# [racket] [htdp] Help with Exercise 12.4.2

 From: Matthias Felleisen (matthias at ccs.neu.edu) Date: Tue Jul 6 17:35:55 EDT 2010 Previous message: [racket] [htdp] Help with Exercise 12.4.2 Next message: [racket] [htdp] Help with Exercise 12.4.2 Messages sorted by: [date] [thread] [subject] [author]

Amen.

-- Make sure you truly understand list vs cons. If you have any
doubts, stick to cons.
-- For your template, spell out what each expression produces before
you finish the definition.
-- Use the examples, especially the ones that fail.
-- Figure out what you need to combine the pieces.
-- Ideally, arrange the thing in a table:

input | (first input) | .... | recursive result | expected result

and keep adding examples until it clicks: i.e., until you know how to
combine recursive result with some other things to get the expected
result.

And do stick to the design recipe all the way down.

On Jul 6, 2010, at 5:30 PM, David Yrueta wrote:

> Hi Sam --
>
> As Jos commented, this is a very tough exercise, but it looks like
> you're off to a good start.
>
> The most obvious problem with your current solution lies with
> 'insert-everywhere/in-single-word.'  My first suggestion is to check
> the function examples to make sure the values they expect are
> consistent with your data definitions.
>
> Dave
>
>
>
> On Tue, Jul 6, 2010 at 1:15 PM, Jos Koot <jos.koot at telefonica.net>
> wrote:
> Exercise 12.4.2 is notorius. When looking to (list 'a 'b 'c), you
> should
> exspect 6 distinct permutations.
> Jos
>
> > -----Original Message-----
> > From: users-bounces at racket-lang.org
> > [mailto:users-bounces at racket-lang.org] On Behalf Of Sam Griff
> > Sent: 06 July 2010 20:00
> > To: users at racket-lang.org
> > Subject: [racket] [htdp] Help with Exercise 12.4.2
> >
> > Hello. Like many others working through HtDP I have hit the
> > wall at this exercise. I'm not sure what I have so far is in
> > line with the "proper" way to do this exercise and would
> > appreciate any help/suggestions. Here is the code I have so far:
> >
> > ;; A list of words is either
> > ;; 1. empty
> > ;; 2. (cons w low)
> > ;;    where w is a word and low is a list of words
> >
> > ;; arrangements : word  ->  list-of-words ;; to create a list
> > of all rearrangements of the letters in a-word (define
> > (arrangements a-word)
> >   (cond
> >     [(empty? a-word) (cons empty empty)]
> >     [else (insert-everywhere/in-all-words (first a-word)
> >             (arrangements (rest a-word)))]))
> >
> > ;; Contract:
> > ;; insert-everywhere/in-single-word : symbol word ->
> > list-of-words ;; Purpose:
> > ;; to insert a symbol everywhere in a single word ;; Examples:
> > ;; (insert-everywhere/in-single-word 'a empty) should produce
> > (list 'a) ;; (insert-everywhere/in-single-word 'b (list 'a))
> > should produce (list (list 'a 'b) (list 'b 'a)) ;;
> > (insert-everywhere/in-single-word 'c (list 'a 'b)) should
> > produce (list (list 'c 'a 'b) (list 'a 'c 'b) (list 'a 'b
> > 'c)) ;; Template:
> > ;; (define (insert-everywhere/in-single-word s w)
> > ;;   (cond
> > ;;     [(empty? w) ...]
> > ;;     [else ... (first w) ...
> > (insert-everywhere/in-single-word s w) ...]))
> > (define (insert-everywhere/in-single-word s w)
> >   (cond
> >     [(empty? w) (list s)]
> >     [else  (list (list s (first w)) (cons (first w)
> > (insert-everywhere/in-single-word s (rest w))))]))
> >
> > ;; insert-everywhere/in-all-words : symbol list-of-words ->
> > list-of-words ;; to insert a symbol everywhere in a list of
> > words (define (insert-everywhere/in-all-words s low)
> >   (cond
> >     [(empty? low) empty]
> >     [else (append (insert-everywhere/in-single-word s (first
> > low)) (insert-everywhere/in-all-words s (rest low)))]))
> >
> >
> > ;; Tests:
> > (define testword1 (list 'a))
> > (define testword2 (list 'a 'b))
> > (check-expect (insert-everywhere/in-single-word 'a empty)
> > (list 'a)) (check-expect (insert-everywhere/in-single-word 'b
> > testword1) (list (list 'b 'a) (list 'a 'b))) (check-expect
> > (insert-everywhere/in-single-word 'c testword2) (list (list
> > 'c 'a 'b) (list 'a 'c 'b) (list 'a 'b 'c)))
> >
> >
> > Thanks,
> >
> > Sam
> > _________________________________________________
> >   http://lists.racket-lang.org/listinfo/users
>
>
> _________________________________________________