Sort polynomials Common Lisp

Question

I'm trying to sort a list of polynomials written in this format: (M [coefficient] [total degree] [Variable List]).

example:

((M 1 1 ((V 1 A))) (M 1 2 ((V 1 A) (V 1 C))) (M 1 2 ((V 2 A))) (M 1 2 ((V 1 A) (V 1 B))))

This is: a + a * c + a ^ 2 + a * b, I need to get a + a * b + c + a * a ^ 2, because a * b < a ^ 2 and a < a ^ 2.

I tried to use the function sort, but my output is:

((M 1 1 ((V 1 A))) (M 1 2 ((V 2 A))) (M 1 2 ((V 1 A) (V 1 B))) (M 1 2 ((V 1 A) (V 1 C))))

that is a + a ^ 2 + a * b + a * c.

I use:

(defun sort-poly (a b)
  (cond 
    (t (sort-poly-helper (varpowers a) (varpowers b)))))

(defun sort-poly-helper (a b)
  (cond 
    ((null a) (not (null b)))
    ((null b) nil)
    ((equal (third(first a)) (third(first b))) (sort-poly-helper (rest a) (rest b)))
    (t (sort (list (third(first a)) (third(first b))) #'string-lessp))))

with:

 (sort '((M 1 1 ((V 1 A))) (M 1 2 ((V 1 A) (V 1 C))) (M 1 2 ((V 2 A))) (M 1 2 ((V 1 A) (V 1 B)))) #'sort-poly)

Some help? Thanks

Answer 1

Your definition of what you want to do is sufficiently opaque that an answer is hard to provide. But the way to start is to stop programming like it is 1956 and use some abstraction.

First of all, let's define how to make a variable and get at its bits:

(defun make-variable (name &optional (degree 1))
  `(v ,name ,degree))

(defun variable-name (v)
  (second v))

(defun variable-degree (v)
  (third v))

Now let's define how to make polynomials from lists of variables. Note that the total degree of the polynomial is computable from the degrees of all the variables, so we do that.

(defun make-polynomial (variables &optional (coefficient 1))
  ;; The total degree of the polynomial can just be computed from the
  ;; degrees of its variables
  `(m ,coefficient ,(reduce #'* variables :key #'variable-degree)
      ,variables))

(defun polynomial-coefficient (p)
  (second p))

(defun polynomical-total-degree (p)
  (third p))

(defun polynomial-variables (p)
  (fourth p))

Now, given lists of polynomials, we can sort them using the abstractions we've built: we don't need to grovel around with list accessors (and indeed we could change the representation of polynomials or variables and nothing would ever know).

I am guessing that what you want to sort on is the highest degree of a variable in a polynomial although it is not really clear, and not the total degree of the polynomial (which would be easier). So let's write a function to pull out the highest variable degree:

(defun highest-variable-degree (p)
  (reduce #'max (mapcar #'variable-degree (polynomial-variables p))))

And now we can sort lists of polynomials.

CL-USER 23 > (sort (list (make-polynomial (list (make-variable 'a)
                                               (make-variable 'b 2)))
                         (make-polynomial (list (make-variable 'c)
                                                (make-variable 'd))))
                   #'<
                   :key #'highest-variable-degree)
((m 1 1 ((v c 1) (v d 1))) (m 1 2 ((v a 1) (v b 2))))

Remember: it is not 1956 any more.