Convert integer to list of digits


You have an integer, and you want a list of all its base-10 digits as integers.

If the integer is negative, the list should start with a minus sign.


Using number->string

(define (integer->list integer)
  (let ((chars (string->list (number->string integer))))
    (map (lambda (char)
           (if (char=? #\- char) '- (string->number (string char))))

Credit: Jakub T. Jankiewicz

Using quotient and remainder

The truncate/ procedure (from R7RS) performs an integer division, and returns both the quotient and the remainder. In some other programming languages, this operation is called divmod.

(define (integer->list integer)
  (let ((neg? (negative? integer)))
    (let loop ((integer (abs integer)) (digits '()))
      (let-values (((integer digit) (truncate/ integer 10)))
        (let ((digits (cons digit digits)))
          (if (> integer 0) (loop integer digits)
              (if neg? (cons '- digits) digits)))))))

Credit: Lassi Kortela


(integer->list 123450)
;; ==> (1 2 3 4 5 0)

(integer->list -123450)
;; ==> (- 1 2 3 4 5 0)

(integer->list 0)
;; ==> (0)

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