Value of counter is not updating in a For Loop - excel

Question

I have data in B Column of an excel file. I have made a loop that If the value in B Cell is greater than a particular (len1) value, then the code puts the Cell (Value-Len1) value in a new cell at the end of the rows.
I increment the counter as lastrow = lastrow+1 everytime when the row is added. Now here is the problem. Iniitially in the input file I had 122 set of data. But by the time the For loop finishes the value of lastrow becomes 160, but the loop exits at 122. WHY?? Any Help will be appreciated.
For i = 1 To lastrow Step 1
If Range("B" & i).Value > len1 Then
Range("A" & lastrow + 1).Value = Range("A" & i).Value
Range("B" & lastrow + 1).Value = Range("B" & i).Value - len1
Range("B" & i).Value = len1
lastrow = lastrow + 1
End If
Next

Answer 1

To get the behaviour you want you need a while loop (or do loop)
i = 1
While i <= lastrow
If Range("B" & i).Value > len1 Then
lastrow = lastrow + 1
Range("A" & lastrow).Value = Range("A" & i).Value
Range("B" & lastrow).Value = Range("B" & i).Value - len1
Range("B" & i).Value = len1
End If
i = i + 1
Wend
I tested it with the sub below:
Sub LoopBeyondLastRow()
Dim i As Long, lastrow As Long, len1 As Long
len1 = 10
With ThisWorkbook.Sheets("Sheet1")
lastrow = .Cells(Rows.Count, "B").End(xlUp).Row
i = 1
While i <= lastrow
If .Range("B" & i).Value > len1 Then
lastrow = lastrow + 1
.Range("A" & lastrow).Value = .Range("A" & i).Value
.Range("B" & lastrow).Value = .Range("B" & i).Value - len1
.Range("B" & i).Value = len1
End If
i = i + 1
Wend
End With
End Sub
Please note the following:
Inside the loop I incremented lastrow first and then used it in the following 2 statements (to reduce the number of addition operations)
In my test code I added With ThisWorkbook.Sheets("Sheet1") to fully qualify all ranges. Not doing this is the source of many bugs that are sometimes very difficult to pinpoint. One should get in the habbit of never to write Range or Cells without a little . before them.

Answer 2

A faster method would be to export the range values to array and then do the comparision. Store the final output into a temp array and write it back to the worksheet.
If you want to follow your approach then is this what you are trying? I have commented the code so you should not have a problem understanding it. Basically you need 2 loops if you want to recheck the data that you are adding at the end of the row.
Option Explicit
Sub Sample()
Dim ws As Worksheet
Dim ComparisionValue As Long
Dim countOfMatchedValues As Long
Dim lRow As Long
Dim i As Long
Dim outputRow As Long
Dim rng As Range
'~~> Change this to the relevant sheet
Set ws = Sheet1
'~~> Change this to the relevant
'~~> comparision value
ComparisionValue = 122
With ws
'~~> Start an indefinite loop
Do
'~~> Find last row
lRow = .Range("A" & .Rows.Count).End(xlUp).Row
'~~> Fix the output row for the new data
outputRow = lRow + 1
'~~> Check if there are any matches for your condition
countOfMatchedValues = Application.WorksheetFunction.CountIf( _
.Range("B1:B" & lRow), ">" & ComparisionValue)
'~~> If not then exit loop
If countOfMatchedValues = 0 Then Exit Do
'~~> Do your stuff
For i = 1 To lRow
If .Range("B" & i).Value > ComparisionValue Then
.Range("A" & outputRow).Value = .Range("A" & i).Value
.Range("B" & outputRow).Value = .Range("B" & i).Value - ComparisionValue
.Range("B" & i).Value = ComparisionValue
outputRow = outputRow + 1
End If
Next i
Loop
End With
End Sub
In Action

Answer 3

for loops use a pre-defined number of iterations. For an unknown number of iterations you need to use a while loop.
Your code uses the value of lastRow at the time it was interpreted, and is never updated again.
This is similar to:
lastRow = 1
Debug.Print lastRow
lastRow = lastRow + 1
Debug.Print lastRow
You will see:
1
2
and not:
2
2
because once the first Debug statement has been executed, changing the value of lastRow doesn't affect this particular output anymore.

Answer 4

Test the next code, please:
Sub TestLoopAddedRowsInclusive()
'..... your code defining LastRow and len1
Dim lastRInit As Long
lastRInit = LastRow
For i = 1 To Rows.count
If Range("B" & i).Value = "" And i >= lastRInit Then Exit For
If Range("B" & i).Value > len1 Then
Range("A" & LastRow + 1).Value = Range("A" & i).Value
Range("B" & LastRow + 1).Value = Range("B" & i).Value - len1
Range("B" & i).Value = len1
LastRow = LastRow + 1
End If
Next
End Sub