simpspns rule in excel vba

[fionbaire]

hi there, very new to excel vba so strugglinh a bit. I am getting an error 13 type mismatch when running the code below:

Function SimpsonInt(n As Integer, dLower As Double, dUpper As Double, sEq As String) As Double
'This function calculates the area under the curve y(x) defined by sEq from
'x=dLower to x=dUpper using Simpson's rule with n intervals
'Note: the independent variable in sEq must be "_x_"
'
'Local variables
Dim h As Double, sum As Double, term As Double
Dim x As Double
Dim i As Integer
'Do error checking
If n = 0 Then
simpson = 0#
MsgBox "Sorry # of intervals has to be > 0"
Exit Function
End If
n = n * 2
h = (dUpper - dLower) / n
x = dLower
sum = 0#
For i = 1 To n Step 2
term = y(sEq, x) + 4 * y(sEq, x + h) + y(sEq, x + 2 * h)
sum = sum + term
x = x + 2 * h
Next i
SimpsonInt = sum * h / 3
End Function
Function y(sEq As String, x As Double) As Double
y = Evaluate(Replace(sEq, "_x_", x))
End Function

all inputs are coming froa a userform through textboxes and basically i am trying to replace the letter x in the entered string e.g x^3 changes to perhaps 3^3 then it gets evaluated etc and you end up with an integral value but im losing the faith, any ideas please?

 

[Andrew Poulsom]

Welcome to MrExcel.

On which line do you get the error?

 

[fionbaire]

Welcome to MrExcel.

On which line do you get the error?

im getting the error on the first use of the y function

y = Evaluate(Replace(sEq, "_x_", x))

 

[Andrew Poulsom]

How are you calling the function SimpsonInt?

 

[fionbaire]

How are you calling the function SimpsonInt?

ive been trying to call the function from a button in the userform code

Private Sub CommandButton1_Click()
Call SimpsonInt(TextBox1.Value, TextBox2.Value, TextBox3.Value, TextBox4.Value)
End Sub

 

[Andrew Poulsom]

Shouldn't it be?

Call SimpsonInt(CInt(TextBox1.Value), CDbl(TextBox2.Value), CDbl(TextBox3.Value), TextBox4.Value)

 

[baziili]

hi,

thanks a lot for your integral function, but I've got an issue with "x^2":
the result for this expression is zero : SimpsonInt(20, 1, 9, x ^ 2 + 3 * x) = 0 (i've also tried to write SimpsonInt(20, 1, 9, x * x + 3 * x) - but also = 0) , but should be 362.666
the problem is when "x" is written twice or more time in the same expression (x^3+2*x^2+3*x)...

please....can somebody help with this?

thanks in advance!!

 

[shg]

The function expects a string.

Try SimpsonInt(20, 1, 9, "x ^ 2 + 3 * x")

 

[baziili]

The function expects a string.

Try SimpsonInt(20, 1, 9, "x ^ 2 + 3 * x")

with a string doesn't work in generally "run-time error '13' Type mismatch"

 

[shg]

The function signature requires the equation to be passed as a string:

Function SimpsonInt(n As Integer, dLower As Double, dUpper As Double, [COLOR="#FF0000"]sEq As String[/COLOR]) As Double

However, the substitution requires the variable to be "_x_", not "x"