Identifying missing data in a range - vba

Question

I'm looking to set up a VBA macro which automatically runs through a list of data in a column and picks out the missing values. The code I've got at the moment (see below) works, but I believe it starts looking in cell A2. I'd like it to start in B1.
How could I make this change?
Apologies, I'm a beginner to VBA!
Sub Check_Sequential()
Dim LR As Long, i As Long
LR = ActiveSheet.Range("A" & Rows.Count).End(xlUp).Row
ALR = ActiveSheet.Range("C" & Rows.Count).End(xlUp).Row + 1
x = 2
Cells(1, 3) = "Missing Numbers"
For i = 2 To LR
0
If Cells(i, 1) <> x Then
Cells(ALR, 3) = x
ALR = ActiveSheet.Range("C" & Rows.Count).End(xlUp).Row + 1
x = x + 1
Else
x = x + 1
End If
If Cells(i, 1) > x Then GoTo 0
If Cells(i, 1) = x Then
x = x + 1
End If
Next i
MsgBox "Done"
End Sub

Answer 1

In order to move from cell A2 to B1 for the search start range you would need the following changes to your code. This just addresses the area you defined your initial search:
Sub Check_Sequential()
Dim LR As Long, i As Long
LR = ActiveSheet.Range("B" & Rows.Count).End(xlUp).Row '// Changed from "A" to "B"
ALR = ActiveSheet.Range("C" & Rows.Count).End(xlUp).Row + 1
x = 2
Cells(1, 3) = "Missing Numbers"
For i = 1 To LR '// Chaged i to one to start at row 1 instead of 2
0
If Cells(i, 2) <> x Then '// Changed from i,1 to i,2
Cells(ALR, 3) = x
ALR = ActiveSheet.Range("C" & Rows.Count).End(xlUp).Row + 1
x = x + 1
Else
x = x + 1
End If
If Cells(i, 2) > x Then GoTo 0 '// Changed from i,1 to i,2
If Cells(i, 2) = x Then '// Changed from i,1 to i,2
x = x + 1
End If
Next i
MsgBox "Done"
End Sub
This should allow your code to work, although as a few others pointed out there are a few things you could look into to further develop your VBA skills:
Declare all variables with a type this will make your code more readable and stop them defaulting to variant (Eg ALR)
Avoid using 0 as a line name as "On Error GoTo 0" is how you restore to default error handling and this could be misleading
It is clearer to edit the cells value by using the ".value" property and it would work exactly the same in this case
It might be worth clearing your output column before writing as this code will stack the output on top of the older ones, not sure if this is intend though.
Although it was not a bad piece of code especially consider you classify yourself as a beginner.

Answer 2

My dear friend Steven Roman tells us that Cells is not an "official" collection or object... but it is really useful. Cells is equivalent to Application.Cells and Activesheet.Cells.
Cells(1,2) is equivalent to Range("B1"). I like Cells() better than Range() because it can take integer variables for row and column like your i and x, while Range doesn't like that.