[plt-scheme] HtDP 26.1.2

Hi Matthias --
Thank you for your response.

I'm still a bit confused.

How can this accomplished --

(check-expect (merge-all-neighbors (list (list 9) (list 1))) (list (list 1
9)))?

-- without some sorting going on within "merge-all-neighbors"?

I would expect a function which only appended contiguous lists would
evaluate like this --

(check-expect (merge-all-neighbors (list (list 1) (list 9))) (list (list 1
9)))
(check-expect (merge-all-neighbors (list (list 9) (list 1))) (list (list 9
1)))

But instead, in both cases above, "merge-all-neighbors" returns (list (list
1 9)), yes?

Where am I going wrong?

Thanks!
Dave



On Sat, Mar 21, 2009 at 4:29 PM, Matthias Felleisen <matthias at ccs.neu.edu>wrote:

>
> On Mar 21, 2009, at 6:26 PM, dave yrueta wrote:
>
>  Hi all --
>>
>> Question about HtDP function "merge-neighbors:"
>>
>> Link to exercise:  http://www.htdp.org/2003-09-26/Book/curriculum-Z-
>> H-33.html#node_sec_26.1
>>
>> The text describes the contract for "merge-neighbors" as a function
>> which "merges pairs of neighboring lists. More specifically, it
>> consumes a list of lists (of numbers) and merges neighbors. For
>> example,
>>
>> (equal? (merge-all-neighbors (list (list 2) (list 5) (list 9) (list
>> 3)))
>>        (list (list 2 5) (list 3 9))) "
>>
>> In the example, however, it seems that in addition to merging pairs of
>> neighboring lists, "merge-neighbors" also sorts each pair of lists in
>> ascending order before combining them.
>>
>
> No!
>
>
>  In other words --
>>
>> (check-expect (merge-all-neighbors (list (list 9) (list 1)) (list (list 1
>> 9))?
>>
>
> This example wouldn't contradict the exercise. Indeed, it jives just fine.
>
>
>  Further, suppose the function input is a list of list-of-numbers in
>> descending order --
>>
>> (define list1 (list (list 9) (list 7) (list 5) (list 3))).
>>
>> In this case, does "merge-all-neighbors" evaluate list1 to (list (list
>> 7 9) (list 3 5)) --
>>
>
> This one obviously. But now run merge-all-neighbors on the result and see
> what you get.
>
> -- Matthias
>
>  -- or to (list (list 3 5) (list 7 9)).
>>
>
> That would be a function that merges neighbors and re-arranges things in
> addition.
>
>
>
>
>> Thanks!
>>
>> Dave Yrueta
>>
>>
>>
>>
>>
>> _________________________________________________
>>  For list-related administrative tasks:
>>  http://list.cs.brown.edu/mailman/listinfo/plt-scheme
>>
>
>
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