Source code in the freebasic programming language

' version 18-08-2017
' compile with: fbc -s console

Type _point_
    As Double x, y
End Type

Function shoelace_formula(p() As _point_ ) As Double

    Dim As UInteger i
    Dim As Double sum

    For i = 1 To UBound(p) -1
        sum += p(i   ).x * p(i +1).y
        sum -= p(i +1).x * p(i   ).y
    Next
    sum += p(i).x * p(1).y
    sum -= p(1).x * p(i).y

    Return Abs(sum) / 2
End Function

' ------=< MAIN >=------

Dim As _point_ p_array(1 To ...) = {(3,4), (5,11), (12,8), (9,5), (5,6)}

Print "The area of the polygon ="; shoelace_formula(p_array())

' empty keyboard buffer
While Inkey <> "" : Wend
Print : Print "hit any key to end program"
Sleep
End