Scheme: Cond "not equal"

Question

I'd like to do this in scheme:

if ((car l) != (car (cdr (order l))) do something

in particular I wrote this:

((eq? (car l) (car (cdr (order l))) ) 
 (cons (count (car (order l)) (order l)) 
       (count_inorder_occurrences (cdr (order l))))) 

but it compares (car l) with (car (cdr (order l)) for equality. I want instead to do something only if eq? is false. How can I do that in my example?

Thanks

Answer 1

You can use not for that.

(cond
 ((not (eq? (car l) (cadr (order l))))
  (cons (count (car (order l)) (order l))
        (count-inorder-occurrences (cdr (order l))))
 ...)

Answer 2

You can use not to negate the value of a predicate.

e.g. in an if statement: (if (not (eq? A B)) <EVAL-IF-NOT-EQ> <EVAL-IF-EQ>)

or in a cond you can do:

(cond ((not (eq? A B))
       <EVAL-IF-NOT-EQ>)
      .
      .
      .
      (else <DEFAULT-VALUE>))