How do I duplicate a whole line in Emacs?

Question

I saw this same question for VIM and it has been something that I myself wanted to know how to do for Emacs. In ReSharper I use CTRL-D for this action. What is the least num

Answer 1

Here's yet another function for doing this. My version doesn't touch the kill ring, and the cursor ends up on the new line where it was on the original. It will duplicate the region if it's active (transient mark mode), or default to duplicating the line otherwise. It will also make multiple copies if given a prefix arg, and comment out the original line if given a negative prefix arg (this is useful for testing a different version of a command/statement while keeping the old one).

(defun duplicate-line-or-region (&optional n)
  "Duplicate current line, or region if active.
With argument N, make N copies.
With negative N, comment out original line and use the absolute value."
  (interactive "*p")
  (let ((use-region (use-region-p)))
    (save-excursion
      (let ((text (if use-region        ;Get region if active, otherwise line
                      (buffer-substring (region-beginning) (region-end))
                    (prog1 (thing-at-point 'line)
                      (end-of-line)
                      (if (< 0 (forward-line 1)) ;Go to beginning of next line, or make a new one
                          (newline))))))
        (dotimes (i (abs (or n 1)))     ;Insert N times, or once if not specified
          (insert text))))
    (if use-region nil                  ;Only if we're working with a line (not a region)
      (let ((pos (- (point) (line-beginning-position)))) ;Save column
        (if (> 0 n)                             ;Comment out original with negative arg
            (comment-region (line-beginning-position) (line-end-position)))
        (forward-line 1)
        (forward-char pos)))))

I have it bound to C-c d:

(global-set-key [?\C-c ?d] 'duplicate-line-or-region)

This should never be re-assigned by a mode or anything because C-c followed by a single (unmodified) letter is reserved for user bindings.