Pascal J. Bourguignon wrote:

and my solutions (still incomplete): http://www.informatimago.com/develop/lisp/l99/index.html

Where we find:

(---------------------------------------------------------------
P13 (**) Run-length encoding of a list (direct solution).

Example:
* (encode-direct '(a a a a b c c a a d e e e e))
((4 A) B (2 C) (2 A) D (4 E))
"

;; Iterative solution, uses only O(r) space:

(defun encode-modified (list)
(let ((result '())
(count 0)
(last-item nil))
(labels ((collect-result ()
(push (if (= 1 count)
last-item
(list count last-item))
result))
(new-item (item)
(setf count 1
last-item item))
(same-item ()
(incf count))
(return-result ()
(when (plusp count)
(collect-result))
(nreverse result)))
(dolist (item list (return-result))
(cond
((zerop count) (new-item item))
((eql item last-item) (same-item))
(t (collect-result)
(new-item item))))))) ---------------------------------------------------------------)

Gauche Scheme

(define (encode List)
(if (null? List)
()
(let
((tmp (fold
(lambda(x accum)
(if (equal? x (caar accum))
(cons (cons x (car accum)) (cdr accum))
(cons (list x) accum)))
`((,(car List)))
(cdr List))))
(reverse
(map
(lambda(xs) (let1 len (length xs)
(if (= 1 len) (car xs) (list len (car xs)))))
tmp)))))

gosh> (encode '(a a a a b c c a a d e e e e))
((4 a) b (2 c) (2 a) d (4 e))

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On 2025-06-21, B. Pym <[email protected]> wrote:

Pascal J. Bourguignon wrote:

and my solutions (still incomplete):
http://www.informatimago.com/develop/lisp/l99/index.html

Where we find:

(---------------------------------------------------------------
P13 (**) Run-length encoding of a list (direct solution).

Example:
* (encode-direct '(a a a a b c c a a d e e e e))
((4 A) B (2 C) (2 A) D (4 E))
"

(flow '(a a a a b c c a a d e e e e)

(partition-by identity)
(map [juxt car len]))
((a 4) (b 1) (c 2) (a 2) (d 1) (e 4))

Oops.

(flow '(a a a a b c c a a d e e e e)

(partition-by identity)
(map [juxt len car]))
((4 a) (1 b) (2 c) (2 a) (1 d) (4 e))

Oops.

(flow '(a a a a b c c a a d e e e e)

(partition-by identity)
(mapcar [iff cdr [juxt len car] car]))
((4 a) b (2 c) (2 a) d (4 e))

I get working code before I'm done properly scanning the requirements and can fix it in a few keystrokes.

Shorter:

;; Iterative solution, uses only O(r) space:

TL; DR.

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