CODEWITHSUNDEEP
lispcommon-lisp
user7267832
user7267832

Reputation:

Lisp recursion not working fine

I'm working on a Lisp project for a college class, so I'm just starting to code in this language.
We were asked to implement a Lisp library to manage monomials and polynomials. Monomials are rappresented as: (M coefficient total-deegre (vars-and-powers)) e.g. 3ywt^3 is (M 3 5 ((V 3 T) (V 1 W) (V 1 Y))).
I'm not sure about how the implementation of the function which has to return a list of the variables in M. I've already defined a function to get the vars-and-powers, called varpowers.
My idea is: I'm using the list returned by varpowers to get the variables using the last function. Here is my code: 

(defun var-of (m)
  (setq a (varpowers m))
  (cond ((null a) nil)
        (t (cons (last (car a))
                 (var-of (cdr a))))))

I guess the recursion is not working because everytime var-of is called in the last line a is redefined and its car is always the same ((V 3 T)). How can I fix this problem?
Thanks in advance.

Upvotes: 0

Views: 280

Answers (1)

Leo
Leo

Reputation: 1934

Change setq by let:

(defun var-of (m)
  (let ((a (varpowers m)))
      (cond ((null a) nil)
            (t (cons (last (car a))
                     (var-of (cdr a)) )))))

Upvotes: 4

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