Excel VBA: Looking for Advice Avoiding an Infinite Loop - vba

Question

Imgur Album with screens of worksheets: http://imgur.com/a/6rFWF
Long story short, I am writing an Excel VBA utility that will assign two types of security shifts (called coverages and weekend duties) to security staff members. Basically, I have a worksheet with all of the staff members and their various availability information in it (the top image in the imgur album) and a worksheet with all of the coverage dates in it (the bottom image in the imgur album). Note that I don't have an image of the weekend duty dates as it looks similar to the coverage dates (but with the Friday and Saturday shifts).
The utility basically assigns a random staff member to each date, checking to make sure it doesn't violate any of their availability requirements. Unfortunately, I realize that I am creating a large chance for an infinite loop to occur. In my own testing, there has only been 1 attempt out of around 15-16 that did not enter an infinite loop near the end. So I'm looking for your help to account for this so the utility doesn't eat itself.
Here is the "pseudo-code" for the procedure in question.
'Loop for Column A in the Coverage Slips sheet (image 2 in imgur album)
Do Until (CoverageRowNumber = LastCoverageSlipRow + 1)
Get a Random Staff Member by RNG
If staff member still needs more shifts (see Requirements columns) Then
If staff member does not have an "X" under the day of the week Then
If staff member does not have a matching date conflict Then
Assign the coverage
Increase CoverageRowNumber
End If
End If
End If
Loop
'Loop for Column B in the coverage slips sheet (image 2 in imgur album)
Do Until...
Same as the loop above
Loop
Edit: Disregard that I have the dates in two columns for now. I'll be fixing that once I solve the problem of this post...it's an easy fix and will cut the code almost in half.
The problem is that as the utility gets near the end of the list of dates, it often runs into the scenario where the only staff members left cannot sit that specific shift (whether because of day of the week or specific date). In the event that it runs into this scenario, I can see a couple of acceptable options (though I don't know how I'd go about programming them):
Undo all of the work that the utility did and start over until it can get lucky and find a solution that works. This would save me some time doing manual placements for the last few shifts but might take a very long time. Additionally, I'd have to store all of the original values and then paste them back into the spreadsheet anytime it starts over.
Simply stop assigning shifts and just exit the procedure. I will be able to manually place the last few shifts by moving a few people around. I sure is a lot less work than manually assigning 200 shifts by hand like I've been doing it the past few years.
Do you guys have any thoughts that could be of help here? I'm not even sure how I could have the procedure check to see if there are any available options or not, but either way there's got to be a way to detect (and deter) this infinite loop before it crashes the program.
Sorry for the novel, and thanks in advance for any help!
Edit: In an effort to provide a little more clarity, I figured I'd copy and paste the actual code below:
'------------------------------------------------------------'
'Create ws variables for each worksheet
Dim wsConflicts As Worksheet
Dim wsCoverageSlips As Worksheet
Dim wsWDSlips As Worksheet
Dim wsCoverageOutput As Worksheet
Dim wsWDOutput As Worksheet
'------------------------------------------------------------'
Public Function SetSheets()
'Assign the worksheets to the ws variables
Set wsConflicts = Worksheets("Conflicts")
Set wsCoverageSlips = Worksheets("Coverage Slips")
Set wsWDSlips = Worksheets("WD Slips")
Set wsCoverageOutput = Worksheets("Coverage Output")
Set wsWDOutput = Worksheets("WD Output")
'Display a message (debugging)
'MsgBox "The sheets have been assigned successfully"
End Function
'------------------------------------------------------------'
Public Function ColumnLetter(ColumnNumber As Integer) As String
Dim n As Long
Dim c As Byte
Dim s As String
n = ColumnNumber
Do
c = ((n - 1) Mod 26)
s = Chr(c + 65) & s
n = (n - c) \ 26
Loop While n > 0
ColumnLetter = s
End Function
'------------------------------------------------------------'
Sub AssignCoverages()
'Fill the ws variables
Call SetSheets
'Set the first and last row numbers
Dim FirstStaffMemberRow As Integer
FirstStaffMemberRow = 3
Dim LastStaffMemberRow As Integer
LastStaffMemberRow = wsConflicts.UsedRange.Rows.Count
'Count the number of required coverages and weekend duties
Dim RequiredCoverages As Integer
Dim RequiredWDs As Integer
For i = FirstStaffMemberRow To LastStaffMemberRow
RequiredCoverages = RequiredCoverages + wsConflicts.Range("B" & i).Value
RequiredWDs = RequiredWDs + wsConflicts.Range("C" & i).Value
Next i
'Display a message (debugging)
MsgBox "You currently have " & RequiredCoverages & " required coverages and " & RequiredWDs & " required weekend duties."
'Count the number of coverage slips and weekend duty slips
Dim FirstCoverageSlipRow As Integer
FirstCoverageSlipRow = 1
Dim LastCoverageSlipRow As Integer
LastCoverageSlipRow = wsCoverageSlips.UsedRange.Rows.Count
Dim NumCoverageSlips As Integer
NumCoverageSlips = (LastCoverageSlipRow - FirstCoverageSlipRow + 1)
Dim FirstWDSlipRow As Integer
FirstWDSlipRow = 1
Dim LastWDSlipRow As Integer
LastWDSlipRow = wsWDSlips.UsedRange.Rows.Count
Dim NumWDSlips As Integer
NumWDSlips = (LastWDSlipRow - FirstWDSlipRow + 1)
'Check to make sure there are enough required shifts for slips
If RequiredCoverages <> NumCoverageSlips Then
MsgBox "The number of shifts you require (Columns B & C on Conflicts sheet) does not match the number of slips you've entered. You have " & RequiredCoverages & " required coverages and " & NumCoverageSlips & " coverage slips. You have " & RequiredWDs & " required weekend duties and " & NumWDSlips & " weekend duty slips. Please correct this error and retry."
Exit Sub
Else
'Debugging
'MsgBox "The number of shifts you require (Columns B & C on Conflicts sheet) matches the number of slips you've entered. You have " & RequiredCoverages & " required coverages and " & NumCoverageSlips & " coverage slips. You have " & RequiredWDs & " required weekend duties and " & NumWDSlips & " weekend duty slips."
End If
'Massive loop to assign coverages to random staff members
Dim NumRemainingCoverages As Integer
NumRemainingCoverages = NumCoverageSlips
Dim SlipRowNumber As Integer
SlipRowNumber = FirstCoverageSlipRow
'Loop for Column A
Do Until (SlipRowNumber = LastCoverageSlipRow + 1)
'Get a random staff member row
StaffMemberRow = GetRandomStaffMemberRow(FirstStaffMemberRow, LastStaffMemberRow)
'Check to make sure the staff member has remaining required coverages
If wsConflicts.Range("B" & StaffMemberRow).Value > 0 Then
'Check to make sure the staff member can sit the day of the week
Dim CurrentDate As Date
CurrentDate = wsCoverageSlips.Range("A" & SlipRowNumber).Value
Dim CurrentDay As Integer
CurrentDay = Weekday(CurrentDate)
Dim CurrentDayColumn As String
If CurrentDay = 1 Then CurrentDayColumn = "D"
If CurrentDay = 2 Then CurrentDayColumn = "E"
If CurrentDay = 3 Then CurrentDayColumn = "F"
If CurrentDay = 4 Then CurrentDayColumn = "G"
If CurrentDay = 5 Then CurrentDayColumn = "H"
If CurrentDay = 6 Then CurrentDayColumn = "I"
If CurrentDay = 7 Then CurrentDayColumn = "J"
If wsConflicts.Range(CurrentDayColumn & StaffMemberRow).Value = "" Then
'Check to make sure the staff member does not have a date conflict
Dim ColumnNumber As Integer
Dim ColumnLetterText As String
Dim CoverageDateConflicts As Integer
CoverageDateConflicts = 0
For ColumnNumber = 11 To 20
ColumnLetterText = ColumnLetter(ColumnNumber)
Dim CoverageSlipDate As Date
If IsDate(wsConflicts.Range(ColumnLetterText & StaffMemberRow).Value) = True Then
CoverageSlipDate = wsConflicts.Range(ColumnLetterText & StaffMemberRow).Value
Else
CoverageSlipDate = DateValue("01/01/1900")
End If
If CurrentDate = CoverageSlipDate Then
CoverageDateConflicts = CoverageDateConflicts + 1
End If
Next ColumnNumber
If CoverageDateConflicts = 0 Then
'Assign the coverage
Dim BlankCoverageOutputRow As Integer
BlankCoverageOutputRow = wsCoverageOutput.UsedRange.Rows.Count + 1
wsCoverageOutput.Range("A" & BlankCoverageOutputRow).Value = wsConflicts.Range("A" & StaffMemberRow).Value
wsCoverageOutput.Range("B" & BlankCoverageOutputRow).Value = CurrentDate
'Reduce the staff member's required coverages by 1
Dim CurrentRequirements As Integer
CurrentRequirements = wsConflicts.Range("B" & StaffMemberRow).Value
wsConflicts.Range("B" & StaffMemberRow).Value = CurrentRequirements - 1
'Reduce the number of remaning coverages by 1
NumRemainingCoverages = NumRemainingCoverages - 1
'Increase the slip row number by 1
SlipRowNumber = SlipRowNumber + 1
'Message box for debugging
'MsgBox "Coverage Date (" & CurrentDate & ") assigned to " & wsConflicts.Range("A" & StaffMemberRow).Value & "."
End If 'End date check
End If 'End day check
End If 'End requirements check
Loop 'End loop for column A
End Sub
'------------------------------------------------------------'
Public Function GetRandomStaffMemberRow(FirstStaffMemberRow As Integer, LastStaffMemberRow As Integer)
'Pick a random number between the first staff member row and the last
Call Randomize
GetRandomStaffMemberRow = Int((LastStaffMemberRow - FirstStaffMemberRow + 1) * Rnd + FirstStaffMemberRow)
End Function

Answer 1

The question is too open for a detailed answer, so I try with some guidelines. I hope it helps.
I would use a class Solution with the following members:
Solution.ReadInputFromSheet() reads the table from the sheet into the class members
Solution.GenerateRandom() creates a new random solution. Try to find a balance between smart (add some logic to avoid totally random solutions) and speed (don't get stuck, exit after trying 10 or 50 random numbers that don't work), but speed is more important
Solution.Quality() As Double calculates the quality of the solution. For example a solution that is not valid returns 0, if Joe has 10 consecutive shifts returns 20, if the shifts are better distributed returns 100.
Solution.WriteOnSheet() write the data from the class members into the sheet.
Solution.Clone() As Solution() creates a new Solution instance with the same data
Make a cycle that creates a solution, checks if its quality is better than the best quality solution found so far, if it is better keep it, otherwise go and calculate another solution.
Set BestS = New Solution
BestS.ReadInputFromSheet
BestS.GenerateRandom()
Set S = New Solution
S.ReadInputFromSheet
For I = 1 To 10000
S.GenerateRandom()
If S.Quality() > BestS.Quality() Then Set BestS = S.Clone()
Next I
BestS.WriteOnSheet
Instead of 10000 you can use Timer to run it for a finite number of seconds, or make a button to interrupt it when you come back from lunch break.
A faster solution generator function is better than risking of getting stuck with one difficult (or impossible) solution.
For a smarter solution generator function I need more details on the rules.

Answer 2

So I went ahead and developed my own solution to this problem--it's not perfect and it's probably not the best way to handle the scenario. But it works, and it solved my problem in a matter of minutes instead of hours learning other methods.
Basically, I created two new "counter" variables. The first is FailedAttempts. Every time the procedure tries a random staff member but runs into a conflict, it increments FailedAttempts by 1. Every time the random staff member is a successful match (no conflicts), it resets FailedAttempts to 0. If at any time FailedAttempts = 100, it immediately exits the loop and starts over. In other words, if it tries 100 random staff members in a row without finding a match, I assume it's not going to find a match and just cut my losses.
The second variable, Assignments, is incremented by 1 every time that the procedure makes a successful assignment. When this number equals the number of shifts that the procedure is supposed to assign, it immediately exits the loop.
To do this, I had to use a couple of forbidden 'GoTo' commands (I wasn't sure how else to exit the loop. You can exit a For loop with Exit For but I believe this is invalid for Do While loops. I ended up only needing two GoTo's, one for exiting the loop and one to go back to the beginning of the procedure. I also made sure that the cells in the worksheet that change during the procedure are reset to their original state before it retries the assignment procedure.
I'll save everyone the trouble of reading through the extended version of the code, but in 'pseudo-code' form it looks like this:
Retry: 'Label for GoTo command
Do Until (CoverageRowNumber = LastCoverageSlipRow + 1)
Get a Random Staff Member by RNG
If staff member still needs more shifts (see Requirements columns) Then
If staff member does not have an "X" under the day of the week Then
If staff member does not have a matching date conflict Then
'Assign the coverage
'Increase CoverageRowNumber
Assignments = Assignments + 1
Else
FailedAttempts = FailedAttempts + 1
End If
Else
FailedAttempts = FailedAttempts + 1
End If
Else
FailedAttempts = FailedAttempts + 1
End If
If FailedAttempts > 100 Then
GoTo ExitLoop
End If
Loop
ExitLoop: 'Label for GoTo command
If Assignments <> NumCoverageSlips Then
GoTo Retry
End If
'Do rest of procedure
Again, there may be (and certainly is) a more elegant and "correct" way of accomplishing the task at hand. This method worked for me with the given environment. Thanks to those who provided solutions--even though I ended up going a different direction they provided great food for thought and helped me learn a bunch of new methods (especially the class idea from #stenci).
Thanks all.