> From: Joe Gilray <jgil...@gmail.com>
> To: Racket mailing list <users@racket-lang.org>
> Subject: [racket] idiomatic and fast
>
> I'm trying to come up with the most idiomatic way to generate triangle
> numbers that is also fast:
>
> (for/list ([i (in-range 2 22)]) (for/sum ([j (in-range i)]) j))
>
> works but is slow because it regenerates the sums each time
>
> (define s 0) (for/list ([i (in-range 1 21)]) (set! s (+ s i)) s)
>
> is fast, but ugly.
>
> How do I keep the sum in an idiomatic fashion?
How about this:
;; Nat -> Listof[Nat]
;; produces the list of the first n triangle numbers, ascending
(define (list-triangles n)
(reverse
(for/fold ([triangles '(1)]) ([i (in-range 2 (+ 1 n))])
(cons (+ i (first triangles))
triangles))))
Cheers,
jmj
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