Newbie lisper looking for hints

andreif_at_mail.dntis.ro
Date: 03/24/05 

Date: 24 Mar 2005 02:00:35 -0800

Greetings,

Back to my magic-square problem I have written something looking like
this :

(defun test-sum (sum lst)
  ;; returns (lst) if list element sum is equal to 'sum', NIL otherwise
  (if (= (apply #'+ lst) sum)
      (list lst)
    ()))

(defun sublist-n-elements-fixed-sum (n sum0 lst lst0)
  (cond
   ((= n 0) (test-sum sum0 lst0))
   (T
    (setf acc NIL)
    (dolist (x lst acc)
      (cond
       ((<= x sum0)
        (setf new-lst (remove x lst))
        (setf new-lst0 (append (list x) lst0))
        (setf val (sublist-n-elements-fixed-sum (1- n) sum0 new-lst
new-lst0))
        (nconc acc (list val))
        (print (append '("after call =>") (list val) (list acc)))
        )
       )
      )
    )
   )
  )

if called with:
  (sublist-n-elements-fixed-sum 3 15 '(1 2 3 4 5 6 7 8 9) ())
it should return all lists of 3 elements from the list (1 2 3 4 5 6 7 8
9) and the sum of all elements in those list must be 15.

My biggest problem is in the recursive call :
        (setf val (sublist-n-elements-fixed-sum (1- n) sum0 new-lst
new-lst0))
        (nconc acc (list val))
        (print (append '("after call =>") (list val) (list acc)))

I did a (trace ...) on the function and at some point the recursive
call returns a list, val is set to a good value but acc remains still
NIL at the print call which I really don't understand why. I used the
simplest example :

  (print (sublist-n-elements-fixed-sum 2 3 '(1 2) ()))

and I get :

   1: (SUBLIST-N-ELEMENTS-FIXED-SUM 2 3 (1 2) NIL)
     2: (SUBLIST-N-ELEMENTS-FIXED-SUM 1 3 (2) (1))
       3: (SUBLIST-N-ELEMENTS-FIXED-SUM 0 3 NIL (2 1))
       3: returned ((2 1))

("after call =>" ((2 1)) NIL)
     2: returned NIL

("after call =>" NIL NIL)
     2: (SUBLIST-N-ELEMENTS-FIXED-SUM 1 3 (1) (2))
       3: (SUBLIST-N-ELEMENTS-FIXED-SUM 0 3 NIL (1 2))
       3: returned ((1 2))

("after call =>" ((1 2)) NIL)
     2: returned NIL

("after call =>" NIL NIL)
   1: returned NIL

NIL

Something is wrong here but I don't see why, I tried replacing (setf
acc ()) with (let ((acc ()) ...) but same result.

Any hints on what I might be doing wrong ?

Thanks,
Andrei

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