(49g 50g) Shoelace algorithm

[Thomas Klemm]

Alternatively we can use

\(A=\frac{1}{2}\sum_{i=1}^{n}x_{i}(y_{i+1}-y_{i-1})\)

where \(y_{n+1}=y_{1}\) and \(y_{0}=y_{n}\).

(xs ys -- area)

Code:

« DUP TAIL OVER HEAD + SWAP
  REVLIST DUP TAIL SWAP HEAD + REVLIST
  - * ∑LIST 2 /
»

The coordinates of the polygon have to be entered as two separate lists xs and ys.
There might be better ways to rotate the elements of ys left and right.

Kind regards
Thomas