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.sp...@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 > <jensa...@soegaard.net> wrote: >> 2012/5/27 Harry Spier <vasishtha.sp...@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)) ____________________ Racket Users list: http://lists.racket-lang.org/users
