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41. Positions

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 42. Markers.

41.1 Point  The special position where editing takes place.
41.2 Motion  Changing point.
41.3 Excursions  Temporary motion and buffer changes.
41.4 Narrowing  Restricting editing to a portion of the buffer.


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41.1 Point

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 41.4 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 38.9 Windows and Point, for more details.

Function: point &optional buffer
This function returns the value of point in buffer, as an integer. buffer defaults to the current buffer if omitted.

 
(point)
     => 175

Function: point-min &optional buffer
This function returns the minimum accessible value of point in buffer. This is normally 1, but if narrowing is in effect, it is the position of the start of the region that you narrowed to. (See section 41.4 Narrowing.) buffer defaults to the current buffer if omitted.

Function: point-max &optional buffer
This function returns the maximum accessible value of point in buffer. This is (1+ (buffer-size buffer)), unless narrowing is in effect, in which case it is the position of the end of the region that you narrowed to. (see section 41.4 Narrowing). buffer defaults to the current buffer if omitted.

Function: buffer-end flag &optional buffer
This function returns (point-min buffer) if flag is less than 1, (point-max buffer) otherwise. The argument flag must be a number. buffer defaults to the current buffer if omitted.

Function: buffer-size &optional buffer
This function returns the total number of characters in buffer. In the absence of any narrowing (see section 41.4 Narrowing), point-max returns a value one larger than this. buffer defaults to the current buffer if omitted.

 
(buffer-size)
     => 35
(point-max)
     => 36

Variable: buffer-saved-size
The value of this buffer-local variable is the former length of the current buffer, as of the last time it was read in, saved or auto-saved.


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41.2 Motion

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 41.1 Point.

41.2.1 Motion by Characters  Moving in terms of characters.
41.2.2 Motion by Words  Moving in terms of words.
41.2.3 Motion to an End of the Buffer  Moving to the beginning or end of the buffer.
41.2.4 Motion by Text Lines  Moving in terms of lines of text.
41.2.5 Motion by Screen Lines  Moving in terms of lines as displayed.
41.2.6 Moving over Balanced Expressions  Moving by parsing lists and sexps.
41.2.7 Skipping Characters  Skipping characters belonging to a certain set.


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41.2.1 Motion by Characters

These functions move point based on a count of characters. goto-char is the fundamental primitive; the other functions use that.

Command: goto-char position &optional buffer
This function sets point in buffer to the value position. If position is less than 1, it moves point to the beginning of the buffer. If position is greater than the length of the buffer, it moves point to the end. buffer defaults to the current buffer if omitted.

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.

Command: forward-char &optional count buffer
This function moves point count characters forward, towards the end of the buffer (or backward, towards the beginning of the buffer, if count is negative). If the function attempts to move point past the beginning or end of the buffer (or the limits of the accessible portion, when narrowing is in effect), an error is signaled with error code beginning-of-buffer or end-of-buffer. buffer defaults to the current buffer if omitted.

In an interactive call, count is the numeric prefix argument.

Command: backward-char &optional count buffer
This function moves point count characters backward, towards the beginning of the buffer (or forward, towards the end of the buffer, if count is negative). If the function attempts to move point past the beginning or end of the buffer (or the limits of the accessible portion, when narrowing is in effect), an error is signaled with error code beginning-of-buffer or end-of-buffer. buffer defaults to the current buffer if omitted.

In an interactive call, count is the numeric prefix argument.


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41.2.2 Motion by Words

These functions for parsing words use the syntax table to decide whether a given character is part of a word. See section 45. Syntax Tables.

Command: forward-word &optional count buffer
This function moves point forward count words (or backward if count is negative). Normally it returns t. If this motion encounters the beginning or end of the buffer, or the limits of the accessible portion when narrowing is in effect, point stops there and the value is nil.

count defaults to 1 and buffer defaults to the current buffer.

In an interactive call, count is set to the numeric prefix argument.

Command: backward-word &optional count buffer
This function is just like forward-word, except that it moves backward until encountering the front of a word, rather than forward. buffer defaults to the current buffer if omitted.

In an interactive call, count is set to the numeric prefix argument.

Variable: words-include-escapes
This variable affects the behavior of 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.


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41.2.3 Motion to an End of the Buffer

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.

Command: beginning-of-buffer &optional count
This function moves point to the beginning of the buffer (or the limits of the accessible portion, when narrowing is in effect), setting the mark at the previous position. If count is non-nil, then it puts point count tenths of the way from the beginning of the buffer.

In an interactive call, count is the numeric prefix argument, if provided; otherwise count defaults to nil.

Don't use this function in Lisp programs!

Command: end-of-buffer &optional count
This function moves point to the end of the buffer (or the limits of the accessible portion, when narrowing is in effect), setting the mark at the previous position. If count is non-nil, then it puts point count tenths of the way from the end of the buffer.

In an interactive call, count is the numeric prefix argument, if provided; otherwise count defaults to nil.

Don't use this function in Lisp programs!


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41.2.4 Motion by Text Lines

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.

Command: goto-line line
This function moves point to the front of the lineth line, counting from line 1 at beginning of the buffer. If line is less than 1, it moves point to the beginning of the buffer. If line is greater than the number of lines in the buffer, it moves point to the end of the buffer--that is, the end of the last line of the buffer. This is the only case in which 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. 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.

Command: beginning-of-line &optional count buffer
This function moves point to the beginning of the current line. With an argument count not nil or 1, it moves forward count-1 lines and then to the beginning of the line. buffer defaults to the current buffer if omitted.

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.

Command: end-of-line &optional count buffer
This function moves point to the end of the current line. With an argument count not nil or 1, it moves forward count-1 lines and then to the end of the line. buffer defaults to the current buffer if omitted.

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.

Command: forward-line &optional count buffer
This function moves point forward count lines, to the beginning of the line. If count is negative, it moves point -count lines backward, to the beginning of a line. If count is zero, it moves point to the beginning of the current line. buffer defaults to the current buffer if omitted.

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.

Function: count-lines start end &optional ignore-invisible-lines-flag
This function returns the number of lines between the positions start and end in the current buffer. If start and end are equal, then it returns 0. Otherwise it returns at least 1, even if start and end are on the same line. This is because the text between them, considered in isolation, must contain at least one line unless it is empty.

With optional ignore-invisible-lines-flag non-nil, lines collapsed with selective-display are excluded from the line count.

N.B. The expression to return the current line number is not obvious:

 
(1+ (count-lines 1 (point-at-bol)))

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 43.1 Examining Text Near Point. These functions do not move point, but test whether it is already at the beginning or end of a line.


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41.2.5 Motion by Screen Lines

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 52.2 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 52.10 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 cache-long-line-scans.

Function: vertical-motion count &optional window pixels
This function moves point to the start of the frame line count frame lines down from the frame line containing point. If count is negative, it moves up instead. The optional second argument window may be used to specify a window other than the selected window in which to perform the motion.

Normally, vertical-motion returns the number of lines moved. The value may be less in absolute value than count if the beginning or end of the buffer was reached. If the optional third argument, pixels is non-nil, the vertical pixel height of the motion which took place is returned instead of the actual number of lines moved. A motion of zero lines returns the height of the current line.

Note that vertical-motion sets window's buffer's point, not window's point. (This differs from FSF Emacs, which buggily always sets current buffer's point, regardless of window.)

Function: vertical-motion-pixels count &optional window how
This function moves point to the start of the frame line pixels vertical pixels down from the frame line containing point, or up if pixels is negative. The optional second argument window is the window to move in, and defaults to the selected window. The optional third argument how specifies the stopping condition. A negative fixnum indicates that the motion should be no more than pixels. A positive value indicates that the motion should be at least pixels. Any other value indicates that the motion should be as close as possible to pixels.

Command: move-to-window-line count &optional window
This function moves point with respect to the text currently displayed in window, which defaults to the selected window. It moves point to the beginning of the screen line count screen lines from the top of the window. If count is negative, that specifies a position -count lines from the bottom (or the last line of the buffer, if the buffer ends above the specified screen position).

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.


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41.2.6 Moving over Balanced Expressions

Here are several functions concerned with balanced-parenthesis expressions (also called sexps in connection with moving across them in XEmacs). The syntax table controls how these functions interpret various characters; see 45. Syntax Tables. See section 45.5 Parsing Balanced Expressions, for lower-level primitives for scanning sexps or parts of sexps. For user-level commands, see section `Lists and Sexps' in XEmacs Reference Manual.

Command: forward-list &optional arg
This function moves forward across arg balanced groups of parentheses. (Other syntactic entities such as words or paired string quotes are ignored.) arg defaults to 1 if omitted. If arg is negative, move backward across that many groups of parentheses.

Command: backward-list &optional count
This function moves backward across count balanced groups of parentheses. (Other syntactic entities such as words or paired string quotes are ignored.) count defaults to 1 if omitted. If count is negative, move forward across that many groups of parentheses.

Command: up-list &optional count
This function moves forward out of count levels of parentheses. A negative argument means move backward but still to a less deep spot.

Command: down-list &optional count
This function moves forward into count levels of parentheses. A negative argument means move backward but still go deeper in parentheses (-count levels).

Command: forward-sexp &optional count
This function moves forward across count balanced expressions. Balanced expressions include both those delimited by parentheses and other kinds, such as words and string constants. count defaults to 1 if omitted. If count is negative, move backward across that many balanced expressions. For example,

 
---------- Buffer: foo ----------
(concat-!- "foo " (car x) y z)
---------- Buffer: foo ----------

(forward-sexp 3)
     => nil

---------- Buffer: foo ----------
(concat "foo " (car x) y-!- z)
---------- Buffer: foo ----------

Command: backward-sexp &optional count
This function moves backward across count balanced expressions. count defaults to 1 if omitted. If count is negative, move forward across that many balanced expressions.

Command: beginning-of-defun &optional count
This function moves back to the countth beginning of a defun. If count is negative, this actually moves forward, but it still moves to the beginning of a defun, not to the end of one. count defaults to 1 if omitted.

Command: end-of-defun &optional count
This function moves forward to the countth end of a defun. If count is negative, this actually moves backward, but it still moves to the end of a defun, not to the beginning of one. count defaults to 1 if omitted.

User Option: defun-prompt-regexp
If non-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.


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41.2.7 Skipping Characters

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 45.4 Motion and Syntax.

Function: skip-chars-forward character-set &optional limit buffer
This function moves point in buffer forward, skipping over a given set of characters. It examines the character following point, then advances point if the character matches character-set. This continues until it reaches a character that does not match. The function returns nil. buffer defaults to the current buffer if omitted.

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 non-letter, and "^a-zA-Z" skips non-letters stopping before the first letter. See section 44.2 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 ----------

Function: skip-chars-backward character-set &optional limit buffer
This function moves point backward, skipping characters that match character-set, until limit. It just like skip-chars-forward except for the direction of motion.


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41.3 Excursions

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 operator. 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 38.16 Window Configurations and see section 39.11 Frame Configurations).

Special Operator: save-excursion forms...
The save-excursion special operator 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 15.5 Nonlocal Exits).

The save-excursion special operator 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 500 times in the Lisp sources of XEmacs.

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 38.16 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)))

Special Operator: save-current-buffer forms...
This special operator is similar to save-excursion but it only saves and restores the current buffer. Beginning with XEmacs 20.3, save-current-buffer is a primitive.

Macro: with-current-buffer buffer forms...
This macro evaluates forms with buffer as the current buffer. It returns the value of the last form.

Macro: with-temp-file filename forms...
This macro creates a new buffer, evaluates forms there, and writes the buffer to filename. It returns the value of the last form evaluated.

Macro: save-selected-window forms...
This macro is similar to save-excursion but it saves and restores the selected window and nothing else.


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41.4 Narrowing

Narrowing means limiting the text addressable by XEmacs 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.

Command: narrow-to-region start end &optional buffer
This function sets the accessible portion of buffer to start at start and end at end. Both arguments should be character positions. buffer defaults to the current buffer if omitted.

In an interactive call, start and end are set to the bounds of the current region (point and the mark, with the smallest first).

Command: narrow-to-page &optional move-count
This function sets the accessible portion of the current buffer to include just the current page. An optional first argument move-count non-nil means to move forward or backward by move-count pages and then narrow. The variable page-delimiter specifies where pages start and end (see section 44.8 Standard Regular Expressions Used in Editing).

In an interactive call, move-count is set to the numeric prefix argument.

Command: widen &optional buffer
This function cancels any narrowing in buffer, so that the entire contents are accessible. This is called widening. It is equivalent to the following expression:

 
(narrow-to-region 1 (1+ (buffer-size)))

buffer defaults to the current buffer if omitted.

Special Operator: save-restriction body...
This special operator saves the current bounds of the accessible portion, evaluates the body forms, and finally restores the saved bounds, thus restoring the same state of narrowing (or absence thereof) formerly in effect. The state of narrowing is restored even in the event of an abnormal exit via throw or error (see section 15.5 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 operator 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 make 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 ((start (point-min-marker))
      (end (point-max-marker)))
  (unwind-protect
      (progn body)
    (save-excursion
      (set-buffer (marker-buffer start))
      (narrow-to-region start 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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