(3,:)-B(3,:)];

end

In the first one I'm supposed to get the coordinates of the vectors, the variable looks like this:

In the second for loop i used the atan2d function combined with norm,cross:

Ang=[];

for l=5:length(W_wek)

Ang(l) = atan2d(norm(cross(W_wek(l-1,:),W_wek(l,:))),dot(W_wek(l-1,:),W_wek(l,:)));

end

I was sure it should work but it keeps giving me this error: Index in position 1 exceeds array bounds. Index must not exceed 3.

Any ideas how to fix this?

[dpb]

for l=5:length(W_wek)
  Ang(l) = atan2d(norm(cross(W_wek(l-1,:),W_wek(l,:))),dot(W_wek(l-1,:),W_wek(l,:)));     

length() is a dangerous function -- it's defined as max(size(x)) so for your case above it will return 50.

You then use it as the upper limit on the l looping index and use l as the row index for the subscripting expression into the array. You've got the indices/limits turned around.

I recommend to not use length here at all and in most cases you'll want a specific size value just like here...

Answer 1

[William Rose]

@Mateusz Soczalski,

(I deleted my comment because I posted it before it was complete, and because I intended to post an answer rather than a comment. I'm not very good at the mechanics of posting.)

W_wek=2*rand(3,50)-1;   %points uniform in the (-1...+1) cube
N=length(W_wek);
angle=zeros(1,N-5);     %allocate array for angles
for i=5:N
    angle(i)=acosd(dot(W_wek(:,i),W_wek(:,i-1))/(norm(W_wek(:,i))*norm(W_wek(:,i-1))));
end
fprintf('Angle:  min=%.2f, max=%.2f, mean=%.2f\n',min(angle),max(angle),mean(angle));

Angle:  min=0.00, max=170.65, mean=78.95

Angle is between 0 and 180.