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

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.
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>>
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