Problem Statement

Given the n + 1 vertices x[0], y[0] .. x[N], y[N] of a simple polygon described in a clockwise direction, then the polygon's area can be calculated by: (Where abs returns the absolute value) Write a function/method/routine to use the the Shoelace formula to calculate the area of the polygon described by the ordered points:

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Let's start with the solution:

proc real Shoelace(N, X, Y);
int     N, X, Y;
int     S, I;
[S:= 0;
for I:= 0 to N-2 do
        S:= S + X(I)*Y(I+1) - X(I+1)*Y(I);
S:= S + X(I)*Y(0) - X(0)*Y(I);
return float(abs(S)) / 2.0;
];

RlOut(0, Shoelace(5, [3, 5, 12, 9, 5], [4, 11, 8, 5, 6]))