i woz lookn thru SICP nd i cin diz 8 kweenz puzzl!! lol!!!

http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-15.html#%_sec_2.2.3

da nx wk i wrk on exasize 2.42 nd 2.43. itz hard nd stuff cuz
no 1 eva sez da answer nd i think exasize 2.43 izza trick kwestshun
nd alelson nd sussman just wanna b kool like mean cats r kool!!!!

i think that exasize 2.42 izza exponential process and da mods 4
exasize 2.43 izza exponential process to. der iz no difference in
da speed. cud som 1 plz confirm dis

thats all i haf to say so i guess ill go hang my self with my socks

bye bye

;; my soln to 2.42
;; SICP (2ed.) pg 125
(define (queens board-size)
  (define (queen-cols k)
    (if (= k 0)
        (list empty-board)
        (filter
         (lambda (positions) (safe? k positions))
         (flatmap
          (lambda (rest-of-queens)
            (map (lambda (new-row)
                   (adjoin-position new-row k rest-of-queens))
                 (enumerate-interval 1 board-size)))
          (queen-cols (- k 1))))))
  (queen-cols board-size))

;; list of prospective board positions for new queen
(define (adjoin-position new-row k rest-of-queens)
  (append rest-of-queens (list (list new-row k))))

; a predicate to determine if two queens threaten each other
; (from Lisp 3ed, Winston and Horn pg289)
(define (threat? i j a b) ; One queen at (i,j) other at (a,b)
  (or (= i a) ; same row
      (= j b) ; same column
      (= (- i j) (- a b)) ; SW-NE diagonal
      (= (+ i j) (+ a b)))) ; NW-SE diagonal

; a predicate to tell us if the position (n,m) for the new
; queen is safe (from Winston and Horn pg289)
(define (conflict? n m board)
  (cond ((null? board) board)
        ((threat? n
                  m
                  (car (car board))
                  (cadr (car board)))
         #t)
        (else (conflict? n m (cdr board)))))

(define (safe? k positions)
  (let ((nk-pair (car (last-pair positions))) ; new queen
        (k-minus-1-board (except-last-pair positions))) ; other queens
    (or (= k 1) ; no conflicts if only one queen
        (not (conflict? (car nk-pair) k k-minus-1-board)))))

(define empty-board ())

(define (filter predicate sequence)
  (cond ((null? sequence) sequence)
        ((predicate (car sequence))
         (cons (car sequence)
               (filter predicate (cdr sequence))))
        (else (filter predicate (cdr sequence)))))

(define (flatmap proc seq)
  (fold-right append () (map proc seq)))

--
__Pascal Bourguignon__                     http://www.informatimago.com/