Eight Queens Problem using VB.NET

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Introduction

The Eight Queens Problem is a famous problem in AI field and a famous example about AI algorithms used to solve such a problem using backtracking method.

Background

The solution shows all unique solutions to the eight queens problem which are exactly 92 solutions, 12 of which are distinct. The application also allows you to play the game yourself and try to find your own solution. The algorithm uses backtracking and depth first limited search to level 8 (8 queens) to find a solution.

Using the Code

The Queen class is the basic object used in the algorithm and all over the project for graphics display too. It's fairly simple:

Public Class Queen
    Private mRow As Integer
    Private mColumn As Integer

    Public Sub New()
        mRow = 0
        mColumn = 0
    End Sub

    Public Sub New(ByVal Row As Byte, ByVal Column As Byte)
        mRow = Row
        mColumn = Column
    End Sub

    Public Property Row() As Integer
        Get
            Return mRow
        End Get
        Set(ByVal value As Integer)
            mRow = value
        End Set
    End Property

    Public Property Column() As Integer
        Get
            Return mColumn
        End Get
        Set(ByVal value As Integer)
            mColumn = value
        End Set
    End Property
End Class	

The ChessBoard user control draws the board and is actually where the algorithm of finding all solutions takes place. The function of interest is the MoveQueen function which moves a queen across the board, then checks if the new position is a good place or that the queen is attacked. The process is repeated recursively until all 8 queens are placed in a safe place.

Private Sub MoveQueen(ByVal Level As Integer)
    If Level > 7 Then
        For j As Integer = 0 To 7
            For i As Integer = 0 To 7
                If (Queens(j).Row = j) And (Queens(j).Column = i) Then
                    mCells(i, j) = True
                Else
                    mCells(i, j) = False
                End If
            Next
        Next
        Solutions.Add(mCells.Clone)
        Exit Sub
    End If
    For j As Integer = 0 To 7
        If Level < 8 Then
            Queens(Level).Row = Level
            Queens(Level).Column = j
            If CheckAll(Level) Then MoveQueen(Level + 1)
        End If
    Next
End Sub

Finally, a call to GetSolutions will initiate the MoveQueen function starting from the first level, which is the depth of the current position in the depth tree. Reaching a level of 8 means we have found a solution. Otherwise, 1 level is decremented to the previous level and the search continues using different values. This process is called Backtracking.

Public Sub GetSolutions()
    mUserPlay = False
    Playing = False
    Queens.Clear()
    ResetCells()
    DrawBoard()
    For j As Integer = 0 To 7
        Queens.Add(New Queen)
    Next
    For i As Integer = 0 To 7
        Queens(0).Row = 0
        Queens(0).Column = i
        MoveQueen(1)
    Next
End Sub

Points of Interest 

What's interesting here is that the solutions are ordered according to the board from top-left to bottom-right so you can have all solutions by repeating click on solve button.

History

• 4^th January, 2009: Initial post