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 shoelace . p[][] res .
   sum = 0
   for i = 1 to len p[][] - 1
      sum += p[i][1] * p[i + 1][2]
      sum -= p[i + 1][1] * p[i][2]
   .
   sum += p[i][1] * p[1][2]
   sum -= p[1][1] * p[i][2]
   res = abs sum / 2
.
data[][] = [ [ 3 4 ] [ 5 11 ] [ 12 8 ] [ 9 5 ] [ 5 6 ] ]
shoelace data[][] res
print res