On 2016-08-30, Steve Graham <solitary.wanderer52@gmail.com> wrote:

The problem states:

Define a function that takes a list and returns a list indicating the number of times each (eql) element appears, sorted from most common
element to least common

$ txr
This is the TXR Lisp interactive listener of TXR 147.
Use the :quit command or type Ctrl-D on empty line to exit.

[group-by identity '(a a b a c b c c a a d e)]

#H(() (e (e)) (d (d)) (a (a a a a a)) (c (c c c)) (b (b b)))

[hash-update [group-by identity '(a a b a c b c c a a d e)] length]

#H(() (e 1) (d 1) (a 5) (c 3) (b 2))

(hash-pairs [hash-update [group-by identity '(a a b a c b c c a a d

e)] length])
((e 1) (d 1) (a 5) (c 3) (b 2))

[sort (hash-pairs [hash-update [group-by identity '(a a b a c b c c

a a d e)] length]) > second]
((a 5) (c 3) (b 2) (d 1) (e 1))

Without using a hash-table or mutation.

Gauche Scheme:

(use srfi-1) ;; lset-difference

(define (update-alist alist item)
(define found (assoc item alist))
(if found
(cons (list item (+ 1 (cadr found)))
(lset-difference (lambda (a b) (equal? (car a) b))
alist (list item)))
(cons (list item 1) alist)))

(define (foo the-list)
(let go ((xs the-list) (counts '()))
(if (null? xs)
(sort counts > cadr)
(go (cdr xs) (update-alist counts (car xs))))))

(foo '(a a b a c b c c a a d e))
===>
((a 5) (c 3) (b 2) (e 1) (d 1))

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