Question

Okay, so i have recently posted a question regarding create a recursive function in Scheme that would generate even functions called from the defined list below:

(define list0 (list 'j 'k 'l 'm 'n 'o 'j) )
(define list1 (list 'a 'b 'c 'd 'e 'f 'g) )
(define list2 (list 's 't 'u 'v 'w 'x 'y 'z) )
(define list3 (list 'j 'k 'l 'm 'l 'k 'j) )
(define list4 (list 'n 'o 'p 'q 'q 'p 'o 'n) )
(define list5 '((a b) c (d e d) c (a b) )
(define list6 '((h i) (j k) l (m n)) )
(define list7 (f (a b) c (d e d) (b a) f) )

which for my evens function i created this recursive function:

(define mylist '(1 2 3 4 5 6 7))
(define (evens lst)
(define (do-evens lst odd)
    (if (null? lst)
        lst
    (if odd
        (do-evens (cdr lst) #f)
        (cons (car lst) (do-evens (cdr lst) #t)))))
        (do-evens lst #t))

but now i am trying to create a ‘oddrev’ function that does as such:
(oddrev 1st) which should return a new list, formed from the odd-numbered elements taken from 1st, but in the reverse of their original order. That is if i typed in:

(oddrev '(a b c d e f g))

which would/should return:
(g e c a)

(oddrev (LIST 's 't 'u 'v 'w 'x 'y 'z))

which would/should return:
(y w u s)

(oddrev '((h i) (j k) l (m n)))

which would/should return:

(l (h i))

and

(oddrev '())

which would/should return a empty list, etc.

I am wondering if someone could show me how this might look. I am trying to just learn scheme for future references and i hear that is a cool programming language but as of right now i am hitting a few bumps in the road. Any help for a new person would be greatly appreciated. Thank You

Answer 1

Ok break it down into parts, firstly get every odd element.

Well since you already have this nice little function to get evens how can we get odds using it? Well what if we were to tell it that it really started on an even number, so instead of (do-evens lst #t) we said (do-evens lst #f). Now we’ll get all the odd elements! For clarity perhaps we want to change even => odd in this body of this function (a simple find-substitute will work just fine)

Next we have to reverse it. There are 2 ways to do this, either by hand or via the Scheme libraries. If you want to do it by hand I’ll let you work out that challenge. Otherwise reverse will do that work for us.

I’ll let you figure out how to combine these 2 steps. It’s not too hard! Happy Scheming