Output argument not assigned during call

I need help to correct this code. While I did not write this code in function, it worked right, but now matlab gives error

[X, Y] = points(t1, U1, U01)

function [X, Y] = points(t, U, U0)

d = U-U0;
s = abs(diff(sign(d))); 
id = find( s>0 );
 

h = zeros(size(id));
f = zeros(size(id));
for k = 1:length(id)
  i = id(k); 
  
  a1 = (U0(i+1)-U0(i))/(t(i+1)-t(i));
  a2 = (U(i+1)-U(i))/(t(i+1)-t(i));
  b1 = U0(i)-a1*t(i);
  b2 = U(i)-a2*t(i);
  
  A = [-a1, 1; 
       -a2, 1];
  B = [b1; b2];
  u = A\B; 
  X(k) = u(1); 
  Y(k) = u(2);
end

end

Output argument "X" (and maybe others) not assigned during call to "points".

Answer 1

[Fangjun Jiang]

X and Y are only assigned inside the for-loop, the for-loop is dependent on the value of "id", which is dependent on the result of find(). What if find() returns empty?

Always assign a default return value to avoid this problem.

[Pavel M]

Ok, id really returns empty, but while I did not write this code in function, it worked right. Why now id returns empty?

[Fangjun Jiang]

It depends on U0 and U. Try to run and understand these three lines

d = U-U0;
s = abs(diff(sign(d))); 
id = find( s>0 )

Answer 2

[Image Analyst]

The function should start like this:

function [X, Y] = points(t, U, U0)
X = []; % Make sure we return SOMETHING, even though it's empty, even if we never assign X.
Y = []; % Make sure we return SOMETHING, even though it's empty, even if we never assign Y.