[racket] Looping with look-behind and look-forward

Sorry Jens, I see I've just recreated the form you originally suggested.
Thanks, Harry

On Sun, May 27, 2012 at 11:46 AM, Harry Spier <vasishtha.spier at gmail.com> wrote:
> I think I've come up with a simpler form to do this.
>
> (define (running-average-of-3 l)
>   (for
>    ( [prior (in-list l)]
>      [x (in-list (rest l))]
>      [next (in-list (rest (rest l)))])
>   (displayln (/ (+ x prior next) 3))))
>
> (running-average-of-3 '(1 2 3 4 5 6))
>
>
> So I can do something like:
>
> (define (f1 l)
>   (for/fold ([result-list '()])
>    ( [prior (in-list (append '() l))]
>      [x (in-list l)]
>      [next (in-list  (append (rest l) '()))])
>    ...
>    ... make new-result-element using x prior and next...
>   (cons new-result-element result-list))
>  (reverse result-list))
>
> The list I'm working on in my application has about 40,000 sub-lists.
> Will the compiler build new lists from scratch as the sequence for
> prior and next in this code or will it use pointers to and from the
> existing list l to do this?
>
> Thanks,
> Harry
>
> On Sun, May 27, 2012 at 10:38 AM, Jens Axel Søgaard
> <jensaxel at soegaard.net> wrote:
>> 2012/5/27 Harry Spier <vasishtha.spier at gmail.com>:
>>> Is the "for" form a macro?  And if so how difficult would it be to
>>> make a new form "for-with-look-around" that had built in look-back and
>>> look-ahead operators (next x) (prior x) (  next?) (prior?).
>>>
>>> So you could do something like:
>>>
>>> ((define (running-average-of-3 l)
>>>   (for-with-look-around
>>>    ([x (in-list l)])
>>>    (unless? (or (prior?) (next?))
>>>                 (displayln (/ (+ x (prior x) (next x)) 3))))))
>>>
>>> From a functional programming perspective is building a form like this
>>> a bad idea?
>>
>> It is a fine idea.
>> Here is variation on the idea.
>>
>> (define (in-triples l)
>>  (define (sublists xs)
>>    (if (empty? xs)
>>        '()
>>        (cons xs (sublists (rest xs)))))
>>  (define (triple xs) (take xs 3))
>>  (sequence-map triple (drop-right (sublists l) 2)))
>>
>> (define (running-average-of-3 l)
>>  (for ([t (in-triples l)])
>>    (match-define (list x y z) t)
>>    (displayln (/ (+ x y z) 3))))
>>
>> (running-average-of-3 '(1 2 3 4 5 6))