I don't think that is quite the right formula.
Using the correct formula, your function would be very similar:
function area = shoelace(x,y)
n = length(x);
xp = [x; x(1)];
yp = [y; y(1)];
area = 0;
for i = 1:n
area = area + det([xp(i), xp(i+1); yp(i), yp(i+1)]);
end
area = 1/2*abs(area);
Then you can call this function with two vectors containing the vertices of any n-gon:
x = rand(100,1);
y = rand(100,1);
shoelace(x,y)
You can check the result by comparing it with MATLAB's built-in function "polyarea"
