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Message from discussion destructively changing the cdr of a sublist in a function
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Pascal J. Bourguignon  
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 More options Nov 9 2012, 6:40 pm
Newsgroups: comp.lang.lisp
From: "Pascal J. Bourguignon" <p...@informatimago.com>
Date: Sat, 10 Nov 2012 00:40:41 +0100
Local: Fri, Nov 9 2012 6:40 pm
Subject: Re: destructively changing the cdr of a sublist in a function
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ccc31807 <carte...@gmail.com> writes:
> On Friday, November 9, 2012 11:40:06 AM UTC-5, Barry Margolin wrote:
>> You're making a silly mistake, see below.

> Yes, thanks, I see that.

>> The syntax of SETF is:
>> (setf <place> <new-value>)
>> Compare that to what you wrote. Fix the discrepancy and your function
>> will work.

>> There's one other thing: you shouldn't perform destructive modification
>> of a data structure created using a literal (i.e. quoted) expression.  
>> So change your DEFPARAMETER to use functions to create fresh object, i.e.
>> (defparameter list-1 (list (cons 0 2) (cons 1 3) ...))

> I'm trying my hand at a mancala type game (by way of explanation). Here is what I finally came up with. I won't ask you for comments, but would appreciate it if you have any.

> -------------code--------------------
> (defun drop-pebbles (state square)
>   (let* ((factor (length state))                                             ;this is the factor for the modulus operation
>             (target (car (subseq state square (1+ square)))) ;finds the target element for further processing
>             (num-pebbles (cdr target))                                               ; get the number of pebbles to drop
>             (start-square (mod (1+ square) factor))
>             (new-state (copy-alist state)))
>    (setf (cdr (nth square new-state)) 0)
>   (drop-pebbles-helper new-state num-pebbles start-square factor)))

> (defun drop-pebbles-helper (new-state num-pebbles start-square factor)
>   (do ((pebbles num-pebbles (1- pebbles))
>        (element start-square (1+ element)))
>     ((zerop pebbles) new-state)
>     (progn
>       (incf (cdr (nth (mod element factor) new-state))))))
> ----------------run twice-------------------
> * list-1

> ((0 . 2) (1 . 1) (2 . 0) (3 . 0) (4 . 3) (5 . 1))
> * (drop-pebbles list-1 2)
>   0: (DROP-PEBBLES ((0 . 2) (1 . 1) (2 . 0) (3 . 0) (4 . 3) (5 . 1)) 2)
>     1: (DROP-PEBBLES-HELPER ((0 . 2) (1 . 1) (2 . 0) (3 . 0) (4 . 3) (5 . 1)) 0
>                             3 6)
>     1: DROP-PEBBLES-HELPER returned
>          ((0 . 2) (1 . 1) (2 . 0) (3 . 0) (4 . 3) (5 . 1))
>   0: DROP-PEBBLES returned ((0 . 2) (1 . 1) (2 . 0) (3 . 0) (4 . 3) (5 . 1))
> ((0 . 2) (1 . 1) (2 . 0) (3 . 0) (4 . 3) (5 . 1))

Try:

(defun display-mancala (state &optional (stream *standard-output*))
  (let* ((n/2 (truncate (length state) 2))
         (above (subseq state 0 n/2))
         (below (reverse (subseq state n/2))))
    (format stream
            "~{+~*-----~}+~%~:*~{|~3D  ~}|~%~{+~*-----~}+~%~:*~{|~3D  ~}|~%~:*~{+~*-----~}+~%"
            (mapcar (function cdr) above)
            (mapcar (function cdr) below))))

(display-mancala '((0 . 2) (1 . 1) (2 . 0) (3 . 0) (4 . 3) (5 . 1)))
+-----+-----+-----+
|  2  |  1  |  0  |
+-----+-----+-----+
|  1  |  3  |  0  |
+-----+-----+-----+

If you encapsulate your lists in clos objects, then you can write a
print-object method so that the debugger prints this ASCII-art instead
of the a-lists:

(defclass mancala ()
  ((state :type list :initarg :state :accessor mancala-state)))

(defmethod print-object ((self mancala) stream)
  (if *print-readably*
      (print-unreadable-object (self stream))
      (display-mancala (mancala-state self) stream))
  self)

(make-instance 'mancala :state '((0 . 2) (1 . 1) (2 . 0) (3 . 0) (4 . 3) (5 . 1)))
+-----+-----+-----+
|  2  |  1  |  0  |
+-----+-----+-----+
|  1  |  3  |  0  |
+-----+-----+-----+

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


 
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