scheme - Convert number to list of digits

Question

How do I convert a number to a list of digits?

I am currently doing:

;; (num->list 12345) -> '(1 2 3 4 5)
(define (num->list n)
  (local 
    ((define (num->list n)
       (map (lambda (c)
              (char->num c))
            (string->list (number->string n))))

    (define (char->num c)
      (- (char->integer c) 48)))

    (num->list n)))

but would like to know if there's a better way.

Answer 1

Here's how I'd do it in Racket:

(require srfi/1 srfi/26)
(define (digits->list num (base 10))
  (unfold-right zero? (cut remainder <> base) (cut quotient <> base) num))

This is the sort of problem unfold was designed for. :-D