Comments on: Looping Through Months

By: Tushar Mehta

Tushar Mehta — Tue, 03 Feb 2009 09:06:00 +0000

It's kinda strange that no one used a data structure appropriate to the problem. For some reason, people seem to have decided to limit themselves to existing loop control structures and functions and operators.

To really do this correctly, introduce a structured data type that includes the year and the month. Add functions to support transfers to/from "real dates" as well as a "next month" function. One could also go all out and use a class module to create an appropriate type.

Realizing that I could "embed" the new data structure in an existing data type, I decided to be somewhat lazy. So, my "data structure" consists of YYYYMM embedded in a Long data type.

I need two functions, one to map a date to YYYYMM and another to return the next month

Function YYYYMM(aDate As Date)
YYYYMM = Year(aDate) * 100 + Month(aDate)
End Function
Function NextYYYYMM(YYYYMM As Long)
If YYYYMM Mod 100 = 12 Then NextYYYYMM = YYYYMM + 100 - 11 _
Else NextYYYYMM = YYYYMM + 1
End Function

Now, the code to implement Dick's requirement is relatively trivial

Sub MonthLoop()
Dim CurrYM As Long, LimYM As Long
CurrYM = YYYYMM(#10/1/2007#)
LimYM = YYYYMM(#9/30/2008#)
Do While CurrYM < = LimYM
Debug.Print CurrYM
CurrYM = NextYYYYMM(CurrYM)
Loop
End Sub

One can also use the DateSerial function to implement the above...kind of. Neither KeepItCool nor Jon pointed out a potential pitfall with DateSerial. If a date doesn't exist in a particular month, it will pick a date in the *next* month. So, if the current date is 31 Jan 2008, then 1 month out will be a date in March! Nonetheless, I had thought of using it in this case as below.

Function MonthsDiff(aDate As Date, Optional NbrMths As Integer = 1)
'It is the caller's responsibility to ensure that _
the day-of-month is meaningful in the desired month!
MonthsDiff DateSerial(Year(aDate), Month(aDate) + NbrMths, Day(aDate))
End Function
Sub MonthLoop1()
Dim I As Date
I = #10/1/2007#
Do While I < = #9/30/2008#
Debug.Print I
I = MonthsDiff(I)
Loop
End Sub

By: Dan Maxey

Dan Maxey — Mon, 02 Feb 2009 21:02:00 +0000

Usually I'm the one on the soapbox about elegant code, but gosh... is it really worth the effort on this one? Here is my 2 minute "beat it with a rock until it goes away" solution.

Sub loopmonths()

Dim startdate As Date
Dim enddate As Date
Dim startmonth As Long
Dim numberofmonths As Long
Dim firstmonth As Long
Dim count As Long
Dim thatmonth As Long

startdate = Now
enddate = startdate + 730 'this example is two years from now, but any future date will do

numberofmonths = DateDiff("m", startdate, enddate)
firstmonth = DatePart("m", startdate)
thatmonth = firstmonth
For count = 1 To numberofmonths
Debug.Print thatmonth 'Here is where your "do stuff" goes
thatmonth = thatmonth + 1
If thatmonth > 12 Then thatmonth = 1
Next

End Sub

By: Ryan

Ryan — Mon, 02 Feb 2009 19:10:00 +0000

so, i lied. I really need to not try to do this so quickly. I made a mistake in my assignment to nMonth. In order the advance the sequence the second line of code inside the loop should be below.

nMonth = ((nMonth mod 12) +1)

By: Ryan

Ryan — Mon, 02 Feb 2009 19:07:00 +0000

okay, last edit I promise. Change the while loop to a do..until loop and you’re money.

By: Ryan

Ryan — Mon, 02 Feb 2009 18:37:00 +0000

My solution assumes that you will not be looping through more than 1 year. Sorry for the repost, but apparently I must use != do indicate not equals when posting

Sub loopMonths
Dim dtStartDate as Date, dtEndDate as Date
Dim x as variant
Dim nMonth as Integer

dtStartDate =#10/1/2007#: dtEndDate = #9/30/2008#

nMonth = month(dtStartDate)
do while nMonth != month(dtEndDate)
x=CallFunction(nMonth)
nMonth = ((nMonth -1) mod 12) +1
Loop

end sub

By: Ryan

Ryan — Mon, 02 Feb 2009 18:34:00 +0000

My solution assumes that you will not be looping through more than 1 year

Sub loopMonths

Dim dtStartDate as Date, dtEndDate as Date
Dim x as variant
Dim nMonth as Integer

dtStartDate =#10/1/2007#: dtEndDate = #9/30/2008#

nMonth = month(dtStartDate)
do while nMonth month(dtEndDate)
x=CallFunction(nMonth)
nMonth = ((nMonth -1) mod 12) +1
Loop

end sub

By: Orlando

Orlando — Mon, 02 Feb 2009 00:13:00 +0000

Sub LoopMonths()

Dim i As Date
Dim dtStartDate As Date, dtEndDate As Date
Dim x As Variant

dtStartDate = #10/1/2007#: dtEndDate = #9/30/2008#
i = dtStartDate
Do
x = CallFunction(Month(i))
i = DateSerial(Year(i), Month(i) + 1, 1)
Loop While i <= dtEndDate

End Sub

By: Chris Akers

Chris Akers — Sun, 01 Feb 2009 21:29:00 +0000

Makes handy use of DateSerial normalizing date parts that are out-of-range:

Sub LoopMonths()

Dim intYear As Integer, intMonth As Integer, i As Integer
Dim dtStartDate As Date, dtEndDate As Date, dtLoopDate As Date
Dim x As Variant

dtStartDate = #10/1/2007#: dtEndDate = #9/30/2008#
intYear = Year(dtStartDate): intMonth = Month(dtStartDate)
dtLoopDate = dtStartDate: i = 0

Do
x = CallFunction(Month(dtLoopDate))
i = i + 1
dtLoopDate = DateSerial(intYear, intMonth + i, 1)
Loop While dtLoopDate <= dtEndDate

End Sub

By: Nigel

Nigel — Sun, 01 Feb 2009 19:56:00 +0000

Annoyingly, the ‘not equals’ sign <> in the branching statement iMonth (not equals) iPrevious isn’t rendered by the VB formatter.

By: Nigel

Nigel — Sun, 01 Feb 2009 19:52:00 +0000

Hmmm... Depends what you're trying to do with the code: list every month included between dtStartDate and dtEndDate?

That's got some complications: anything that uses DateAdd - either explicitly, or implied in a modulo division - runs into the bug that adding a month to the last day of February lands you on the 28th (or 29th) of March instead of the last day of March. Ditto for any month 30 days in length followed by one of 31 days. The workaround of hopping back one day from the first of the month is inelegant and needs a lot of coding.

Bluntly, I don't think there's any escape from the need to loop through each and every day between dtStartDate and dtEndDate. Not without a truly horrible perversion of the loop structure and rather kludgy 'magic number':

Sub LoopMonths()

Dim i As Long
Dim dtStartDate As Date, dtEndDate As Date
Dim iMonth As Integer, iPrevious As Integer

dtStartDate = "5 Oct 2007"
dtEndDate = "21 Sept 2008"

iPrevious = VBA.Month(dtStartDate)

Debug.Print iPrevious

For i = dtStartDate To dtEndDate

iMonth = VBA.Month(i)

If iMonth iPrevious Then

iPrevious = VBA.Month(i)
Debug.Print iPrevious
i = i + 27 'skip unnecessary iterations
End If

Next i

End Sub