Permutations in Visual Basic -- Generating All Possible Combinations


Explanation and Usage

It is very useful to have all possible combinations of a string. For example: In a search engine, if the user types "mp3 rock hard," the engine should be smart enough to search for all combinations of the user input; otherwise, it will not be a powerful searching tool. These combinations would look like the following:

"mp3 rock hard"
"mp3 hard rock"
"rock mp3 hard"
"rock hard mp3"
"hard mp3 rock"
"hard rock mp3"

The Idea

I've decided to code a function that would generate all possible combinations out of a given string.

The Research

First of all, I tried searching about it in CodeGuru's forums. But all I found was that there was a function that could do that job, but in VC++. And because I know nothing of C & Cia, I didn't even look at it.

The Decision

So, what could I do? I could develop my own algorithm in Visual Basic. And that is what I've decided to do. First of all, while developing the algorithm, I asked my whole family and my neighbor (a judge) for help with the algorithm; no one could get even close. As time passed, after rewriting the whole thing from scratch for more than 20 times, I was getting closer and closer. With significant but buggy outputs, I've noticed that I would have to develop (for the 1st time in my little programmer life) a callback function. And that worked! The function generates all the 40,320 combinations of an 8-element string (in other words, "1 2 a 4 b 6 c 8") in 1.34 seconds.

The Result

' Generates all combination possibilities out of a string
Public Function PermuteString(ByVal Ztring As String, _
       Optional Base As String = "") As String

Dim TmpStrArray() As String, I As Long

' If there's only 1 element, then
If InStr(1, Ztring, " ", vbTextCompare) = 0 Then
    PermuteString = Base & " " & Ztring & vbCrLf
    Exit Function
End If

' If more than 1 element: split elements in one array of elements
TmpStrArray = Split(Ztring, " ", , vbTextCompare)

If Base = "" Then
    ' Loop trough each element and do callbacks to permute again
    For I = LBound(TmpStrArray) To UBound(TmpStrArray)
            PermuteString = PermuteString & _
            PermuteString(ReturnAllBut(TmpStrArray, I),_
    ' Loop trough each element and do callbacks to permute again
    For I = LBound(TmpStrArray) To UBound(TmpStrArray)
            PermuteString = PermuteString & " " & _
            PermuteString(ReturnAllBut(TmpStrArray, I), _
            Base & " " & TmpStrArray(I))
End If

End Function

' Return all items in a array but 1
Public Function ReturnAllBut(ByRef Arrai() As String, _
       But As Long) _
       As String
    Dim I As Long
    For I = LBound(Arrai) To UBound(Arrai)
        If I <> But Then
           ReturnAllBut = ReturnAllBut & Arrai(I) & " "
        End If
    ReturnAllBut = RTrim(ReturnAllBut)
End Function

To Test the Speed, Use This

Public Sub TestPermutationSpeed()

Dim I As Long   ' Used in loops
Dim Nou         ' Used to calc delay
Dim NumberOfElements As Long
                ' Used to calc number of elements in PermutyString

Const NumberOfPermutations = 1
      ' Number of permutations to be done
Const PermutyString = "A B C D E F G H"
      ' String to be permuted

NumberOfElements = UBound(Split(PermutyString, " ", , _
                          vbTextCompare)) + 1
                   ' Calc number of elements in PermutyString

Nou = Timer  ' Get start time
For I = 1 To NumberOfPermutations
    ' Loop #NumberOfPermutations times
    PermuteString (PermutyString)
    ' Do permutation
Next    ' End of loop

' Display the results.
MsgBox NumberOfPermutations & " permutations of " & _
       NumberOfElements & " elements in " & _
       Timer - Nou & " seconds"
End Sub

That's all for now!


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