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Forum => newLISP in the real world => Topic started by: Fritz on January 28, 2010, 11:11:08 AM

Title: Other ways to pass variable by reference
Post by: Fritz on January 28, 2010, 11:11:08 AM
I`m trying to find some easy way to cut lists to the limited size.


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(0 100 '("Some" "long" "list")) returns me first 100 dwellers of the list.



(By the way, expression
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(-100 100 '("Some" "long" "list")) says: "ERR: list index out of bounds" if the list is not long enough. But same expression works ok with strings).



Then I have tried to create a function, that will push an item to the list and cut it to the size I need.


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(define (push-cut what where how-long)
  (push what where)
  (if (> (length where) how-long)
    (set 'where (0 how-long where)))
  0)

And... it won`t work. I have found some method in "Code Patterns": make a context and transfer variables like db:db.



But I`m pretty sure, there should be some other way to expain to the function, which variable I want to be pushed.
Title: Re: Other ways to pass variable by reference
Post by: Kazimir Majorinc on January 28, 2010, 01:34:06 PM
Your error is that you assume that, if you call



(push-cut 7 L 3)



L and variable where can have *same* value. They cannot. It is ORO, one reference only. So, where is the solution? You must pass original reference, L, to the function, so function will "work" with original reference.


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(define (push-cut what where how-long)
  (push what (eval where))
  (if (> (length (eval where)) how-long)
      (set where (0 how-long (eval where)))
     0))
 
(set 'L '(1 2 3 4))
(push-cut 7 'L 3)
(println L) ;(7 1 2)

If you do not like that apostrophe in (push-cut 7 'L 3) you can use macro


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(define-macro (push-cut what where how-long)
  (push (eval what) (eval where))
  (if (> (length (eval where)) (eval how-long))
      (set (eval 'where) (0 (eval how-long) (eval where)))
      0))

(set 'L '(1 2 3 4))
(push-cut (+ 1 6) L (+ 1 2))
(println L) ;(7 1 2)

 And this is if you want it to work with more general references:


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(define-macro (push-cut what where how-long)
  (eval (expand '(begin (push (eval what) where)
                        (if (> (length where (eval how-long)))
                            (setf where (0 (eval how-long) where))
                            0))
                'where)))

(set 'L '(1 2 3 4))
(push-cut (+ 1 6) L (+ 1 2))
(println L) ;(7 1 2)

(set 'L '((1 2 3 4)(3 4 5 6) (5 6 7 8)))
(push-cut 12 (L 1) 2)
(println L) ;((1 2 3 4) (12 3) (5 6 7 8))
(exit)

Heavy use of eval might look strange, but, in my opinion, it is how it should be in Newlisp. If you do not use contexts.
Title: Re: Other ways to pass variable by reference
Post by: Fritz on January 30, 2010, 09:02:40 AM
Thank you. I`ll try these variants now.
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