IFERROR in in this macro? - excel

Question

The problem is that when I change the value in I16 or I17 I get an error. How
can I prevent this error from happening?
I check in I16 and I17 for the sheetnames, because every week an updated sheet comes available.
Thank you
Sub Compare()
Call compareSheets(range("I16").Value, range("I17").Value)
End Sub
Sub compareSheets(Sofon As String, Sofon2 As String)
Dim mycell As range
Dim mydiffs As Integer
For Each mycell In ActiveWorkbook.Worksheets(Sofon2).range("M:M")
If Not mycell.Value = ActiveWorkbook.Worksheets(Sofon).Cells(mycell.Row, mycell.Column).Value Then
mycell.Interior.Color = vbYellow
mydiffs = mydiffs + 1
End If
Next
MsgBox mydiffs & " differences found in Column M (Salesman)", vbInformation
ActiveWorkbook.Sheets(Sofon2).Select
End Sub

Answer 1

Just to show what I was thinking.
I agree with puzzlepiece87 that On Error is finicky, but with something this simple I would use it to avoid the excess loops.
Sub compareSheets(Sofon As String, Sofon2 As String)
Dim mycell As Range
Dim mydiffs As Integer
On Error GoTo nosheet
For Each mycell In ActiveWorkbook.Worksheets(Sofon2).Range("M:M")
If Not mycell.Value = ActiveWorkbook.Worksheets(Sofon).Cells(mycell.Row, mycell.Column).Value Then
mycell.Interior.Color = vbYellow
mydiffs = mydiffs + 1
End If
Next
MsgBox mydiffs & " differences found in Column M (Salesman)", vbInformation
ActiveWorkbook.Sheets(Sofon2).Select
Exit Sub
nosheet:
If Err.Number = 9 Then
MsgBox "One or both sheets do not exist"
Else
MsgBox Err.Description
End If
End Sub

Answer 2

Since the OP wanted an ISERROR type of solution, I decided to post the code which incorporates a function to check if a sheet exists in a workbook. The concept is similar to answers already posted, but it keeps any On Error statements strictly inside the function and uses regular code blocks to evaluate errors.
Sub Compare()
Dim bGo As Boolean
Dim s1 As String, s2 As String
s1 = Range("I16").Value2
s2 = Range("I17").Value2
If Not WorksheetExist(s1) Then
bGo = False
MsgBox "The sheet " & s1 & " does not exist in this workbook."
End If
If Not WorksheetExist(s2) Then
bGo = False
MsgBox "The sheet " & s2 & " does not exist in this workbook."
End If
If bGo Then compareSheets s1, s2
End Sub
Function WorksheetExist(sName As String, Optional wb As Workbook) As Boolean
Dim wbCheck As Workbook, ws As Worksheet
If wb Is Nothing Then Set wbCheck = ThisWorkbook Else: Set wbCheck = wb
On Error Resume Next
Set ws = wbCheck.Sheets(sName)
On Error GoTo 0
If Not ws Is Nothing Then WorksheetExist = True Else: WorksheetExist = False
End Function
And, based on #puzzlepiece87 methodology, here is an improved WorksheetExist Function that eliminates of On Error statements altogether.
Function WorksheetExist(sName As String, Optional wb As Workbook) As Boolean
Dim wbCheck As Workbook, ws As Worksheet
If wb Is Nothing Then Set wbCheck = ThisWorkbook Else: Set wbCheck = wb
WorksheetExist = False
For Each ws In wbCheck.Worksheets
If ws.Name = sName Then
WorksheetExist = True
Exit For
End If
Next
End Function

Answer 3

You could use something similar to this to call compareSheets. It will warn you if either of the two ranges do not correspond to sheet names and won't call compareSheets if true.
Dim Sheet1 As Worksheet
Dim boolI16SheetCheck As Boolean
Dim boolI17SheetCheck As Boolean
boolI16SheetCheck = False
boolI17SheetCheck = False
For Each Sheet1 in ActiveWorkbook.Worksheets
If Sheet1.Name = Activesheet.Range("I16").Value Then boolI16SheetCheck = True
If Sheet1.Name = Activesheet.Range("I17").Value Then boolI17SheetCheck = True
If boolI16SheetCheck = True And boolI17SheetCheck = True Then
Call compareSheets(range("I16").Value, range("I17").Value)
Exit Sub
End If
Next Sheet1
If boolI16SheetCheck = False Then
If boolI17SheetCheck = False Then
Msgbox "Neither I16 nor I17 sheet found."
Else
Msgbox "I16 sheet not found."
End If
Else
Msgbox "I17 sheet not found."
End If
End Sub