www.delorie.com/gnu/docs/emacs-lisp-intro/emacs-lisp-intro_102.html   search  
 
Buy the book!


Programming in Emacs Lisp

[ < ] [ > ]   [ << ] [ Up ] [ >> ]         [Top] [Contents] [Index] [ ? ]

5.3.3 The Complete beginning-of-buffer

Here is the complete text of the beginning-of-buffer function:

 
(defun beginning-of-buffer (&optional arg)
  "Move point to the beginning of the buffer;
leave mark at previous position.
With arg N, put point N/10 of the way
from the true beginning.
Don't use this in Lisp programs!
\(goto-char (point-min)) is faster
and does not set the mark."
  (interactive "P")
  (push-mark)
  (goto-char
   (if arg
       (if (> (buffer-size) 10000)
           ;; Avoid overflow for large buffer sizes!
           (* (prefix-numeric-value arg)
              (/ (buffer-size) 10))
         (/ (+ 10 (* (buffer-size)
                     (prefix-numeric-value arg)))
            10))
     (point-min)))
  (if arg (forward-line 1)))

Except for two small points, the previous discussion shows how this function works. The first point deals with a detail in the documentation string, and the second point concerns the last line of the function.

In the documentation string, there is reference to an expression:

 
\(goto-char (point-min))

A `\' is used before the first parenthesis of this expression. This `\' tells the Lisp interpreter that the expression should be printed as shown in the documentation rather than evaluated as a symbolic expression, which is what it looks like.

Finally, the last line of the beginning-of-buffer command says to move point to the beginning of the next line if the command is invoked with an argument:

 
(if arg (forward-line 1)))

This puts the cursor at the beginning of the first line after the appropriate tenths position in the buffer. This is a flourish that means that the cursor is always located at least the requested tenths of the way through the buffer, which is a nicety that is, perhaps, not necessary, but which, if it did not occur, would be sure to draw complaints.


[ < ] [ > ]   [ << ] [ Up ] [ >> ]         [Top] [Contents] [Index] [ ? ]

  webmaster   donations   bookstore     delorie software   privacy  
  Copyright 2003   by The Free Software Foundation     Updated Jun 2003