Eight Queens Plus

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Problem Description



Plus (exclude diagonals)

This is an extension of the  "Eight Queens" problem: "Place 8 queens on a chessboard so that no queen threatens another". In this version we add the restriction "and no queen lies on either main diagonal"

In a single turn a queen can move multiple squares in a straight line horizontally, vertically, or diagonally.   So the problem may be rephrased as "place 8 markers on a standard chess board so that no two are in line horizontally, vertically or diagonally and no marker lies on either of the two main diagonals.

Background & Techniques

This is a classic example of a problem that can be solved with a technique known as "graph searching" or "path finding", with backtracking.   

We start placing queens on the board in some systematic manner and for each trial placement, check to see that no other queen is in the same row, column or diagonal.  If there  is a conflicting queen, retract the move in some manner that will prevent us from trying it again and try the next location.   If there is no conflicting queen, try placing the next.  If we run out of places to try, we have to notify the previous queen that she is in the wrong place, etc.  Eventually we will either place all eight queens and succeed or will have tried all locations and conclude that there is no solution.

Sometimes "rules of thumb" or heuristics can dramatically reduce the search space and make such problems solvable (see "The Knights Tour" for example).    In this problem, the brute force method works fine to find a single solution, although it may not find all solutions. 

I've loaded 3 versions of this program, because that's the way the program evolved. Version 1 solves the basic puzzle. Version 2 finds multiple solutions by stepping through the grid and finding a solution for each valid starting position (48 solutions =   64 squares minus the 16 diagonal positions). One solution is returned  each time the user presses the Solve button. Note that some are duplicates and others  represent rotations or mirror images.   Also note that there is no guarantee that we will find all solutions this way since there may  be multiple solutions that have the same starting position.  

In fact, I believe that there are only 2 unique solutions of the "Plus" version, each with 8 variations  for a total of 16.  The traditional version of the puzzle has 92 solutions of which 12 are unique.  These can also be displayed with their  rotations and mirror images.  Version 3 identifies and displays these reversed and rotated images. It also  adds an OnDrawCell exit to the StringGrid to make a graphic display and adds a user play facility. 

PlaceCounter(n) in the recursively called function which looks for a place to position the nth counter.  A search is made for a candidate square in the Board array, indicated by a 0 value.   Three functions test whether the candidate's row, column, and diagonals are unoccupied.  If so, the value n is placed at that location and if n is less than 8, we call ourselves with a value of n+1.    If no square can be found for queen n, we return false.  A false return triggers a reversion to the previous board at the previous level and we continue searching.

Addendum May 1, 2002:   I recently received a feedback message from a viewer asking if would be possible to cover a chessboard with 8 solutions to the original Eight Queens problem  (with main diagonals included).    Not knowing the answer, I wrote a little program to find out.  (the answer is no.  Of the 92 solutions, 6 at most can co-exist on the board. )    The really interesting part was implementing Niklaus Wirth's algorithm for solving the problem.   Wirth, inventor of Delphi's ancestor, Pascal, came up with a most ingenious technique:  He used three small boolean arrays, 

bulletone with 8 members with False set when the row corresponding to that entry is occupied,  
bulletOne with 15 members indexed from 2 to 16 with False entries  when the corresponding  sum of a column and row is occupied,   These correspond to the top-right to bottom-left diagonals  (for example locations (1,4), (2,3), (3,2), and (4,1) all fall on the same diagonal of that type, represented by ColPlusRow[5].   
bulletAnd an array  with 15 members indexed from -7 to +7 with  False entries when the corresponding difference of a column and row is occupied.   These of course represent the 15 top-left to bottom-right diagonals.   

So for any column we now have an easy way check  whether diagonals are empty for any given column and row.  Here's the  recursive depth first Search procedure that finds all 92 solutions:

procedure TForm1.Search (column: integer);
    row: integer;
    rowFlag,plusFlag,minusFlag: Boolean;
    for row := 1 to 8 do
        rowFlag := gRow[row];
        plusFlag := ColPlusRow[column + row];
        minusFlag := ColMinusRow[column - row];
        if rowFlag and plusFlag and minusFlag then
           Solution[column] := row;
           SetQueen(row, column);
           if column < 8 then Search(column + 1)
           else ShowSolution;
           RemoveQueen(row, column);
end; {Search}

Procedure SetQueen sets the 3 array positions we just checked to False and RemoveQueen sets the 3 array positions to True.   The Solution field ends up with 8 numbers corresponding to the row numbers for each column.  

A second button recursively searches solutions derived as above and tracks the maximum number that can co-exist without overlapping.  Best solution is displayed as found. 

It's not very polished or documented, but here is a link that will download the source code for EightQueens_Wirth if anyone is interested.   

Addendum April 8, 2003:  I cleaned up Eight Queens Wirth today and added code to answer one more viewer's question "What is the smallest set of solutions which will completely cover the board?"   My answer is 12, but I cannot prove it.  Viewers are invited to let me know if you know of a smaller set. 

Running/Exploring Eight Queens Wirth 

bulletBrowse source extract
bulletDownload source
bulletDownload  executable


Addendum December 27, 2004:  While answering a viewer's question recently, I had a chance to review the original Eight Queens code posted here 3 years ago and realized that the traditional version of the puzzle was not included.  I modified the 3 versions today to allow users to select whether or not a queen can sit on a diagonal.  So the Plus 3 version will now display not only two unique solutions for the "Plus" version but also the 12 unique solutions for the traditional version of the puzzle.   Each solution has 7 additional variations (rotations and mirrors) which are also displayed.   How does 8 versions of 12 unique solutions square with the 92 solutions found by Eight Queens Wirth and others?  It turns out that solution #8 is unchanged after 180 degree rotation, so there are only 4 unique versions of this solution, reducing the total potentially unique solutions from 96 to 92.        


Running/Exploring Eight Queens Plus 

bulletBrowse source extract
bulletDownload source
bulletDownload  executable

Suggestions for Further Explorations

December, 2004 Done: If you remove the initialization  of the diagonals to -1 in  InitBoard,  you can find solutions for the original version of the problem.   I tried it and found 88 total and 11 unique solutions.  Searching the Web, I've found sites that say that there are 92 solutions with 12 unique.  So there probably are cases where multiple solutions have the same starting position.  Niklaus Wirth, the originator of  Delphi's predecessor, Pascal, has published an optimized search algorithm for all solutions that is available online.  If I had known about it before starting, it would probably be incorporated here.  I didn't, so it's up to you for now.
Search the Internet for Eight Queens and you will turn up lots of sites with more good information.  You could modify this program to solve the N x N problem, perhaps find the maximum number of queens that can be placed on an N x N board without capture.    Is there a way to find the total number of solutions for a given size board without finding all the solutions?


Originally posted: 12/13/2001

Modified: 05/15/2018

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