CODEWITHSUNDEEP
listrecursionschemeracket
Aspen van Wood
Aspen van Wood

Reputation: 33

How do you return all the elements in a list of lists? (PLT Scheme)

For example, If the input is: (list (list 1) (list 2) (list 3)) Output will be: (list 1 2 3)

Why doesn't this code work?

(define (listoutput mylist)
  (cond
    [(empty? mylist) empty]
    [(cons? mylist) (append (list (first list)) (listoutput (rest mylist)))]))

(check-expect (listoutput (list (list 1) (list 2) (list 3)))
              (list 1 2 3)

Upvotes: 1

Views: 170

Answers (3)

Óscar López
Óscar López

Reputation: 236004

You're asking for the flatten procedure, try this:

(define (flatten lst)
  (cond ((empty? lst) null)
        ((not (list? lst)) (list lst))
        (else (append (flatten (first lst))
                      (flatten (rest lst))))))

You can test it:

(flatten  (list (list 1) (list 2) (list 3)))
> (1 2 3)
(flatten '(1 (2 (3)) (4)))
> (1 2 3 4)

Upvotes: 0

Keith Irwin
Keith Irwin

Reputation: 5668

[(cons? mylist) (append (list (first list)) (listoutput (rest mylist)))]))

That line has two errors. The most obvious is that it should be (first mylist) rather than (first list) (although I expect that you actually know that and mistyped it). The second is that (first mylist) returns a list (since you are getting a list of lists), but then you wrap it with the list function before passing it to append. As a result, in your example, you're not passing (list 1) to append the first time you call it, but rather (list (list 1)). You should simply remove the list function from that expression.

Upvotes: 0

Ryan Culpepper
Ryan Culpepper

Reputation: 10643

You're actually fairly close. Here are some questions that might help:

What's the contract/signature of listoutput?

What's the contract/signature of append?

Also, I recommend building your examples simplest first, each example building on the previous ones. For example,

(check-expect (listoutput empty)) ???)
(check-expect (listoutput (cons (list 1) empty)) ???)

If the first test passes but the second one fails, what does that imply about where the problem is, given the structure of the function?

Upvotes: 2

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