Category : Miscellaneous Language Source Code
Archive   : PCLISP30.ZIP
Filename : QUEENS.L

Output of file : QUEENS.L contained in archive : PCLISP30.ZIP

; Place n queens on a board (graphical version)
; See Winston and Horn Ch. 11
; Usage:
; (queens )
; where is an integer -- the size of the board - try (queens 4)
; I do not know who the original Author of this is but it was found with some
; XLISP example lisp programs. This has been slightly modified to run on
; PC-LISP V2.13.
; Peter Ashwood-Smith
; August 22nd, 1986

; Do two queens threaten each other ?

(defun threat (i j a b)
(or (= i a) ;Same row
(= j b) ;Same column
(= (- i j) (- a b)) ;One diag.
(= (+ i j) (+ a b)))) ;the other diagonal

; Is poistion (n,m) on the board safe for a queen ?

(defun conflict (n m board)
(cond ((null board) nil)
((threat n m (caar board) (cadar board)) t)
(t (conflict n m (cdr board)))))

; Place queens on a board of size SIZE

(defun queens (size)
(prog (n m board soln)
(setq soln 0) ;Solution #
(setq board ())
(setq n 1) ;Try the first row
(setq m 1) ;Column 1
(cond ((conflict n m board) (go un-do-m))) ;Check for conflict
(setq board (cons (list n m) board)) ; Add queen to board
(cond ((> (setq n (1+ n)) size) ; Placed N queens ?
(print-board (reverse board) (setq soln (1+ soln))))) ; Print it
(go loop-n) ; Next row which column?
(cond ((null board) (return 'Done))) ; Tried all possibilities
(setq m (cadar board)) ; No, Undo last queen placed
(setq n (caar board))
(setq board (cdr board))
(cond ((> (setq m (1+ m)) size) ; Go try next column
(go un-do-n))
(t (go loop-m)))))

;Print a board

(defun print-board (board soln)
(prog (size)
(setq size (length board)) ;we can find our own size
(princ "\f\n\t\tSolution: ")
(princ soln)
(princ "\n\n\t")
(print-header size 1)
(princ "\n")
(print-board-aux board size 1)
(princ "\n")

; Put Column #'s on top

(defun print-header (size n)
(cond ((> n size) (princ "\n"))
(t (prog () (patom n)
(princ " ")
(print-header size (1+ n))))))

(defun print-board-aux (board size row)
(princ "\n")
(cond ((null board) ())
(t (prog ()
(princ row) ;print the row #
(princ "\t")
(print-board-row (cadar board) size 1) ;Print the row
(print-board-aux (cdr board) size (1+ row)))))) ;Next row

(defun print-board-row (column size n)
(cond ((> n size)())
(t (prog ()
(cond ((equal column n) (princ "Q"))
(t (princ ".")))
(princ " ")
(print-board-row column size (1+ n))))))

  3 Responses to “Category : Miscellaneous Language Source Code
Archive   : PCLISP30.ZIP
Filename : QUEENS.L

  1. Very nice! Thank you for this wonderful archive. I wonder why I found it only now. Long live the BBS file archives!

  2. This is so awesome! 😀 I’d be cool if you could download an entire archive of this at once, though.

  3. But one thing that puzzles me is the “mtswslnkmcjklsdlsbdmMICROSOFT” string. There is an article about it here. It is definitely worth a read: