VBA EXCEL : Why do I keep getting an error for yfunc ? Option Explicit Const XTO

Question

VBA EXCEL : Why do I keep getting an error for yfunc ?

Option Explicit

Const XTOL = 0.0001

Const YTOL = 0.0001

Const RGAS = 8314.33

Function NRsolve(xguess As Double, MAXITER As Integer) As Double

Dim iter As Integer

Dim xnew As Double

Dim xold As Double

Dim y As Double

Dim dydx As Double

Dim status As Integer

'Copy initial estimate into xold

'-------------------------------

xold = xguess

'For loop going from 1 to maximum allowable # of iterations

'----------------------------------------------------------

For iter = 1 To MAXITER

y = yfunc(xold) 'Calculate current value of y with function <yfunc>

If Abs(y) < YTOL Then Exit For 'Test if current value of y is less than tolerance

dydx = fprime(xold) 'Calculate current value of dydx with function <fprime>

xnew = xold - y / dydx 'Update xnew ... this is the NR algorithm

If (Abs(xnew - xold) < XTOL) Then Exit For 'Convergence test #2: Is x changing enough?

xold = xnew

Next

NRsolve = xnew

status = MsgBox(iter, vbOKOnly, "# of iterations")

End Function

Function yfunc(x As Double, T As Double, P As Double, A As Double, B As Double) As Double

Dim status As Integer

Dim A As Double

Dim T As Double

Dim B As Double

Dim P As Double

Dim alias As String

Call SRK_EOS_Params(alias, T, A, B)

yfunc = x ^ 3 - ((RGAS * T) / P) * x ^ 2 + (1 / P) * (A - B * RGAS * T - P * B ^ 2) * x - ((A * B) / P)

End Function

Function fprime(x As Double)

Call SRK_EOS_Params(alias, T, A, B)

Dim alias As String

Dim A As Double

Dim T As Double

Dim B As Double

Dim P As Double

fprime = 3 * x ^ 2 * (P / (RGAS * T)) ^ 2 - 2 * x * (P / (RT)) + (((A * P) / (RGAS ^ 2 * T ^ 2)) - ((B * P) / (RGAS * T)) - ((B * P) / (RGAS * T)) ^ 2)

fprime = dy / dx

End Function

Explanation / Answer

Your code has several errors.

First error bein with function yfunc.You have clearly declared 5 double variables in it as arguments. However , While calling yfunc, there is only one argument. Because of which you are getting error for Yfunc. Kindly provide necessary arguments.

Secondly you have made duplicate declaration for variable A as double. It is passed as an argument in yfunc & declared it seperately under the yfunc function. This is causing the code to produce errors.

I suggest you use Keyword F8 to debug your code line by line. You shall get some idea as in where your code is getting stuck.

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