[plt-scheme] HtDP newbie question, 12.4.2

On Mar 28, 2008, at 1:24 PM, Cooke Kelsey wrote:

> Oh right, in order to include all the words I need more than an  
> append.  Here's a function that adds a letter to ALL the words:
>
> (define (add-first-letter first-letter low)
>   (cond
>     [(empty? (rest low)) (list (cons first-letter low))]
>     [else (cons (cons first-letter (list (first low))) (add-first- 
> letter first-letter (rest low)))]))
> ;example:
> (add-first-letter 'X (list 'A 'T 'E 'Y 'W 'Z))
> ;result:
> (list (list 'X 'A) (list 'X 'T) (list 'X 'E) (list 'X 'Y) (list 'X  
> 'W) (list 'X 'Z))


So if you apply add-first-letter to (first word) and the result of  
(insert-everywhere/in-one-word (rest word)), you get back all but one  
of the word you want.  Which brings me to the second question:


>  That was a lot more complicated than I thought.  I guess that  
> should be a helper function.
>
> However, when I try to make a main function (insert-in-single-word)  
> that calls add-first-letter, I end up going around in the same  
> circle as before.


Because you failed to read and answer my second question. -- Matthias





>
> Matthias Felleisen <matthias at ccs.neu.edu> wrote:
>
> 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
>>>>
>>>>
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>>>
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