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

score 11 · Accepted Answer · answered Mar 28 '13 at 15:58

11

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))))
 ...)

answered Mar 28 '13 at 15:58

C. K. Young

219,335
46
382
435

score 1 · Answer 2 · answered Mar 28 '13 at 15:58

1

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>))

answered Mar 28 '13 at 15:58

robbyphillips

961
5
14

thanks to you too, I can't use "if" if not it would be simpler as you wrote – Frank Mar 28 '13 at 16:04

score 1 · Answer 3 · answered May 25 '15 at 17:46

You don't really need the cond or if if you don't have a list of else cases. when might be what you are looking for. It's basically just the true case of an if.

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

Scheme: Cond "not equal"

3 Answers3