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(emacs-lisp-intro.info)what-line

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`what-line'
===========

   The `what-line' command tells you the number of the line in which
the cursor is located.  The function illustrates the use of the
`save-restriction' and `save-excursion' commands.  Here is the text of
the function in full:

     (defun what-line ()
       "Print the current line number (in the buffer) of point."
       (interactive)
       (save-restriction
         (widen)
         (save-excursion
           (beginning-of-line)
           (message "Line %d"
                    (1+ (count-lines 1 (point)))))))

   The function has a documentation line and is interactive, as you
would expect.  The next two lines use the functions `save-restriction'
and `widen'.

   The `save-restriction' special form notes whatever narrowing is in
effect, if any, in the current buffer and restores that narrowing after
the code in the body of the `save-restriction' has been evaluated.

   The `save-restriction' special form is followed by `widen'.  This
function undoes any narrowing the current buffer may have had when
`what-line' was called.  (The narrowing that was there is the narrowing
that `save-restriction' remembers.)  This widening makes it possible
for the line counting commands to count from the beginning of the
buffer.  Otherwise, they would have been limited to counting within the
accessible region.  Any original narrowing is restored just before the
completion of the function by the `save-restriction' special form.

   The call to `widen' is followed by `save-excursion', which saves the
location of the cursor (i.e., of point) and of the mark, and restores
them after the code in the body of the `save-excursion' uses the
`beginning-of-line' function to move point.

   (Note that the `(widen)' expression comes between the
`save-restriction' and `save-excursion' special forms.  When you write
the two `save- ...' expressions in sequence, write `save-excursion'
outermost.)

   The last two lines of the `what-line' function are functions to
count the number of lines in the buffer and then print the number in the
echo area.

     (message "Line %d"
              (1+ (count-lines 1 (point)))))))

   The `message' function prints a one-line message at the bottom of the
Emacs screen.  The first argument is inside of quotation marks and is
printed as a string of characters.  However, it may contain `%d', `%s',
or `%c' to print arguments that follow the string.  `%d' prints the
argument as a decimal, so the message will say something such as `Line
243'.

   The number that is printed in place of the `%d' is computed by the
last line of the function:

     (1+ (count-lines 1 (point)))

What this does is count the lines from the first position of the
buffer, indicated by the `1', up to `(point)', and then add one to that
number.  (The `1+' function adds one to its argument.)  We add one to
it because line 2 has only one line before it, and `count-lines' counts
only the lines _before_ the current line.

   After `count-lines' has done its job, and the message has been
printed in the echo area, the `save-excursion' restores point and mark
to their original positions; and `save-restriction' restores the
original narrowing, if any.