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Infinite loop

Posted on 2013-12-25
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Last Modified: 2013-12-25
Can someone please point out why this code gets stuck of an infinite loop.

Demo(31)

Sub Demo(intDay As Integer)
    Dim strTmpDate As String
    Dim i As Integer

    For i = 1 To 6
        strTmpDate = Format(intDay & "/" & DatePart("M", Date + 30 * i) & "/" & DatePart("YYYY", Date + 30 * i), "DD/MM/YYYY")
        Do While Not IsDate(strTmpDate)
            strTmpDate = Format(intDay - 1 & "/" & DatePart("M", Date + 30 * i) & "/" & DatePart("YYYY", Date + 30 * i), "DD/MM/YYYY")
            Debug.Print strTmpDate
        Loop
    Next
End Sub

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Question by:MacroShadow
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10 Comments
 
LVL 75

Expert Comment

by:käµfm³d 👽
ID: 39739372
My guess would be that strTmpDate does not contain a valid date. Have you tried stepping through the code?
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LVL 28

Author Comment

by:MacroShadow
ID: 39739376
Yes I have, the problem seems to be that although the code is supposed to subtract 1 day for every loop, it only does it once.
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LVL 14

Expert Comment

by:Alexander Eßer [Alex140181]
ID: 39739378
The variable strTmpDate is never changed in the inner while loop. Either you should alter the value of this variable for the checks and/or add an exit condition for the inner loop.
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Assisted Solution

by:Alexander Eßer [Alex140181]
Alexander Eßer [Alex140181] earned 800 total points
ID: 39739381
Yes I have, the problem seems to be that although the code is supposed to subtract 1 day for every loop, it only does it once.
Then you should add a new variable which holds the calculated day(s) to subtract and put something like "varDay = varDay - 1" in your inner loop...
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LVL 28

Author Comment

by:MacroShadow
ID: 39739384
Doesn't this line
strTmpDate = Format(intDay - 1 & "/" & DatePart("M", Date + 30 * i) & "/" & DatePart("YYYY", Date + 30 * i), "DD/MM/YYYY")

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change the strTmpDate variable? That's what I intended it to do.
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LVL 81

Accepted Solution

by:
byundt earned 1200 total points
ID: 39739390
Alex140181 is correct. You are subtracting 1 from a constant value. The following is working:
Sub Demo(intDay As Integer)
    Dim strTmpDate As String
    Dim i As Integer, j As Integer

    For i = 1 To 6
        strTmpDate = Format(intDay & "/" & DatePart("M", Date + 30 * i) & "/" & DatePart("YYYY", Date + 30 * i), "DD/MM/YYYY")
        j = 1
        Do While Not IsDate(strTmpDate)
            strTmpDate = Format(intDay - j & "/" & DatePart("M", Date + 30 * i) & "/" & DatePart("YYYY", Date + 30 * i), "DD/MM/YYYY")
            Debug.Print strTmpDate
            j = j + 1
        Loop
    Next
End Sub

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Expert Comment

by:Alexander Eßer [Alex140181]
ID: 39739391
What does the "Debug.Print strTmpDate" show?! Is it always the same value?!
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LVL 81

Expert Comment

by:byundt
ID: 39739393
I predict that you will get unexpected results, however, when the current day of the month is 12 or less. This is because VBA assumes date strings are mm/dd/yyyy unless the mm part is bigger than 12.
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LVL 14

Expert Comment

by:Alexander Eßer [Alex140181]
ID: 39739394
That's what I intended to say, byundt, but he should have gotten to that solution by himself ;-)
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LVL 81

Expert Comment

by:byundt
ID: 39739400
If you want to return the last day of the month for 6 consecutive months, then consider:
Sub LastOfMonths()
Dim dat As Date
Dim i As Integer
dat = Date
For i = 1 To 6
    dat = DateSerial(Year(dat), Month(dat) + 2, 0)
    Debug.Print Format(dat, "DD/MM/YYYY")
Next
End Sub

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