A position is the index of a character in the text of a buffer. More precisely, a position identifies the place between two characters (or before the first character, or after the last character), so we can speak of the character before or after a given position. However, we often speak of the character "at" a position, meaning the character after that position.
Positions are usually represented as integers starting from 1, but can also be represented as markers---special objects that relocate automatically when text is inserted or deleted so they stay with the surrounding characters. See section Markers.
Point is a special buffer position used by many editing commands, including the self-inserting typed characters and text insertion functions. Other commands move point through the text to allow editing and insertion at different places.
Like other positions, point designates a place between two characters (or before the first character, or after the last character), rather than a particular character. Usually terminals display the cursor over the character that immediately follows point; point is actually before the character on which the cursor sits.
The value of point is a number between 1 and the buffer size plus 1. If narrowing is in effect (see section Narrowing), then point is constrained to fall within the accessible portion of the buffer (possibly at one end of it).
Each buffer has its own value of point, which is independent of the value of point in other buffers. Each window also has a value of point, which is independent of the value of point in other windows on the same buffer. This is why point can have different values in various windows that display the same buffer. When a buffer appears in only one window, the buffer's point and the window's point normally have the same value, so the distinction is rarely important. See section Windows and Point, for more details.
(point)
=> 175
(1+ (buffer-size)), unless narrowing is
in effect, in which case it is the position of the end of the region
that you narrowed to. (See section Narrowing).
(point-min) if flag is less than 1,
(point-max) otherwise. The argument flag must be a number.
point-max returns a value one larger than this.
(buffer-size)
=> 35
(point-max)
=> 36
Motion functions change the value of point, either relative to the current value of point, relative to the beginning or end of the buffer, or relative to the edges of the selected window. See section Point.
These functions move point based on a count of characters.
goto-char is the fundamental primitive; the other functions use
that.
If narrowing is in effect, position still counts from the
beginning of the buffer, but point cannot go outside the accessible
portion. If position is out of range, goto-char moves
point to the beginning or the end of the accessible portion.
When this function is called interactively, position is the numeric prefix argument, if provided; otherwise it is read from the minibuffer.
goto-char returns position.
beginning-of-buffer or end-of-buffer.
In an interactive call, count is the numeric prefix argument.
beginning-of-buffer or end-of-buffer.
In an interactive call, count is the numeric prefix argument.
These functions for parsing words use the syntax table to decide whether a given character is part of a word. See section Syntax Tables.
If it is possible to move count words, without being stopped by
the buffer boundary (except perhaps after the last word), the value is
t. Otherwise, the return value is nil and point stops
at the buffer boundary.
In an interactive call, count is set to the numeric prefix argument.
forward-word, except that it moves
backward until encountering the front of a word, rather than forward.
In an interactive call, count is set to the numeric prefix argument.
This function is rarely used in programs, as it is more efficient to
call forward-word with a negative argument.
forward-word and everything
that uses it. If it is non-nil, then characters in the
"escape" and "character quote" syntax classes count as part of
words. Otherwise, they do not.
To move point to the beginning of the buffer, write:
(goto-char (point-min))
Likewise, to move to the end of the buffer, use:
(goto-char (point-max))
Here are two commands that users use to do these things. They are documented here to warn you not to use them in Lisp programs, because they set the mark and display messages in the echo area.
nil, then it
puts point n tenths of the way from the beginning of the buffer.
In an interactive call, n is the numeric prefix argument,
if provided; otherwise n defaults to nil.
Warning: Don't use this function in Lisp programs!
nil, then it puts
point n tenths of the way from the end of the buffer.
In an interactive call, n is the numeric prefix argument,
if provided; otherwise n defaults to nil.
Warning: Don't use this function in Lisp programs!
Text lines are portions of the buffer delimited by newline characters, which are regarded as part of the previous line. The first text line begins at the beginning of the buffer, and the last text line ends at the end of the buffer whether or not the last character is a newline. The division of the buffer into text lines is not affected by the width of the window, by line continuation in display, or by how tabs and control characters are displayed.
goto-line does not
necessarily move to the beginning of a line.
If narrowing is in effect, then line still counts from the
beginning of the buffer, but point cannot go outside the accessible
portion. So goto-line moves point to the beginning or end of the
accessible portion, if the line number specifies an inaccessible
position.
The return value of goto-line is the difference between
line and the line number of the line to which point actually was
able to move (in the full buffer, before taking account of narrowing).
Thus, the value is positive if the scan encounters the real end of the
buffer before finding the specified line. The value is zero if scan
encounters the end of the accessible portion but not the real end of the
buffer.
In an interactive call, line is the numeric prefix argument if one has been provided. Otherwise line is read in the minibuffer.
nil or 1, it moves forward
count-1 lines and then to the beginning of the line.
If this function reaches the end of the buffer (or of the accessible portion, if narrowing is in effect), it positions point there. No error is signaled.
nil or 1, it moves forward
count-1 lines and then to the end of the line.
If this function reaches the end of the buffer (or of the accessible portion, if narrowing is in effect), it positions point there. No error is signaled.
If forward-line encounters the beginning or end of the buffer (or
of the accessible portion) before finding that many lines, it sets point
there. No error is signaled.
forward-line returns the difference between count and the
number of lines actually moved. If you attempt to move down five lines
from the beginning of a buffer that has only three lines, point stops at
the end of the last line, and the value will be 2.
In an interactive call, count is the numeric prefix argument.
Here is an example of using count-lines:
(defun current-line ()
"Return the vertical position of point..."
(+ (count-lines (window-start) (point))
(if (= (current-column) 0) 1 0)
-1))
Also see the functions bolp and eolp in section Examining Text Near Point.
These functions do not move point, but test whether it is already at the
beginning or end of a line.
The line functions in the previous section count text lines, delimited only by newline characters. By contrast, these functions count screen lines, which are defined by the way the text appears on the screen. A text line is a single screen line if it is short enough to fit the width of the selected window, but otherwise it may occupy several screen lines.
In some cases, text lines are truncated on the screen rather than
continued onto additional screen lines. In these cases,
vertical-motion moves point much like forward-line.
See section Truncation.
Because the width of a given string depends on the flags that control
the appearance of certain characters, vertical-motion behaves
differently, for a given piece of text, depending on the buffer it is
in, and even on the selected window (because the width, the truncation
flag, and display table may vary between windows). See section Usual Display Conventions.
These functions scan text to determine where screen lines break, and thus take time proportional to the distance scanned. If you intend to use them heavily, Emacs provides caches which may improve the performance of your code. See section Truncation.
vertical-motion returns the number of screen lines over which it
moved point. The value may be less in absolute value than count
if the beginning or end of the buffer was reached.
The window window is used for obtaining parameters such as the
width, the horizontal scrolling, and the display table. But
vertical-motion always operates on the current buffer, even if
window currently displays some other buffer.
If count is nil, then point moves to the beginning of the
line in the middle of the window. If the absolute value of count
is greater than the size of the window, then point moves to the place
that would appear on that screen line if the window were tall enough.
This will probably cause the next redisplay to scroll to bring that
location onto the screen.
In an interactive call, count is the numeric prefix argument.
The value returned is the window line number point has moved to, with the top line in the window numbered 0.
The coordinate arguments frompos and topos are cons cells of
the form (hpos . vpos).
The argument width is the number of columns available to display
text; this affects handling of continuation lines. Use the value
returned by window-width for the window of your choice;
normally, use (window-width window).
The argument offsets is either nil or a cons cell of the
form (hscroll . tab-offset). Here hscroll is
the number of columns not being displayed at the left margin; most
callers get this by calling window-hscroll. Meanwhile,
tab-offset is the offset between column numbers on the screen and
column numbers in the buffer. This can be nonzero in a continuation
line, when the previous screen lines' widths do not add up to a multiple
of tab-width. It is always zero in a non-continuation line.
The window window serves only to specify which display table to
use. compute-motion always operates on the current buffer,
regardless of what buffer is displayed in window.
The return value is a list of five elements:
(pos vpos hpos prevhpos contin)
Here pos is the buffer position where the scan stopped, vpos is the vertical screen position, and hpos is the horizontal screen position.
The result prevhpos is the horizontal position one character back
from pos. The result contin is t if the last line
was continued after (or within) the previous character.
For example, to find the buffer position of column col of screen line
line of a certain window, pass the window's display start location
as from and the window's upper-left coordinates as frompos.
Pass the buffer's (point-max) as to, to limit the scan to
the end of the accessible portion of the buffer, and pass line and
col as topos. Here's a function that does this:
(defun coordinates-of-position (col line)
(car (compute-motion (window-start)
'(0 . 0)
(point-max)
(cons col line)
(window-width)
(cons (window-hscroll) 0)
(selected-window))))
When you use compute-motion for the minibuffer, you need to use
minibuffer-prompt-width to get the horizontal position of the
beginning of the first screen line. See section Minibuffer Miscellany.
Here are several functions concerned with balanced-parenthesis expressions (also called sexps in connection with moving across them in Emacs). The syntax table controls how these functions interpret various characters; see section Syntax Tables. See section Parsing Balanced Expressions, for lower-level primitives for scanning sexps or parts of sexps. For user-level commands, see section `Lists Commands' in GNU Emacs Manual.
---------- Buffer: foo ----------
(concat-!- "foo " (car x) y z)
---------- Buffer: foo ----------
(forward-sexp 3)
=> nil
---------- Buffer: foo ----------
(concat "foo " (car x) y-!- z)
---------- Buffer: foo ----------
nil, this variable holds a regular expression that
specifies what text can appear before the open-parenthesis that starts a
defun. That is to say, a defun begins on a line that starts with a
match for this regular expression, followed by a character with
open-parenthesis syntax.
The following two functions move point over a specified set of characters. For example, they are often used to skip whitespace. For related functions, see section Motion and Syntax.
The argument character-set is like the inside of a
`[...]' in a regular expression except that `]' is never
special and `\' quotes `^', `-' or `\'. Thus,
"a-zA-Z" skips over all letters, stopping before the first
nonletter, and "^a-zA-Z" skips nonletters stopping before the
first letter. See section Regular Expressions.
If limit is supplied (it must be a number or a marker), it specifies the maximum position in the buffer that point can be skipped to. Point will stop at or before limit.
In the following example, point is initially located directly before the `T'. After the form is evaluated, point is located at the end of that line (between the `t' of `hat' and the newline). The function skips all letters and spaces, but not newlines.
---------- Buffer: foo ----------
I read "-!-The cat in the hat
comes back" twice.
---------- Buffer: foo ----------
(skip-chars-forward "a-zA-Z ")
=> nil
---------- Buffer: foo ----------
I read "The cat in the hat-!-
comes back" twice.
---------- Buffer: foo ----------
skip-chars-forward except for the direction of motion.
The return value indicates the distance traveled. It is an integer that is zero or less.
It is often useful to move point "temporarily" within a localized
portion of the program, or to switch buffers temporarily. This is
called an excursion, and it is done with the save-excursion
special form. This construct saves the current buffer and its values of
point and the mark so they can be restored after the completion of the
excursion.
The forms for saving and restoring the configuration of windows are described elsewhere (see section Window Configurations, and see section Frame Configurations).
save-excursion special form saves the identity of the current
buffer and the values of point and the mark in it, evaluates
forms, and finally restores the buffer and its saved values of
point and the mark. All three saved values are restored even in case of
an abnormal exit via throw or error (see section Nonlocal Exits).
The save-excursion special form is the standard way to switch
buffers or move point within one part of a program and avoid affecting
the rest of the program. It is used more than 4000 times in the Lisp
sources of Emacs.
save-excursion does not save the values of point and the mark for
other buffers, so changes in other buffers remain in effect after
save-excursion exits.
Likewise, save-excursion does not restore window-buffer
correspondences altered by functions such as switch-to-buffer.
One way to restore these correspondences, and the selected window, is to
use save-window-excursion inside save-excursion
(see section Window Configurations).
The value returned by save-excursion is the result of the last of
forms, or nil if no forms are given.
(save-excursion forms)
==
(let ((old-buf (current-buffer))
(old-pnt (point-marker))
(old-mark (copy-marker (mark-marker))))
(unwind-protect
(progn forms)
(set-buffer old-buf)
(goto-char old-pnt)
(set-marker (mark-marker) old-mark)))
Warning: Ordinary insertion of text adjacent to the saved point value relocates the saved value, just as it relocates all markers. Therefore, when the saved point value is restored, it normally comes before the inserted text.
Although save-excursion saves the location of the mark, it does
not prevent functions which modify the buffer from setting
deactivate-mark, and thus causing the deactivation of the mark
after the command finishes. See section The Mark.
Narrowing means limiting the text addressable by Emacs editing commands to a limited range of characters in a buffer. The text that remains addressable is called the accessible portion of the buffer.
Narrowing is specified with two buffer positions which become the beginning and end of the accessible portion. For most editing commands and most Emacs primitives, these positions replace the values of the beginning and end of the buffer. While narrowing is in effect, no text outside the accessible portion is displayed, and point cannot move outside the accessible portion.
Values such as positions or line numbers, which usually count from the beginning of the buffer, do so despite narrowing, but the functions which use them refuse to operate on text that is inaccessible.
The commands for saving buffers are unaffected by narrowing; they save the entire buffer regardless of any narrowing.
In an interactive call, start and end are set to the bounds of the current region (point and the mark, with the smallest first).
nil means to move forward or backward by
move-count pages and then narrow to one page. The variable
page-delimiter specifies where pages start and end
(see section Standard Regular Expressions Used in Editing).
In an interactive call, move-count is set to the numeric prefix argument.
(narrow-to-region 1 (1+ (buffer-size)))
throw or error (see section Nonlocal Exits).
Therefore, this construct is a clean way to narrow a buffer temporarily.
The value returned by save-restriction is that returned by the
last form in body, or nil if no body forms were given.
Caution: it is easy to make a mistake when using the
save-restriction construct. Read the entire description here
before you try it.
If body changes the current buffer, save-restriction still
restores the restrictions on the original buffer (the buffer whose
restrictions it saved from), but it does not restore the identity of the
current buffer.
save-restriction does not restore point and the mark; use
save-excursion for that. If you use both save-restriction
and save-excursion together, save-excursion should come
first (on the outside). Otherwise, the old point value would be
restored with temporary narrowing still in effect. If the old point
value were outside the limits of the temporary narrowing, this would
fail to restore it accurately.
The save-restriction special form records the values of the
beginning and end of the accessible portion as distances from the
beginning and end of the buffer. In other words, it records the amount
of inaccessible text before and after the accessible portion.
This method yields correct results if body does further narrowing.
However, save-restriction can become confused if the body widens
and then makes changes outside the range of the saved narrowing. When
this is what you want to do, save-restriction is not the right
tool for the job. Here is what you must use instead:
(let ((beg (point-min-marker))
(end (point-max-marker)))
(unwind-protect
(progn body)
(save-excursion
(set-buffer (marker-buffer beg))
(narrow-to-region beg end))))
Here is a simple example of correct use of save-restriction:
---------- Buffer: foo ----------
This is the contents of foo
This is the contents of foo
This is the contents of foo-!-
---------- Buffer: foo ----------
(save-excursion
(save-restriction
(goto-char 1)
(forward-line 2)
(narrow-to-region 1 (point))
(goto-char (point-min))
(replace-string "foo" "bar")))
---------- Buffer: foo ----------
This is the contents of bar
This is the contents of bar
This is the contents of foo-!-
---------- Buffer: foo ----------
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