Scheme - Recursion - Need list as output

Question

This is my code, which is just a try...

(define-struct cupboard (upperdrawer middledrawer lowerdrawer))

(define-struct room (name furniture))

(define myrooms
  (list
   (make-room "Kitchen"
              (list
               (make-cupboard "Salt" "Sugar" "Pepper")
               (make-cupboard "Tofu" "Sausage" "Beef")))
   (make-room "Bathroom"
              (list
               (make-cupboard "Toothbrush" "Soap" "Shampoo")))
   (make-room "Sleeping room"
              (list
               (make-cupboard "Red Socks" "Blue Socks" "Yellow Socks")
               (make-cupboard "Suits" "Dresses" "Shoes")))))

(define cupboardcontents
  (lambda (myrooms)
    (cond((room? (first myrooms))
          (cupboard? (first myrooms))
          (cupboardcontents (rest myrooms))
         ((cupboard?(first myrooms)) (list
                               (cupboard-upperdrawer myrooms)
                               (cupboard-middledrawer myrooms)
                               (cupboard-lowerdrawer myrooms)))

          ))))

(cupboardcontents myrooms)

Now the task is it to use recursion get a list as output which includes the cupboard contents. Can anybody help here?

uselpa · Accepted Answer · 2015-12-10 17:48:31Z

1

That's actually more tricky that it seems. Here's a try, tell me if you've seen concepts such as named let, then I can expand on it:

(define (cupboardcontents elt)
  (reverse
   (let loop ((elt elt) (res null))
     (cond
       ((null? elt)     res)
       ((room? elt)     (loop (room-furniture elt) res))
       ((cupboard? elt) (list* (cupboard-lowerdrawer elt)
                               (cupboard-middledrawer elt)
                               (cupboard-upperdrawer elt)
                               res))
       ((list? elt)     (loop (cdr elt) (loop (car elt) res)))
       (else            (error "wot?" elt))))))

Testing:

> (cupboardcontents myrooms)
'("Salt" "Sugar" "Pepper" "Tofu" "Sausage" "Beef" "Toothbrush" "Soap" "Shampoo" "Red Socks" "Blue Socks" "Yellow Socks" "Suits" "Dresses" "Shoes")

answered Dec 10, 2015 at 17:48

uselpa

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4 Comments

Rajib Hossain Pavel Over a year ago

We as a reader want to know on named let. It will be awesome I think.

uselpa Over a year ago

@observo Have a look at cs.rpi.edu/courses/fall00/ai/scheme/reference/schintro-v14/…

JinX213 Over a year ago

Hey, thanks for your help and sorry for my late answer. Actually it really helped me a little, but the problem was, that in your solution you used commands we are not allowed to use because they were not on our schedule yet (e.g loop or cdr) ;) Maybe next time i will mention that. Thank a lot though!!

uselpa Over a year ago

loop is not a keyword here, it's the name of an inner procedure; see the link I put into the comments. And I'd be very surprised if cdr was not allowed.

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