[plt-scheme] HtDP newbie question, 12.4.2

On Mar 28, 2008, at 12:34 PM, Cooke Kelsey wrote:

> OK, here's a simple function that adds a letter to a word:
>
> (define (add-first-letter first-letter word)
>   (append (list first-letter) word))


That's not what I asked you to _design_.  Please read the suggestion  
carefully. And again, (append (list foo) bar) is equal to (cons foo  
bar). Why do yu keep using append?



> (add-first-letter 'A (list 'X 'T 'E))=(list 'A 'X 'T 'E')
> So my question is, where does (list 'A 'T 'E) come from?

It is your sample input to the function. It says "Given" in that column.


> In your table below, you say "the recursion: "X" "TE" --> "XTE"  
> "TXE" "TEX".  Actually I have never been able to create such a  
> recursion.  That's the big mystery to me.


The purpose statement of the function tells you what the recursive  
call should produce. So when you code, you may assume that the  
recursive call works properly.


>
> I just read Dave Yrueta's encouraging note, and he says I need to  
> focus on your hint about adding 'A to the list, "in the simplest  
> and most natural way."  Well, all I can think of is to append it,  
> like I did above.  But there is no recursive element, and I cannot  
> call it or plug it into the main recursive function.  It is  
> floating out in space.
>
> Matthias Felleisen <matthias at ccs.neu.edu> wrote:
>
> Okay you have gotten to the point where you're jumping even faster.  
> Slogan:
>
>  furiously fast programming leads to nowhere.
>
> ;; ---
>
> All I was getting at is a simple point:
>
> Your recursive calls in insert-everywhere/one-word produced almost  
> all the work in the recursion. Indeed, it always produces all but  
> one word, except that the first letter from the original word is  
> missing. Example:
>
>    given:          the recursion                           in the  
> final result I want                    plus
>   'X   "ATE"     "X" "TE" --> "XTE" "TXE" "TEX"     "AXTE" "ATXE"  
> "ATXE" "ATEX"           "XATE"
>
> Given the recursive result (the list of words that are too short by  
> the first letter in the orginal word) can you design a function  
> that adds this letter (no matter what it is) to the front of all  
> these words you have gotten back?
>
> DONE?
>
> After that: what is missing?
>
> -- Matthias
>
>
>
>
>
> On Mar 28, 2008, at 11:51 AM, Cooke Kelsey wrote:
>> OK, that helps me a little.  If I insert "cons (first word)," then  
>> I can rebuild the word:
>>
>> (define (cons-function word)
>>   (cond
>>     [(empty? (rest word)) word]
>>     [else (cons (first word) (cons-function (rest word)))]))
>>
>> (cons-function (list 'A 'T 'W)) = (list 'A 'T 'W)
>>
>> The problem is how to add this to a shrinking function:
>>
>> (define (shrinking-function s word)
>>   (cond
>>     [(empty? (rest word)) empty]
>>     [else (list (append (list s) word) (shrinking-function s (rest  
>> word)))]))
>> (This shrinking function isn't exactly what I want, but at least  
>> it gives me (list 'X 'A 'T 'W) and (list 'X 'T 'W)).
>>
>> So I append them together:
>>
>> (append (cons-function (list 'A 'T 'W)) (shrinking-function 'X  
>> (list 'A 'T 'W)))
>>
>> And I get this:
>>
>> (list (list 'X 'A 'T 'W) (list (list 'X 'T 'W) empty)) (list 'A 'T  
>> 'W (list 'X 'A 'T 'W) (list (list 'X 'T 'W) empty))
>>
>> Garbage!
>>
>> As a last resort, I try to combine cons and shrinking functions  
>> together:
>>
>> (define (cons+shrinking-function s word)
>>   (cond
>>     [(empty? (rest word)) word]
>>     [else (append (cons (first word) (cons+shrinking-function 'X  
>> (rest word))) (list s) (rest word))]))
>>
>> (cons+shrinking-function 'X (list 'A 'T 'W)) = (list 'A 'T 'W 'X  
>> 'W 'X 'T 'W)
>>
>> It looks kind of nice, but it is even further from the expected  
>> result.
>>
>>
>> Matthias Felleisen <matthias at ccs.neu.edu> wrote:
>>
>> How about (cons 'A (list 'X 'T)) = (list 'A 'X 'T)
>>
>>
>>
>>
>> On Mar 28, 2008, at 10:53 AM, Cooke Kelsey wrote:
>>> (append (list 'A) (list 'X 'T)) = (list 'A 'X 'T)
>>>
>>> Matthias Felleisen <matthias at ccs.neu.edu> wrote:
>>>
>>> Here is a definition of 'adding in the missing element':
>>>
>>> * If a list X misses the symbol 'A (at the front), how do you get  
>>> a list that has the 'A and all of X?
>>>
>>> * X = (list 'X 'T) and 'A --> I want (list 'A 'X 'T). How do I do  
>>> that?
>>>
>>> -- Matthias
>>>
>>>
>>> Be a better friend, newshound, and know-it-all with Yahoo!  
>>> Mobile. Try it now.
>>
>>
>>
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>
>
>
> Never miss a thing. Make Yahoo your homepage.

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