Emacs how to show only the lines on or before the cursor in a file

Question

I am a high school computer science teacher and would like to use emacs to present programs to my students. I would like to be able to present the programs, even short ones, one line at a time and not have the whole program show in the emacs buffer from the start.

Thus I would like emacs to hide all the lines below the current line and reveal each line when I move the cursor down to that line.

Answer 1

Try this:

(defun narrow-next-line ()
  (interactive)
  (widen)
  (call-interactively 'move-end-of-line)
  (forward-char)
  (call-interactively 'move-end-of-line)
  (narrow-to-region 1 (point)))

(global-set-key (kbd "C-x n i") 'narrow-next-line)

Choose your own hotkey for this command. See also http://www.gnu.org/software/emacs/manual/html_node/emacs/Narrowing.html