What's wrong with my polygon area calculator code

Hi, I wrote this code. It essentially calculates area of a polygon when given coordinates by shoelace formula. But it gives me wrong answer. Couldn't figure out why.

function AreaOfPolygon = gaussarea (xymatrice)
[r,c]=size(xymatrice);
x=xymatrice(:,1);
y=xymatrice(:,2);
calculation=0;
x1=xymatrice(1,1);
y1=xymatrice(1,2);
xLast=xymatrice(r,1);
yLast=xymatrice(r,2);
for i=1:r
    if i==1
        calculation=calculation+(x1*(y(i+1)-yLast));
    elseif i==r
        calculation=calculation+(xLast*(y1-y(i-1)));
    else
        calculation=calculation+(x(i)*y(i+1)-y(i-1));
    end
end
AreaOfPolygon=abs(calculation)/2
end

[John D'Errico]

I suppose there is a good reason why you do not just use polyarea?

Answer 1

[John D'Errico]

It seems to work for me. A simple case:

x = [0 0 1 1 0];
y = [0 1 1 0 0];
xymatrice = [x',y'];

AreaOfPolygon
AreaOfPolygon =
     1

polyarea(x,y)
ans =
     1

When I ran your code, it yields 1 as the result, which is indeed the area of that square.

So perhaps you can show an example where you think there is a problem. It may be a case where you have a complex polygon that is self intersecting. In that case

xymatrice = rand(10,2);

AreaOfPolygon =
       1.0281

polyarea(xymatrice(:,1),xymatrice(:,2))
ans =
     0.060961

So in this rather nasty case the two give different results.

plot(x,y,'o-')

But in that case, the "area" is a complicated thing.