Recalculating Excel VBA Function on one sheet disrupts other sheets - excel

Question

I made a function that counts the number of items in a given month.
Column A is the month, and Column B is the number of items in that month.
Cell B1 has:
=countItems(A1)
Excel data:
Code:
Function countItems(month)
Application.Volatile
Dim count As Integer
If Not (month = 0) Then
count = 0
Do While Not (Cells(month.row + count + 1, 3) = 0)
count = count + 1
Loop
countItems = count
Else
countItems = ""
End If
End Function
I dragged the formula down from B1 to B500 and it works properly for every month. The formula returns nothing if there is no month in the corresponding A cell.
I have multiple sheets using the same formula on similarly-structured data sets.
Whenever the values in column B update for this Sheet 1, the other sheets will change too. However, Sheet 2 will update using Column C from Sheet 1.
If I have Sheet 2 recalculate, Sheet 1 will update using Column C from Sheet 2.
The function counts the number of items in a given month by checking how far down it can read in Column C before it finds the blank cell, indicating that the month is over.
Sheet 2 has 1 item in the first month, but it will still return 3 due to Sheet 1 having 3 items (counts Row 1 through 3 and stops at Row 4).
The second month of Sheet 2 begins on Row 3. But since the function is reading Column C from Sheet 1, it will run into the blank cell after counting 1 more item (counts Row 3 and stops at Row 4). Therefore no matter how many items are in Sheet 2 Month 2, it will return 1.
The function always uses the correct Column A, and only displays a number in Column B where there is date in Column A.
The consequence is that only 1 sheet can have the correct values, and doing that disrupts the other sheets.
I cannot solve this at the moment because I am new to VBA.
I have thought of making all of the function's cell references include a self-reference to the current cell's sheet, but I don't know how to do that and I don't know if it would work.
Edit: I couldn't make it work this way, but Application.Caller.Offset() with relative cell position worked as a solution. I am still wondering if there is a way to use absolute cell position though.
The sheets are not grouped together.

Answer 1

it's because there's a "time-space shift" between the range passed to the function and the range "felt" as the "caller" one by the function
you can see this behavior by modifying the function code as follows
Function countItems(month)
Application.Volatile
Dim count As Integer
Dim r As Range
Dim p As Variant, a As Variant
Set r = Application.Caller '<~~ retrieve the actual "calling cell" of the current function "instance"
p = r.Parent.Name
a = r.Address
MsgBox p & " vs " & month.Parent.Name & vbCrLf & a & " vs " & month.Address '<~~ compare the function "calling cell" vs the "passed cell"
If Not (month = 0) Then
count = 0
Do While Not (Cells(month.Row + count + 1, 3) = 0)
count = count + 1
Loop
countItems = count
Else
countItems = ""
End If
End Function
and you'll see msgboxs prompts showing you differences between the function "calling cell" and "passed cell" addresses and/or sheets
so to avoid this behavior you could rely on the "calling range only", like follows:
Option Explicit
Function countItems(month)
Application.Volatile
Dim r As Range
Set r = Application.Caller '<~~ retrieve the actual "calling cell" of the current function "instance"
'the "calling cell" is the one with the called function in its formula, i.e. in column "B" as per your data structure. then ...
If Not IsEmpty(r.Offset(, -1)) Then '<~~ ... the column with dates are one column to the left, and ...
Do While Not IsEmpty(r.Offset(countItems + 1, 1)) '<~~ ... the values to be checked for are one column to the right
countItems = countItems + 1
Loop
End If
End Function