Loop through Column restarting when value changes - vba

Question

I've found a couple threads with similar titles but weren't really what I am looking to do. What I'm trying to do is go through the list of numbers in Col A, and calculate the time difference using NetworkDays for the first instance the number appears in Col B ' Received On ' and the last instance the number appears in Col C ' Processed On '. After the NetworkDays calculation is done I'd like to put that value repeating in Col D on every respective row. The number of times a value will appear in Col A constantly varies, and Col A itself is several thousand lines long and constantly growing. Once that is done I need to loop through all the other different sets of numbers in Col A and repeat the process. As an example, ***39430 first appears on Row 2 and last appears on Row 7. Using Networkdays(B2,C7) gives 11 days, and so forth. After that move onto ***39383. Sample below.
Sample data
Below is the code I have so far. From the sample above I have to put a blank row under ***39430 in order to get the code to work, otherwise it just continues on to the bottom of the list and calculates that difference (not what I want obviously). What I'm stumped on is how to tell the loop to restart whenever the value changes in Col A and then continue on. I suspect it might be something close to Do Until ActiveCell.Value <> Activecell.Offset(-1,0).Value but I can't quite figure it out. Also how to get the Networkdays value to repeat on every respective row.
Dim counter As Integer
Dim CycleTime As Long
counter = 0
Do Until ActiveCell.Value = ""
counter = counter + 1
ActiveCell.Offset(1, 0).Select
Loop
'Gives the number of rows to offset
MsgBox counter
'Shows the correct number of days difference
MsgBox WorksheetFunction.NetworkDays(Range("B2"), Range("B2").Offset(counter - 1, 1))
CycleTime = WorksheetFunction.NetworkDays(Range("B2"), Range("B2").Offset(counter - 1, 1))
Range("D2").Value = CycleTime
Any help would be greatly appreciated. Thanks in advance.
Update
After using the code provided for a couple of weeks I've noticed a complication that I had not thought of before. Previously, I had thought that there was always only one output doc for each input doc (not considered in scope of original question), however as shown in Sample-New image in the top box there can be more than one output doc per input doc. For the new screenshot below I've included two additional columns, Col. C 'Output Doc #' and Col. D 'Output Doc Created On'. What I'd like to be able to do, amending the code that #YowE3K provided below, is to nest another loop that goes through Col. D 'Output Doc #' and uses NetworkDays to calculate the difference from B1 and D1 for the first group, and then B1 and D8 for the second group. As it is now, the code isn't written to handle the change and calculates everything as shown in Column F, with the ideal code resulting in Column G. The second box (in dark blue) shows a typical example where the code performs perfectly. Loops are something I'm struggling with to understand and not really sure how to even take a stab at this. Any comments to the code in a response would be very helpful. Thanks in advance.
Sample - New

Answer 1

The following code loops using endRow as the loop "counter".
startRow is set to the row containing the start of the current "Doc Number", and endRow is incremented until it is pointing at the last row for that "Doc Number".
Once endRow is pointing at the correct place, CycleTime is calculated and written to column D of each row from startRow to endRow. startRow is then set to point to the beginning of the next "Doc Number".
The loop ends when a blank cell is found in column A.
Sub Calc()
Dim startRow As Long
Dim endRow As Long
Dim CycleTime As Long
startRow = 2
endRow = 2
Do
If Cells(startRow, "A").Value <> Cells(endRow + 1, "A").Value Then
CycleTime = WorksheetFunction.NetworkDays(Cells(startRow, "B"), Cells(endRow, "C"))
Range(Cells(startRow, "D"), Cells(endRow, "D")).Value = CycleTime
startRow = endRow + 1
End If
endRow = endRow + 1
If Cells(endRow, "A").Value = "" Then
Exit Do
End If
Loop
End Sub
Edited to keep track of the first and last "Approved" record, and only update column D if one is found:
Sub Calc()
Dim startRow As Long 'Start of the Doc Number
Dim firstRow As Long 'First "approved" row
Dim lastRow As Long 'Last "approved" row
Dim endRow As Long 'End of the Doc Number
Dim CycleTime As Long
startRow = 2
endRow = 2
firstRow = -1
lastRow = -1
Do
If Cells(endRow, "Q").Value = "Approved" Then
'Found an "Approved" record
'Set the first row if not already set
If firstRow = -1 Then
firstRow = endRow
End If
'Set the last row (will replace this if we find another record)
lastRow = endRow
End If
If Cells(startRow, "A").Value <> Cells(endRow + 1, "A").Value Then
If firstRow > 0 Then ' (If it is -1 then we never found an "Approved" record)
CycleTime = WorksheetFunction.NetworkDays(Cells(firstRow, "B"), Cells(lastRow, "C"))
Range(Cells(startRow, "D"), Cells(endRow, "D")).Value = CycleTime
End If
'Set up for next Doc Number
startRow = endRow + 1
firstRow = -1
lastRow = -1
End If
'Go to next row
endRow = endRow + 1
'Exit when we hit a blank Doc Number
If Cells(currentRow, "A").Value = "" Then
Exit Do
End If
Loop
End Sub