An Elisp attempt at a little engineering problem--converting a decimal number to binary.

dec-to-bin.emacs

(defun dec-to-bin (dec)
  (interactive "nDecimal number to convert: ")
  (setq debug-on-error t)
  (let ((bin-string "")
        (current-dec dec))
    (while (/= 0 current-dec)
      (setq bin-string (concat (number-to-string (% current-dec 2)) bin-string))
      (setq current-dec (truncate (/ current-dec 2))))
    (message "%d in binary is %s" dec bin-string)))