Is there a split-at-reverse anywhere, equivalent to split-at except that the first return value is reversed? If not, should that be added somewhere like srfi/1?
I’m asking because wanted to be able to write functions like: (define ((drop-lens n) lst) (define-values [fst-lst rst-lst] (split-at-reverse lst n)) (values rst-lst (λ (x) (append-reverse fst-lst x)))) (define ((list-ref-lens i) lst) (match-define-values [fst-lst val:rst-lst] (split-at-reverse lst i)) (match-define (cons val rst-lst) val:rst-lst) (values val (λ (x) (append-reverse fst-lst (cons x rst-lst))))) These are lenses as used in this package: https://github.com/jackfirth/lenses I wrote my own version of split-at-reverse, but I’m wondering if it should be put in something like srfi/1 as a companion to append-reverse, or if it perhaps already is in some other library. Alex Knauth On Jun 19, 2015, at 4:28 PM, Jens Axel Søgaard <jensa...@soegaard.net> wrote: > A more efficient version using append-reverse from srfi/1. > > #lang racket > (require (only-in srfi/1 append-reverse)) > > (define (list-splits xs) > (define (loop ys zs) ; xs = (append (reverse ys) yz) > (match zs > ['() '()] > [(cons z zs*) (cons (list ys zs) > (loop (cons z ys) zs*))])) > (loop '() xs)) > > > (define (map-once f xs) > (for/list ([ys+zs (list-splits xs)]) > (match ys+zs > [(list ys '()) '()] > [(list ys (cons z zs)) (append-reverse ys (cons (f z) zs))]))) > > (list-splits '(1 2 3)) > (map-once sqr '(1 2 3)) > > > 2015-06-19 22:07 GMT+02:00 Jens Axel Søgaard <jensa...@soegaard.net>: > #lang racket > (define (list-splits xs) > (define (loop ys zs) ; xs = (append (reverse ys) yz) > (match zs > ['() '()] > [(cons z zs*) (cons (list ys zs) > (loop (cons z ys) zs*))])) > (loop '() xs)) > > > (define (map-once f xs) > (for/list ([ys+zs (list-splits xs)]) > (match ys+zs > [(list ys '()) '()] > [(list ys (cons z zs)) (append (reverse ys) (cons (f z) zs))]))) > > (list-splits '(1 2 3)) > (map-once sqr '(1 2 3)) > > > 2015-06-19 22:05 GMT+02:00 Jon Zeppieri <zeppi...@gmail.com>: > It's unlikely that an implementation using continuations would be > faster than one that does not. > > An idiomatic solution might look like: > > (define (map-once fn xs) > (for/list ([i (in-range (length xs))]) > (for/list ([(x j) (in-indexed (in-list xs))]) > (cond [(= i j) (fn x)] > [else x])))) > > > But it's not terribly fast. > If you're willing to use vectors instead of lists, then maybe: > > (define (map-once fn xs) > (build-vector (vector-length xs) > (λ (i) > (define v (vector-copy xs)) > (vector-set! v i (fn (vector-ref v i))) > v))) > > > On Fri, Jun 19, 2015 at 3:24 PM, Luke Miles <rashreportl...@gmail.com> wrote: > > Say I have a list ls and I want to produce a list of > > lists where the i'th list has the i'th element of ls tripled, > > but all other elements are the same. > > > > e.g. '(3 5 7) => '((9 5 7) (3 15 7) (3 5 21)) > > > > What is a fast way to do this? > > > > > > I could do a loop with appending. > > (define (map-once f ls) > > (let M ([sooner null] [later ls]) > > (if (null? later) null > > (cons (append sooner (list (f (car later))) (cdr later)) > > (M (append sooner (list (car later))) (cdr later)))))) > > > > -> (map-once sqr '(4 5 6)) > > '((16 5 6) (4 25 6) (4 5 36)) > > > > Unfortunately, this is very slow & messy. > > I have to do 2 big appends for every element is the return list. > > > > > > Here is a cleaner-looking, but still slow way: > > (define (list-set ls i new-val) > > (let-values ([(sooner later) (split-at ls i)]) > > (append sooner (list new-val) (cdr later)))) > > > > (define (map-once f ls) > > (for/list ([i (in-naturals)] > > [elm (in-list ls)]) > > (list-set ls i (f elm)))) > > > > > > I'm thinking a good implementation might use continuations somehow? > > > > Maybe of vector-set (without the exclamation point) existed, I could use it? > > > > -- > > 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. > > > > -- > -- > Jens Axel Søgaard > > > > > -- > -- > Jens Axel Søgaard > > > -- > 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.
