Would any of the functions in unstable/list help? For example,
#lang racket
(require unstable/list)
(define (gather lst)
(for/hash ([g (group-by car lst)])
(values (caar g) (append-map cdr g))))
(gather '((a c) (a d) (b e) (b f)))
; => '#hash((a . (c d)) (b . (e f)))
On Thu, Jun 11, 2015 at 12:25 PM, 'John Clements' via users-redirect
<us...@plt-scheme.org> wrote:
> I write this kind of code all the darn time:
>
> ;; take (listof (list a b)) into (hashof a (listof b))
> (define (gather l)
> (for/fold ([ht (make-hash)])
> ([pr l])
> (hash-set ht (first pr) (cons (second pr)
> (hash-ref ht (first pr) empty)))))
>
> ;; gather the responses into a table:
> (define rating-table (gather responses2))
>
> ;; compute the mean rating for each student
> (for/list ([(student ratings) (in-hash rating-table)])
> (list student (mean ratings)))
>
> … that is: given an association list, gather them together, then compute
> (say) the mean of the values. In this case, I’m trying to compute the mean
> rating for a bunch of students. Very simple.
>
> So… why does it take so much code? I feel like these should be built-in
> abstractions. There are (at least) two possibilities:
>
> 1) There’s an obvious, built-in way to do this, and I’m just missing it.
> 2) There’s an abstraction that we’re missing, and I feel like Clojure might
> have it. Or am I just making this up? Getting clojure running is such a pain….
>
> Thanks for any advice!
>
> John
>
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