dec2bin.el

dec2bin.el

; Example:
; * M-x dec2bin -> Decimal: 10 [Enter] -> 1010
; * (dec2bin 10) -> "1010"
; * C-u 10 M-x dec2bin -> 1010
; Ref.:
; * Basic syntax : http://xahlee.org/emacs/elisp_basics.html
; * concat : http://www.gnu.org/software/emacs/manual/html_node/elisp/Creating-Strings.html
; * called-interactively-p : http://www.gnu.org/software/emacs/manual/html_node/elisp/Distinguish-Interactive.html
(defun dec2bin (n)
  "Returns binary expression of decimal number N."
  (interactive "NDecimal: ")
  (let (answer)
    (setq answer "")
    (if (= n 0)
        (setq answer "0")
      (let (log2n digit)
        (setq log2n (floor (/ (log n) (log 2.0))))
        (setq digit log2n)
        (while (>= digit 0)
          (let (place)
            (setq place (expt 2 digit))
            (setq answer (concat answer
                                 (if (>= n place)
                                     (progn (setq n (- n place)) "1")
                                   "0")))
            (setq digit (- digit 1))))))
    (if (called-interactively-p 'any)
        (message "%s" answer)
      answer)
    ))