The following function returns a list of bits (number 1 and 0) from a program, and prints a message if used interactively.

(defun bits (num) (interactive "nNumber: ") (unless (integerp num) (error "%S must be an integer" num)) (let (result (n num)) (while (> n 0) (setq result (cons (mod n 2) result) n (/ n 2))) (if (interactive-p) (message "%d is %s" num (mapconcat 'number-to-string result "")) result)))

M-x bits RET 5 RET prints “5 is 101”, while (bits 5) evaluates to (1 0 1).

SimpleCalculator provides a different, *interactive-only solution* for converting numbers: The keys B,H,O switch between binary, octal and hex-mode. M-x calculator RET 5 B prints “Calc=B⇒ 101”

Library Lisp:hexrgb.el converts between hex and decimal RGB values (and also between RGB and HSV color components). – DrewAdams