Question

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LISP – Global variable keep their old value after reinitialization

I’m currently doing some coursework with Lisp (using Common Lisp) and have nowhere else to turn with this problem I’m having.

It’s tricky to explain but here goes..

The situation:
I’ve got two global variables, p1 & p2, which are polynomial expressions. My task is to create a polynomial calculator, which is going well so far.

I keep my polynomial elements in a specific format: 3x^2 == ((x 2) 3), and I’ve created two functions which recursively run through both lists of polynomial elements.

If I wanted to add together 3x^2 and 2x^2, the answer would be 5x^2 (or “((x 2) (3+2))”).

I have a third list, the result, which is appended to whenever an element is calculated/can’t be calculated.

The problem:
With two elements I can add together, I create a temporary variable to be added to the result. However, the global variable is changed despite what I do.

I’ve tried let, lambda, various functions, etc. I’ve been stuck for a couple of days now and would greatly appreciate any help you could give me 🙂

Here’s a small example of what I mean:

(setf p1 '((x 2) 2))
    ;2x^2

(setf p2 '((x 2) 3))
    ;3x^2 

(if (equal (first p1) (first p2)) 
    (progn
      (setf temp p1)
      (setf (second temp) (+ (second p1) (second p2)))
      (append-to-result temp)
      (print p1)
      (print temp)))

Output:

((x 2) 5)

((x 2) 5)

Answer 1

When you do:

(setf temp p1)

you are not_making a copy of the list structure that p1 refers to. Both variables now refer to the same cons cells. Then when you do:

(setf (second temp) ...)

you’re modifying that list structure, which both variables still refer to. Change to:

(setf temp (copy-tree p1))