Page 250 - Excel for Scientists and Engineers: Numerical Methods

CHAPTER 10 ORDINARY DIFFERENTIAL EQUATIONS. PART I 227

I
Dim T As String, temp As String
Dim NRepl As Integer, J As Integer
Dim dummy As Double
T = FormulaText
'First, do substitution of all instances of x address with value
NRepl = (Len(T) - Len(Application.Substitute(T, XRef, I"'))) I Len(XRef)
For J = NRepl To 1 Step -1
temp = Application.Substitute(T, XRef, XValue & " ", J)
On Error GoTo ErrorHandlerl
dummy = Evaluate(temp)
T = temp
ptl: Next J
'Then do substitution of all instances of y address with value
NRepl = (Len(T) - Len(Application.Substitute(T, YRef, ""))) I Len(YRef)
For J = NRepl To 1 Step -1
temp = Application.Substitute(T, YRef, YValue 81 " ", J)
On Error GoTo ErrorHandlerZ
dummy = Evaluate(temp)
T = temp
pt2: Next J
result = Evaluate(T)
Exit Sub
'ERROR HANDLER ROUTINES.
ErrorHandlerl :
'Trappable error number 13 (Type mismatch) is expected.
If Err.Number = 13 Then
On Error GoTo 0 'Disable the error handler.
Resume ptl 'and continue execution.
Else
End 'Some other error, so quit completely
End If
ErrorHandlerZ:
If Err.Num ber = 13 Then
On Error GoTo 0
Resume pt2
Else
End
End If
End Sub
Figure 10-7. Custom function for Runge-Kutta calculation.
(folder 'Chapter 10 Examples', workbook 'ODE Examples', module 'RungeKuttal')
In Figure 10-8, the custom function is applied to the same first-order reaction
kinetics problem that was calculated on a worksheet in the preceding sections.
The formulas in cells C6 and D7 are, respectively,
=-k*D6

and = R u nge 1 (A6, D6, C6, A7-A6)