Adding freshly created formula into new module - excel

Question

I've just created a brand new macro. Took function down below from internet (all credits goes to trumpexcel.com), code down below
Function CONCATENATEMULTIPLE(Ref As Range, Separator As String) As String
Dim Cell As Range
Dim Result As String
For Each Cell In Ref
Result = Result & Cell.Value & Separator
Next Cell
CONCATENATEMULTIPLE = Left(Result, Len(Result) - 1)
End Function
Then I proceed to extract data from various columns and into the one (my table is 20 rows x 10 columns)
Sub conact_data()
Dim i As Integer
For i = 2 To Cells(Rows.Count, "A").End(xlUp).Row
Cells(i, "M").Value = Cells(i, "A").Value & " " & _
Cells(i, "B").Value & " / " & Cells(i, "D").Value & "; "
Next i
End Sub
Thanks to that I've got combined data from column A, B and D, so its 20 rows. All I want to do now is to concatenate data from M2:M21 using CONCATENATEMULTIPLE function therefore I try various approach (I want this huge line in P2 cell) like :
Cells(2, 16).Value = CONCATENATEMULTIPLE (M2:M21, " ")
or
Range("P2") = "CONCATENATEMULTIPLE (M2:M21, " ")"
I don't really know how to apply that
Secondly, I'd like withdraw the Cells(i, "B").Value as percentage. Can I do that in one line like Cells(i, "B").NumberFormat="0.00%".Value (which is not working for me obviously) else I need to copy column B into another column with number format and then combine the new column, properly formatted instead of column B?
Thanks in advance

Answer 1

Percent format: Range("B" & i).NumberFormat = "0.00%"
CONCATENATEMULTIPLE
In VBA, CHR(32) = " "
In Excel, CHAR(32) = " "
With that being said...
'Value
Range("P2").Value = CONCATENATEMULTIPLE(Range("M2:M21"), CHR(32))
'Formula
Range("P2").Formula = "=CONCATENATEMULTIPLE(M2:M21, CHAR(32))"
You should really qualify all of your ranges with a worksheet
Say your workbook has 10 sheets. When you say Range("P2"), how do we (VBE) know what sheet you mean? Objects need to be properly qualified. Sometimes this is not a huge issue, but when you are working across multiple sheets, not qualifying ranges can lead to some unexpected results.
You can qualify with a worksheet a few ways.
Directly: ThisWorkbook.Sheets("Sheet1").Range("P2").Copy
Or use a variable like so
Dim ws as Worksheet: Set ws = ThisWorkbook.Sheets("Sheet1")
ws.Range("P2").Copy
Now there is no room for ambiguity (potential errors) as to the exact location of Range("P2")

Answer 2

First of all, remove your ConcatenateMultiple() code, and instead use Excel worksheet function CONCAT(), which takes a range and a delimiter as parameters.
Here is how you can handle the percentage issue and supply a default for non-numeric items. I've also cleaned up the way you reference your data range.
Sub concat_data()
Dim rngRow As Range, vResult As Variant
Const DEFAULT = 0 'Can also be set to a text value, eg. "Missing"
For Each rngRow In [A2].CurrentRegion.Rows
If IsNumeric(rngRow.Cells(, 4)) Then vResult = rngRow.Cells(, 4) * 100 & "%" Else vResult = DEFAULT
Range("M" & rngRow.Row) = rngRow.Cells(, 1) & rngRow.Cells(, 2) & "/" & vResult & ";"
Next
[M2].End(xlDown).Offset(1).Formula = "=CONCAT(M2:M" & [M2].End(xlDown).Row & ",TRUE,"" "")"
End Sub
I'm not a fan of hard-coding range references, like the [A2] or Range("M"), but will leave that for another time.