[plt-scheme] HTDP 12.4.2

On Jun 23, 2006, at 8:08 AM, wooks wooks wrote:

>
> I guess I could be persuaded that (interperse 'd empty) should give 'd.

Close but no cigar.

You could also consider changing the data definition for words so that 
a word is never empty. :-)

>  I'll have to think about how that helps.
>
>
> ----Original Message Follows----
> From: Matthias Felleisen <matthias at ccs.neu.edu>
> To: "wooks wooks" <wookiz at hotmail.com>
> CC: dyoo at hkn.eecs.berkeley.edu, plt-scheme at list.cs.brown.edu
> Subject: Re: [plt-scheme] HTDP 12.4.2
> Date: Fri, 23 Jun 2006 07:21:03 -0400
>
> Yes now you're on a roll and 90% there except that one example is 
> wrong.
>
> On Jun 23, 2006, at 12:45 AM, wooks wooks wrote:
>
>>
>>
>> I should have said  (????? sym (first alow)) where ????? is a 
>> function that inserts sym into every position in a word and 
>> heretofore i will name interperse  (instead of infiltrate - more 
>> appropriate).
>>
>>
>> The full definition is
>>
>> (define (insertEverywhere sym alow)
>>  (cond
>>    [(empty? alow) empty]
>>    [else (append (interperse sym (first alow))
>>                  (insertEverywhere sym (rest alow)))]
>>    )
>>  )
>>
>> Now I can't see a way to write interperse with the design recipe used 
>> so far.
>>
>> ;CONTRACT
>> ;interperse : symbol word - list-of-words
>> ; produces a list of words by interpersing symbol into each position 
>> in word (including before and after)
>> ;
>> ;EXAMPLES
>> ;(interperse 'd (list 'e 'r)  gives
>> ;(list (list 'd 'e 'r)
>> ;       (list 'e 'd 'r)
>> ;       (list 'e 'r 'd))
>>
>> ;(interperse d empty) gives empty
>> ;(intereperse d 'x) gives (list (list 'd 'x)
>> ;                               (list 'x 'd))
>> ;
>> ;TEMPLATE
>> ;(define (interperse sym word)
>> ;   (cond
>> ;      [(empty? word) ....]
>> ;      [else ....sym ... (first word)
>> ;                        (interperse sym (rest word))]
>> ;    )
>> ;  )
>>
>> It still looks to me that I will need some auxiliary function similar 
>> to what I did with the original infiltrate though (split the word 
>> into 2 parts and generate words by inserting symbol between each 
>> respective split combination.
>>
>>
>>
>>
>> ----Original Message Follows----
>> From: Danny Yoo <dyoo at hkn.eecs.berkeley.edu>
>> To: wooks wooks <wookiz at hotmail.com>
>> CC: plt-scheme at list.cs.brown.edu
>> Subject: Re: [plt-scheme] HTDP 12.4.2
>> Date: Thu, 22 Jun 2006 20:47:44 -0700 (PDT)
>>
>>
>>
>> On Thu, 22 Jun 2006, wooks wooks wrote:
>>
>>>
>>> So, if we are holding onto:
>>>
>>>   (list (list 'd 'r 'e)
>>>         (list 'r 'd 'e)
>>>         (list 'r 'e 'd))
>>>
>>> and we want to mess with it enough so we get:
>>>
>>>    (list (list 'd 'e 'r)
>>>          (list 'e 'd 'r)
>>>          (list 'e 'r 'd)
>>>          (list 'd 'r 'e)
>>>          (list 'r 'd 'e)
>>>          (list 'r 'e 'd))
>>>
>>> do you see what the difference between what we're holding onto and 
>>> what we want is?  Can you give a concept or name to it?
>>>
>>>
>>> The difference is (first alow) - Right.
>>
>>
>> Hi Wooks Wooks,
>>
>> I'm not quite sure if we've done that mind meld thing yet, where we 
>> both are sharing the same thoughts.  *grin* Can you be more specific?
>>
>>
>> How can (first alow), which is going to be the list
>>
>>          (list 'e 'r)
>>
>> then account for the difference between:
>>
>>    (list (list 'd 'r 'e)
>>          (list 'r 'd 'e)
>>          (list 'r 'e 'd))
>>
>>
>>
>> which is what we have on hand, and the result that we really want to 
>> get back:
>>
>>    (list (list 'd 'e 'r)
>>          (list 'e 'd 'r)
>>          (list 'e 'r 'd)
>>          (list 'd 'r 'e)
>>          (list 'r 'd 'e)
>>          (list 'r 'e 'd))
>>
>> I'm not saying that it isn't, but that if we're going to say that it 
>> is, we have to fill in a few more ...'s till we can tell the 
>> non-thinking computer how to do what we think we mean.
>>
>>
>>
>> That is, recalling that portion of the template we were looking at 
>> earlier,
>>
>> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
>>     [else ... sym ... (first alow) ...
>>           ... (insertEverywhere sym (rest alow)) ...]
>> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
>>
>> and knowing what the expected return value is now, can you think of 
>> how to complete the definition now?
>>
>>
>>
>> If not, one way to make things easier is to fill the template will 
>> concrete values, based on the test case you worked out, and see if 
>> that will help:
>>
>>     (... sym ... (first alow) ...
>>          ... (insertEverywhere sym (rest alow)) ...)
>>
>> ==>
>>
>>     (... 'd  ... (list 'e 'r) ...
>>          ...  (list (list 'd 'r 'e)
>>                     (list 'r 'd 'e)
>>                     (list 'r 'e 'd)) ...)
>>
>> Does this help?
>>
>>
>> Note that the steps we've done haven't been particularly brilliant.  
>> One could rephrase the steps we're doing here as:
>>
>>     1.  Shaping the general program pattern.
>>     2.  Cooking up simple test cases.
>>     3.  Mixing in those test case situations into the pattern.
>>
>> We aren't aren't doing anything that couldn't be really codified in 
>> some strategy, some... well, let's call this thing a "design recipe" 
>> for lack of a better term.  *wink*
>>
>>
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