Adding integer variables to a Lisp list

Question

I'm creating a function that multiplies to polynomials together. Part of this involves determining whether I'm multiplying two of the same variables (5x * 3x) or two different variables (5x * 3y).

The function takes two parameters (p1 and p2) which are the polynomials to be multiplied. And they are entered in the form:

'(5 (x 2)) '(3 (y 3))

= 5x^2 * 3y^3.

(These could be anything though..)

So i have an if statement to determine whether the variables are the same:

(if (EQUAL (car(car (cdr p1))) (car(car (cdr p2)))).

Where (car(car (cdr p1)) and (car(car (cdr p2)) are the x and y.

I now want to add these two variable into a list so that the list looks like (x y).

Here is what I've tried:

(setq pList (append (car(car ( cdr p1 )))))
(setq pList (append (car(car ( cdr p2 )))))

But what seems to happen is the first line of code add's the X into the list, but the second line of code replaces the x with a y, so I'm just left with (y). I've tried all sorts of things but nothing that seems to work!

Any help would be much appreciated!

uselpa · Accepted Answer · 2013-12-31 11:04:42Z

2

(defun multp (p1 p2)
  (let ((x1 (caadr p1)) (x2 (caadr p2)))
    (format t "x1 is ~a, x2 is ~a~%" x1 x2)
    (let ((plist (if (equal x1 x2) (list x1) (list x1 x2))))
      (format t "plist is ~a~%" plist))))

then

(multp '(5 (x 2)) '(3 (x 3)))
=> 
x1 is X, x2 is X
plist is (X)
NIL

and

(multp '(5 (x 2)) '(3 (y 3)))
=> 
x1 is X, x2 is X
plist is (X Y)
NIL

edited Dec 31, 2013 at 11:04

answered Dec 31, 2013 at 10:38

uselpa

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5 Comments

hunterge Over a year ago

How come if I try and print x1 and x2 within the function, it returns the error: Undefined Variables : X1 X2. Surely they've just been defined?

hunterge Over a year ago

Works perfectly thanks. How would I add the (X Y) to my list 'pList'? Would I have to do it within the let statement?

uselpa Over a year ago

I would use let, but it depends on what's next in your code. See my edit.

hunterge Over a year ago

How come when I try to print pList out of the let, it returns 'pList is unbound'?

hunterge Over a year ago

Think I've got it, thank you very much for you help!

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