[plt-scheme] Re: HTDP Exercise 12.4.2 ... Help!

This is what you posted  initially (insert-everywhere/in-all-words 's (list
(list 'm 'e) (list 'x))) . Try it out in your in function, I would like to
know how it plays out.



Emeka


On Mon, Apr 27, 2009 at 10:29 PM, S Brown <ontheheap at gmail.com> wrote:

> What I ended up doing was creating a helper function to insert-
> everywhere, which takes 3 arguments, the front of the word, the
> symbol, and the back of the word. insert-everywhere doesn't do
> anything except make the initial call to the insert-everywhere-helper
> function, which does the actual work of inserting the letter into the
> word. So, what I ended up with is basically this:
>
> ;; insert-everywhere:  symbol word  ->  list-of-words
> ;; creates a list of words from symbol s, and word w, where
> ;; s has been inserted into each position of w
> (define (insert-everywhere s w) ... )
>
> and also
>
> ;; insert-everywhere-helper: word symbol word  ->  list-of-words
> ;; creates a list of words where symbol is between word f and word b
> (define (insert-everywhere-helper f s b) ... )
>
> I don't know how correct this is, at least as far as what the book
> wants, but it does pass the following check-expect calls:
>
>  (check-expect (insert-everywhere 'x empty)
>     (list (list 'x)))
>
>  (check-expect (insert-everywhere 'x (list 'a 'b))
>     (list (list 'x 'a 'b)
>           (list 'a 'x 'b)
>           (list 'a 'b 'x)))
>
>  (check-expect (insert-everywhere 'x (list 'a 't 'o 'm))
>     (list (list 'x 'a 't 'o 'm)
>           (list 'a 'x 't 'o 'm)
>           (list 'a 't 'x 'o 'm)
>           (list 'a 't 'o 'x 'm)
>           (list 'a 't 'o 'm 'x)))
>
>  (check-expect (arrangements (list 'h 'o 't))
>                (list (list 'h 'o 't)
>                      (list 'o 'h 't)
>                      (list 'o 't 'h)
>                      (list 'h 't 'o)
>                      (list 't 'h 'o)
>                      (list 't 'o 'h)))
>
> Thank you to everyone who provided input!
>
> On Apr 26, 6:02 pm, Nadeem Abdul Hamid <nad... at acm.org> wrote:
> > Consider having the purpose of your insert-everywhere function:
> >
> > > (insert-everywhere 'x (list 'h 'o 't))
> >
> > to be to produce this result by the time it's done:
> >
> > >    =>   (list (list 'x 'h 'o 't)
> > >               (list 'h 'x 'o 't)
> > >               (list 'h 'o 'x 't)
> > >               (list 'h 'o 't 'x))
> >
> > and consider what function you wish you could use in the body of
> > insert-everywhere itself to achieve this (as opposed to thinking about
> > applying your helper function only after insert-everywhere has
> > produced the result that you currently have it producing).
> >
> > On Apr 26, 2009, at 5:37 PM, S Brown wrote:
> >
> >
> >
> > > I've been working on this for the last few hours, and my insert-
> > > everywhere function is now working like so:
> >
> > > (insert-everywhere 'x (list 'h 'o 't))
> >
> > > =>   (list (list 'x 'h 'o 't)
> > >           (list 'x 'o 't)
> > >           (list 'x 't)
> > >           (list 'x))
> >
> > > So, I wish I had some function that would prefix each list with the
> > > "remainder" of the word, such that:
> >
> > >    (list (prefix remainder (list 'x 'h 'o 't))
> > >          (prefix remainder (list 'x 'o 't))
> > >          (prefix remainder (list 'x 't))
> > >          (prefix remainder (list 'x)))
> >
> > >    =>   (list (list 'x 'h 'o 't)
> > >               (list 'h 'x 'o 't)
> > >               (list 'h 'o 'x 't)
> > >               (list 'h 'o 't 'x))
> >
> > > So, based on this I want a function, prefix, which basically takes two
> > > words, appends them, and returns the new list. So, I guess in order to
> > > use this "prefix" function, I need to figure out how to define
> > > "remainder," but now I seem to be stuck again because remainder
> > > depends on information that isn't being passed along (ie, for (prefix
> > > remainder (list 'x 't)), remainder needs to be (list 'h 'o), but I
> > > can't figure out how to do this!
> >
> > > Am I at least on the right path with this? Any tips/hints??
> >
> > > On Apr 26, 1:25 pm, Matthias Felleisen <matth... at ccs.neu.edu> wrote:
> > >> Yes, and don't forget, you may need *more* auxiliary functions than
> > >> just insert-everywhere
> >
> > >> On Apr 26, 2009, at 1:19 PM, S Brown wrote:
> >
> > >>> Thanks for the input so far. I don't know how I didn't see this
> > >>> before! Instead of calling append on (list s) (first a-low), what I
> > >>> need is to call append on another function which takes as it's
> > >>> arguments a symbol and a single word, which returns a list of words
> > >>> where the symbol has been been inserted before and after each letter
> > >>> in the word. In other words, where insert-everywhere/in-all-words is
> > >>> concerned with a list of words, my new function is only concerned
> > >>> with
> > >>> a single word.
> >
> > >>> So, now it's a matter of going through the design process for my new
> > >>> function, which I'm going to call insert-everywhere.
> >
> > >>> ;; insert-everywhere: symbol word  ->  list-of-words
> > >>> ;; to create a list-of-words where symbol s is inserted before
> > >>> ;; and after each letter in the word w
> > >>> (define (insert-everywhere s w)
> > >>>    (cond
> > >>>       ((empty? w) ... s ...)
> > >>>       (else ... s (first w) ...
> > >>>             ... (insert-everywhere s (rest w)) ...)))
> >
> > >>> I think this makes sense. Now it's a matter of completing each cond
> > >>> line in my new function insert-everywhere.. and figuring out how to
> > >>> actually insert a letter into each position of a word.
> > >>> _________________________________________________
> > >>>   For list-related administrative tasks:
> > >>>  http://list.cs.brown.edu/mailman/listinfo/plt-scheme
> >
> > >> _________________________________________________
> > >>   For list-related administrative tasks:
> > >>  http://list.cs.brown.edu/mailman/listinfo/plt-scheme
> > > _________________________________________________
> > >  For list-related administrative tasks:
> > >  http://list.cs.brown.edu/mailman/listinfo/plt-scheme
> >
> > _________________________________________________
> >   For list-related administrative tasks:
> >  http://list.cs.brown.edu/mailman/listinfo/plt-scheme
> _________________________________________________
>  For list-related administrative tasks:
>  http://list.cs.brown.edu/mailman/listinfo/plt-scheme
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.racket-lang.org/users/archive/attachments/20090428/26bf3ac8/attachment.html>