# [plt-scheme] HtDP newbie question, 12.4.2

 From: Matthias Felleisen (matthias at ccs.neu.edu) Date: Thu Mar 27 08:49:55 EDT 2008 Previous message: [plt-scheme] HtDP newbie question, 12.4.2 Next message: [plt-scheme] HtDP newbie question, 12.4.2 Messages sorted by: [date] [thread] [subject] [author]

```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.

>
> 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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