# How do I code a function that's supposed to be an infinite pattern?

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SnooSquare on 2 Sep 2019
Answered: Alex Mcaulley on 9 Sep 2019
I have a question as follows,
Let x = [x1 x2 x3 x4…..xn] and y = [y1 y2 y3 y4…….yn]. Write a MATLAB function to determine the following: (x1y2-x2y1+x2y3-x3y2+…..xny1-x1yn) Now assume x = [1 2 3 4 5] and y = [10 20 30 40 50]. Evaluate the above in this case.
However, I don't know how to code for a function that has "....yn" or "...xn",
What I have right now is literally this,
function [z] = Assignment1Question6b(x,y)
z = x(1)*y(2)-x(2)*y(1)+x(2)*y(3)-x(3)*y(2)+x(3)*y(4)-x(4)*y(3) + x(5)*y(1)-x(1)*y(5);
end
If anyone can point me in the right direction it would be a huge help, thank you!
##### 2 CommentsShowHide 1 older comment
Alex Mcaulley on 2 Sep 2019
As an alternative of Adam's suggestion, see circshift

Rajani Mishra on 9 Sep 2019
As mentioned in the comment by Adam, Formula may be theoretically for infinite pattern but for arrays ‘x’ and ‘y’ size will be finite and can be found out using numel(x).
For mentioned pattern you can try below mentioned code:
z = 0;
n = numel(x);
for i=1:n-1
z = z + (x(i)*y(i+1) - x(i+1)*y(i));
end
z = z + x(n)*y(1) -x(1)*y(n);

Alex Mcaulley on 9 Sep 2019
Another possibility:
res = sum(x.*circshift(y,-1) - y.*circshift(x,-1))