How can i remove the side effects of cons?

Question

I can get the odd elements of a list using the following code:

(define (odds lis)
(cond
    ((null? lis) '())
    ((not (list? lis)) (quote (Usage: odds(list))))
    ((null? (car lis)) '())
    ((= (length lis) 1) (car lis))
    (else (cons (car lis) (odds (cddr lis))))))

but when input list of odd length For example: (odds '(a b c d e)) It will return (a c . e)

How can i get rid of this obnoxious period?

Answer 1

Try this, it's a change in only one line:

(define (odds lis)
  (cond
    ((null? lis) '())
    ((not (list? lis)) (quote (Usage: odds(list))))
    ((null? (car lis)) '())
    ((= (length lis) 1) lis) ; change here
    (else (cons (car lis) (odds (cddr lis))))))

In the highlighted line, you were returning a single element instead of a proper list (a null-terminated list), that's what was causing the problem.