[plt-scheme] HtDP newbie question, 12.4.2

On Mar 27, 2008, at 1:20 PM, Cooke Kelsey wrote:

> Dear Dr. Felleisen,
> thanks so much for responding!  Your suggestions really helped me  
> to think about the problem.
>
> Here's a table of examples, as you suggested:
>
> ;;Examples
> ; s | low              |(first..)|(rest..)    | result

Q: What do you mean with (rest ..) ?  This doesn't look like my table.


> ;--------------------------------------------------------------------- 
> -----------------------------------
> ; X | '(AT)            |'(AT)    |empty       | '((XAT) (AXT) (ATX))


[This isn't quite right! 'AT is NOT the representation of the English  
word "at".]


> ; X | '((AB) (CD) (EF))|'(AB)    |'((CD) (EF))| '((XAB) (AXB) (ABX)  
> (XCD) (CXD) (CDX) (XEF) (EXF) (EFX))

This second row looks hyper-complicated.

Q: Why not take one step from row 1, which uses a list with one word.  
What would be the next case?




> Here's an attempt to "go from the first columns to the last":
>
> ; Definitions:
> (define (insert-everywhere/in-all-words s low)
>   (cond
>     [(empty? (rest low)) empty]
>     [else (append (insert-in-single-word s (first low)) (insert- 
> everywhere/in-all-words s (rest low)))]))

Good the first leap went fine.


>  (define (insert-in-single-word s word)
>   (cond
>     [(empty? (rest word)) empty]
>     [else (append (list (first word) s (rest word)) (insert-in- 
> single-word s (rest word)))]))

The second leap went wrong. Why?

Is it possible that you JUMPED?  Why don't you turn the above  
examples into examples for this function. And *please* do provide a  
contract, a purpose statement.

-- Matthias



>
> Unfortunately, the list of words gets shorter and shorter (sATX,  
> sTX, sX).  I tried to think of a way to add a recursive "prefix"  
> that gets longer, but each time the function cycles the first  
> letters seem to disappear into the void...
>
> Matthias Felleisen <matthias at ccs.neu.edu> wrote:
>
> On Mar 25, 2008, at 1:48 PM, Cooke Kelsey wrote:
>>
>> I am trying to learn HtDP by myself, and like Dave Yrueta I am  
>> stumped by 12.4.2.
>
> [Oops I forgot to reply to his last message.]
>
>
>> ; Purpose: to insert a symbol between every letter of every word,  
>> e.g. X, AT-->XAT, AXT, ATX.
>
> Please reformulate the example in terms of symbols and lists.
>
>
>>  ;Function Template:
>> (define (insert-everywhere/in-all-words s low)
>>   (cond
>>     [(empty? (rest low))...]
>>     [else ...(first low)...insert-everywhere/in-all-words s (rest  
>> low)]))
>> Note:
>> It is suggested to use "append", which is easy one or two times--- 
>> (append x word) or (append (append (first word) x) (rest word))---- 
>> but I can't see how to use this recursively for an arbitrary  
>> number of letters and words.
>
> So far so good. Let's try what HtDP/2e will introduce:
>
> Please make a table of the following shape:
>
>  s   |   low  |   (first low)  |  (insert-everywhere/in-all-words s  
> (rest low) | expected result for (insert-... s low)
> ---------------------------------------------------------------------- 
> ----------------------------------
>  X   |   AT   |     ?             |  use the purpose statement to  
> determine this |   (list `XAT' `AXT' `ATX')
>
>
> As I said, you need to reformulate this in terms of symbols and  
> lists. If it doesn't jump out at you how to go from the four  
> columns to the last one, then make a couple of more examples. I am  
> pretty sure it will. Here is the rule of thumb:
>
>  --> you know of a built-in function/primitive that does the work
>  --> you don't. in this case, you make a wish: a purpose statement  
> and a contract for a helper function that does it for you.
>
> You may also have to combine a primitives with functions.
>
> Please report back -- Matthias
>
>
>
>
>
>
>
>>
>> The fact that the "hint" refers to the keyword list, which the  
>> book hasn't covered yet, makes me wonder if it's even possible to  
>> answer this problem, given what we know so far...
>>
>> Thanks very much,
>>
>> Cooke
>>
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>
>
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