Re: exercise 2019/01

[Otto Diesenbacher-Reinmüller]

Hi Rowan,

⍨ operator: you are using it to avoid parentheses?

Beside of restructuring code, avoiding duplicate code-fragment and
parentheses, ⍨ has no "functional" impact?

br & many thanks - Otto

Rowan Cannaday <address@hidden> writes:

> addendum, forget an extra ⍳≢
>
> {(⍵⍴⍨≢⍵)⊂⍨(≢⍵)↑⍺/⍳≢⍵}
>
> Cheers.
>
> On Sat, Apr 11, 2020 at 9:19 PM Rowan Cannaday <address@hidden> wrote:
>
>> quick attempt:
>>
>>       3 {(⍵⍴⍨≢⍵)⊂⍨(≢⍵)↑⍺/⍵} 1 2 3 4 5 6 7
>> ┏→━━━━━━━━━━━━━━━━━━┓
>> ┃┏→━━━━┓ ┏→━━━━┓ ┏→┓┃
>> ┃┃1 2 3┃ ┃4 5 6┃ ┃7┃┃
>> ┃┗━━━━━┛ ┗━━━━━┛ ┗━┛┃
>> ┗∊━━━━━━━━━━━━━━━━━━┛
>>
>>
>> On Sat, Apr 11, 2020 at 9:02 PM Otto Diesenbacher-Reinmüller <
>> address@hidden> wrote:
>>
>>> Hi APLers!
>>>
>>> as I had a look into the archives, it seems ok, to post non bugs and
>>> other topics not closely related to gnu-apl, but APL in general (for
>>> sure, utilizing gnu-apl)? If my following post is offtopic please
>>> excuse, and please let me know, where would be right place to discuss.
>>>
>>> So - i am quite new to APL, my road to APL? Some weeks ago, being a
>>> hobbyist/partly professional Common Lisper, I stumpled upon April
>>> (https://github.com/phantomics/april) and was fascinated by
>>> APL. Always hungry for /something/ to get rid of Excel, I am starting
>>> to explore the world of arrays ;).
>>>
>>> Having browsed through "Mastering Dyalog APL"(Legrand) and "APL_ The
>>> Language and Its Usage"(Polivka/Pakin), to get some experience, I am
>>> trying to solve (the easy parts) of the APL Dyalog Problem Solving
>>> Competition (https://www.dyalog.com/student-competition.htm).
>>>
>>> I would like to share my experience, discuss the obstacles I
>>> encounter, and especially get some advice/hints from you, the
>>> experienced APLers! Perhaps this will lead to some blog-posts...
>>>
>>> Let's start with the first one:
>>>
>>> 1 chunk monkey
>>> (
>>> https://www.dyalog.com/uploads/files/student_competition/2019_problems_phase1.pdf
>>> )
>>>
>>> "Write a function that, given a scalar or vector as the right argument
>>> and a positive (>0)integer chunk size n as the left argument, breaks
>>> the array's items up into chunks of sizen. If the number of elements
>>> in the array is not evenly divisible by n, then the last chunkwill
>>> have fewer than n elements. Hint: The partitioned enclose function ⊂
>>> could be helpful for this problem."(from the exercise)
>>>
>>> hmmm, as in the exercise stated, dyadic ⊂ (partition) is to be used.
>>> So the real challenge is to generate the correct vector as left
>>> argument for ⊂, right?
>>>
>>>
>>> ⍝ first try:
>>> ∇ z← size chunkmonkey1 a
>>>   z←((⍴a)↑(size/⍳⍴a)) ⊂ a
>>> ∇
>>> ⍝ BUT
>>> ⍝ - doesn't work with a scalar as asked
>>> ⍝ - ((⍴a)↑(size/⍳⍴a)) seems inefficient, as we make a to big
>>> ⍝   vector and cut it down afterwards... Is this a common/fine
>>> ⍝   pattern?
>>>
>>>
>>> ⍝ second try:
>>> ∇ z← size chunkmonkey2 a
>>>   →((0=≢⍴a)∧1=≢a)/skalar        ⍝ matches a skalar, but not ''
>>>   z←((≢a)↑(size/⍳≢a))⊂a         ⍝ ≢ or ⍴ in this case?
>>>   →0
>>> skalar:
>>>   z←a
>>> ∇
>>>
>>> ⍝ correct, but feels clumsy...
>>> ⍝ - ≢ seems more acurately than ⍴ in this case?
>>> ⍝ - should be doable without jump?
>>> ⍝ - still the inefficient vector
>>>
>>> Final thoughts for now:
>>>
>>> What I learned/understood with this exercise: / is not only reduce
>>> (operator), but also dyadic functions filter and especially expand
>>> (not all APL-variants?).
>>>
>>> Checking for types is not easy. 0=⍴N is a scalar. Howto check for a
>>> string?
>>>
>>> br & and many thanks for any hints, Otto
>>>
>>> --
>>> Dipl. Ing. (FH) Otto Diesenbacher-Reinmüller
>>> diesenbacher.net
>>> Salzburg, Österreich
>>>
>>>


-- 
Dipl. Ing. (FH) Otto Diesenbacher-Reinmüller
diesenbacher.net
Salzburg, Österreich