How do I partition a list into (sub-)lists of random length?

Question

Input, by example (4 is the maximum length):

(my-partition-randomly '(a b c d e f g h i j k l) 4)

Output:

'((a b c) (d) (e f g h) (i j k) (l))

The code should run in the Emacs Lisp interpreter.

@AlexOtt No attempt yet. I could write something rather complex, but am looking for a simple solution. — feklee
– feklee, Commented Nov 16, 2018 at 10:20

choroba · Accepted Answer · 2018-11-16 10:38:13Z

My elisp-fu is weak, but I was able to write the following function:

(defun my-partition-randomly (list max-length) "" (let ((result '())) (while list (push (seq-take-while (lambda (x) x) (map 'list (lambda (x) (pop list)) (number-sequence 1 (+ 1 (random max-length))))) result)) (reverse result)))

It extracts random initial sequences of the input list and adds them to result (the seq-take-while is needed not to include nils when the last subsequence wants to be longer than the remaining list). push adds elements to the left, so the result has to reversed.

Thanks, your elisp-fu is better than mine, and this does the trick! I learned new functions, seq-take-while and map, and this was kind of the intention of my question. Let’s see if there is an even simpler solution.
You might get more attention at emacs.stackexchange.com with your future questions.
I chose Stack Overflow because my question is about Elisp the programming language. (not a fan of having so many overlapping platforms now)

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