Thanks, Dan. Working with shifted lists is an interesting technique that I'll keep in my quiver. I appreciate you taking the time.
On Fri, Dec 2, 2016 at 1:56 PM, Daniel Prager <daniel.a.pra...@gmail.com> wrote: > Perhaps this is a more elegant approach to the tactical problem: > simultaneously iterating over displaced versions of the original list. E.g. > > _x '(1 1 2 3 5 7 200 201 202) ; shifted right > x '(1 2 3 5 7 200 201 202 203) ; original list > x_ '(2 3 5 7 200 201 202 203 203) ; shifted left > > When (= (+ _x 1) x (- x_ 1)) we are mid-run. Otherwise we're at the start > or end of a run. Code below. > > A further displacement is needed to deal with extended runs: x__ to skip > elements in long runs. > > > #lang racket > > ; Given a strictly ascending list of numbers, summarise runs > ; (... b b+1 ... b+n c ...) -> (... a b - b+n c ...) > (define (runify xs) > (define MIN (first xs)) > (define MAX (last xs)) > (for/list ([_x (cons MIN xs)] > [x xs] > [x_ (append (rest xs) (list MAX))] > [x__ (append (drop xs 2) (list MAX MAX))] > #:unless (= (+ _x 2) x_ (- x__ 1))) > (if (= (+ _x 1) x (- x_ 1)) > '- > x))) > > ; Replace runify-ed lists with lists of lists > ; E.g. '(1 - 3 8 12 15 - 100) -> '((1 3) (8) (12) (15 100)) > ; > (define (chunkify xs [acc null]) > (cond [(null? xs) acc] > [(equal? (first xs) '-) > (chunkify (drop xs 2) > (cons (list (caar acc) (second xs)) > (rest acc)))] > [else > (chunkify (rest xs) (cons (list (first xs)) acc))])) > > > (define xs '(1 2 3 5 7 200 201 202 203)) > > (runify xs) ; '(1 - 3 5 7 200 - 203) > > (reverse (chunkify (runify xs))) ; '((1 3) (5) (7) (200 203)) > > > * * * > > Racket question: How do I define a sequence generator (in-shifted-list xs > shift) to make runify work in true single pass fashion? > > I would like to be able to write the following version of runify, without > generating auxiliary lists: > > (define (runify-efficient xs) > (for/list ([_x (in-shifted-list xs -1)] > [x xs] > [x_ (in-shifted-list xs 1)] > [x__ (in-shifted-list xs 2)] > #:unless (= (+ _x 2) x_ (- x__ 1))) > (if (= (+ _x 1) x (- x_ 1)) > '- > x))) > > Dan > > -- > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to racket-users+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
