Calculate area under curve (Integration)

Toady · April 2, 2007

;=======================================================
; Function: _AreaUnderCurve
; Author: Toady
; 
; Uses Trapezoidal rule to give move accurate results
; Action: Performs an evaluated calculus integral over
;         a given range on a single variable expression
;         Read NOTES below before using
;
; $expr: String - Correctly formatted math expression
; $var: String - Independent variable
; $lower_limit: Float - starting location
; $upper_limit: Float - ending location
; $numIntvs: Integer - Number of trapezoids under curve
;                    - Higher number = more accurate
; $Places: Integer - Number of places to round result
; Returns: Area in float
;
; NOTE: upper_limit must be greator than lower_limit
; NOTE: Does not take into account integration x-intercepts
;       Therefore must integrate on those intercepts 1 by 1
; NOTE: A higher $numIntvls is more accurate but too high will
;       take longer to calculate
;
; Ex 1) Calculate Pi
;    4*_AreaUnderCurve("Sqrt(1-((x)^2))","x",0,1,10000)
; Ex 2) Quadatic polynomial 2(x)^2 + 7x + 3
;   _AreaUnderCurve("2*(x)^2 + 7*x + 3","x",0,1,10000)
; Ex 3) Logorithm
;   _AreaUnderCurve("Log(x)","x",0,1,10000)
;=======================================================

Func _AreaUnderCurve($expr,$var,$lower_limit,$upper_limit,$numIntvs,$Places = 4)
    If Not StringInStr($expr,$var) Then
        Return "Variable not found in expression"
    ElseIf Not IsInt($numIntvs) Or $numIntvs < 0 Then
        return "Interval is < 0 or not an integer"
    EndIf   
    Local $dx = ($upper_limit-$lower_limit)/$numIntvs
    Local $area, $x, $y = 0 
    If $upper_limit < $lower_limit Then
        return "upper limit must be greater than lower limit"
    ElseIf $lower_limit >= 0 Or $Upper_Limit <= 0 Then
        $y = Execute(StringReplace($expr,$lower_limit,"$x"))
        For $i = 1 To $numIntvs - 1
            $x = ($i*$dx)
            $y += 2*Execute(StringReplace($expr,$var,"$x"))
        Next
        $y += Execute(StringReplace($expr,$upper_limit,"$x"))
        return Round(Abs($dx*$y)/2,$Places)
    ElseIf $lower_limit < 0 And $upper_limit >= 0 Then
        Local $y_Neg, $y_Pos = 0
        $y_Neg = Execute(StringReplace($expr,0,"$x"))
        For $i = 1 To $numIntvs - 1
            $x = ($i*((Abs($lower_limit))/$numIntvs))
            $y_Neg += 2*Execute(StringReplace($expr,$var,"$x"))
        Next
        $y_Neg += Execute(StringReplace($expr,$lower_limit,"$x"))
        $y_Pos = Execute(StringReplace($expr,0,"$x"))
        For $i = 1 To $numIntvs - 1
            $x = ($i*(($upper_limit)/$numIntvs))
            $y_Pos += 2*Execute(StringReplace($expr,$var,"$x"))
        Next
        $y_Pos = Execute(StringReplace($expr,$upper_limit,"$x"))
        return Round(Abs(($y_Pos+$y_Neg)*$dx)/2,$Places)
    EndIf
EndFunc ;_AreaUnderCurve()==>

Had to write this script for one of my courses in college.

It performs an integration of an expression over a specified interval and returns

the area under the curve.

Calculate Pi: _AreaUnderCurve("4*Sqrt(1-((x)^2))","x",0,1,10000)

Polynomial: _AreaUnderCurve("2*(x)^2 + 7*x + 3","x",0,1,10000)

Logorithm: _AreaUnderCurve("Log(x)","x",0,1,10000)

Edited April 4, 2007 by Toady

jvanegmond · April 2, 2007

Good job. I really had no idea how to do this programming-wise.. :whistle:

tmo · April 2, 2007

Good job. I really had no idea how to do this programming-wise..

just use the definition of an integral. like he did

jvanegmond · April 2, 2007

just use the definition of an integral. like he did

I would never add values when you take tiny steps in a formula, lol. I'd just calculate it the way everyone does it. Edited April 2, 2007 by Manadar

Toady · April 2, 2007

I would never add values when you take tiny steps in a formula, lol. I'd just calculate it the way everyone does it.

Why not? rofl

jvanegmond · April 3, 2007

Why not? rofl

:|

Ok... Integral for f(x) = x between 0 and 1 .. We all know this is 1. Now, what you do is this:

Result = 0