How to modify the code for different type of input?

[Eric]

Does anyone have any suggestions on how to modify following code?
I would like to set a formula to determine the value in cell A1 rather than
manually type any value in this cell. In this case, if I set a formula to
return a value in cell A1, such as, in cell A1, =A2+A3, when I change any
value in cells A2 or A3, the return prime number in cell B1 cannot be
automatically updated. Does anyone have any suggestions?
Thank everyone for any suggestions
Eric

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Coding
===========================================

Select the sheet you want to use and right-click the sheet tab. Select view
code and paste this in exacly as below:-

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Cells.Count > 1 Or IsEmpty(Target) Then Exit Sub
If Target.Address = "$A$1" Then
If IsNumeric(Target) Then
On Error Resume Next
For x = 1 To 10000
Dim i As Long
If x < 2 Or (x <> 2 And x Mod 2 = 0) Or x <> Int(x) Then GoTo 100
For i = 3 To Sqr(x) Step 2
If x Mod i = 0 Then GoTo 100
Next
foundprime = foundprime + 1
If foundprime = Target.Value Then
Target.Offset(0, 1).Value = x
Exit Sub
100
End If
Next
On Error GoTo 0
End If
End If
End Sub

Close the VB editor and then enter 50 in a1 and the 50th prime will be
displayed in B1.

 

[Jim Thomlinson]

Give this a whirl... (it assumes that all of the precidint cells of A1 will
be on the same page as A1)

Private Sub Worksheet_Change(ByVal Target As Range)
Dim x As Long
Dim foundprime As Long

If Target.Cells.Count > 1 Or IsEmpty(Target) Then Exit Sub
If Not Intersect(Target, Range("A1").Precedents) Is Nothing Then
If IsNumeric(Target) Then
On Error Resume Next
For x = 1 To 10000
Dim i As Long
If x < 2 Or (x <> 2 And x Mod 2 = 0) Or x <> Int(x) Then GoTo 100
For i = 3 To Sqr(x) Step 2
If x Mod i = 0 Then GoTo 100
Next
foundprime = foundprime + 1
If foundprime = Target.Value Then
Range("B1").Value = x
Exit Sub
100
End If
Next
On Error GoTo 0
End If
End If
End Sub

 

[Eric]

Thank you for your suggestions
This line gets error
If Not Intersect(Target, Range("A1").Precedents) Is Nothing Then
Do you have any suggestions?
Thank you for any suggestions
Eric