(mmap_free_1): Avoid a compiler warning.

[emacs.git] / man / basic.texi

@c This is part of the Emacs manual.
@c Copyright (C) 1985, 86, 87, 93, 94, 95, 97, 2000 Free Software Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Basic, Minibuffer, Exiting, Top
@chapter Basic Editing Commands

@kindex C-h t
@findex help-with-tutorial
  We now give the basics of how to enter text, make corrections, and
save the text in a file.  If this material is new to you, you might
learn it more easily by running the Emacs learn-by-doing tutorial.  To
use the tutorial, run Emacs and type @kbd{Control-h t}
(@code{help-with-tutorial}).

  To clear the screen and redisplay, type @kbd{C-l} (@code{recenter}).

@menu

* Inserting Text::      Inserting text by simply typing it.
* Moving Point::        How to move the cursor to the place where you want to
                          change something.
* Erasing::             Deleting and killing text.
* Undo::                Undoing recent changes in the text.
* Files: Basic Files.   Visiting, creating, and saving files.
* Help: Basic Help.     Asking what a character does.
* Blank Lines::         Commands to make or delete blank lines.
* Continuation Lines::  Lines too wide for the screen.
* Position Info::       What page, line, row, or column is point on?
* Arguments::           Numeric arguments for repeating a command.
* Repeating::           A short-cut for repeating the previous command.
@end menu

@node Inserting Text
@section Inserting Text

@cindex insertion
@cindex graphic characters
  To insert printing characters into the text you are editing, just type
them.  This inserts the characters you type into the buffer at the
cursor (that is, at @dfn{point}; @pxref{Point}).  The cursor moves
forward, and any text after the cursor moves forward too.  If the text
in the buffer is @samp{FOOBAR}, with the cursor before the @samp{B},
then if you type @kbd{XX}, you get @samp{FOOXXBAR}, with the cursor
still before the @samp{B}.

   To @dfn{delete} text you have just inserted, use @key{DEL}.  @key{DEL}
deletes the character @emph{before} the cursor (not the one that the cursor
is on top of or under; that is the character @var{after} the cursor).  The
cursor and all characters after it move backwards.  Therefore, if you type
a printing character and then type @key{DEL}, they cancel out.

@kindex RET
@cindex newline
   To end a line and start typing a new one, type @key{RET}.  This
inserts a newline character in the buffer.  If point is in the middle of
a line, @key{RET} splits the line.  Typing @key{DEL} when the cursor is
at the beginning of a line deletes the preceding newline, thus joining
the line with the preceding line.

  Emacs can split lines automatically when they become too long, if you
turn on a special minor mode called @dfn{Auto Fill} mode.
@xref{Filling}, for how to use Auto Fill mode.

  If you prefer to have text characters replace (overwrite) existing
text rather than shove it to the right, you can enable Overwrite mode,
a minor mode.  @xref{Minor Modes}.

@cindex quoting
@kindex C-q
@findex quoted-insert
  Direct insertion works for printing characters and @key{SPC}, but other
characters act as editing commands and do not insert themselves.  If you
need to insert a control character or a character whose code is above 200
octal, you must @dfn{quote} it by typing the character @kbd{Control-q}
(@code{quoted-insert}) first.  (This character's name is normally written
@kbd{C-q} for short.)  There are two ways to use @kbd{C-q}:@refill

@itemize @bullet
@item
@kbd{C-q} followed by any non-graphic character (even @kbd{C-g})
inserts that character.

@item
@kbd{C-q} followed by a sequence of octal digits inserts the character
with the specified octal character code.  You can use any number of
octal digits; any non-digit terminates the sequence.  If the terminating
character is @key{RET}, it serves only to terminate the sequence; any
other non-digit is itself used as input after terminating the sequence.
(The use of octal sequences is disabled in ordinary non-binary Overwrite
mode, to give you a convenient way to insert a digit instead of
overwriting with it.)
@end itemize

@cindex 8-bit character codes
@noindent
When multibyte characters are enabled, if you specify a code in the
range 0200 through 0377 octal, @kbd{C-q} assumes that you intend to use
some ISO 8859-@var{n} character set, and converts the specified code to
the corresponding Emacs character code.  @xref{Enabling Multibyte}.  You
select to @emph{which} of the ISO 8859 character sets should Emacs
convert the 8-bit code through your choice of language environment
(@pxref{Language Environments}).

@vindex read-quoted-char-radix
To use decimal or hexadecimal instead of octal, set the variable
@code{read-quoted-char-radix} to 10 or 16.  If the radix is greater than
10, some letters starting with @kbd{a} serve as part of a character
code, just like digits.

A numeric argument to @kbd{C-q} specifies how many copies of the
quoted character should be inserted (@pxref{Arguments}).

@findex newline
@findex self-insert
  Customization information: @key{DEL} in most modes runs the command
@code{delete-backward-char}; @key{RET} runs the command @code{newline}, and
self-inserting printing characters run the command @code{self-insert},
which inserts whatever character was typed to invoke it.  Some major modes
rebind @key{DEL} to other commands.

@node Moving Point
@section Changing the Location of Point

@cindex arrow keys
@kindex LEFT
@kindex RIGHT
@kindex UP
@kindex DOWN
@cindex moving point
@cindex movement
@cindex cursor motion
@cindex moving the cursor
  To do more than insert characters, you have to know how to move point
(@pxref{Point}).  The simplest way to do this is with arrow keys, or by
clicking the left mouse button where you want to move to.

  There are also control and meta characters for cursor motion.  Some
are equivalent to the arrow keys (these date back to the days before
terminals had arrow keys, and are usable on terminals which don't have
them).  Others do more sophisticated things.

@kindex C-a
@kindex C-e
@kindex C-f
@kindex C-b
@kindex C-n
@kindex C-p
@kindex M->
@kindex M-<
@kindex M-r
@findex beginning-of-line
@findex end-of-line
@findex forward-char
@findex backward-char
@findex next-line
@findex previous-line
@findex beginning-of-buffer
@findex end-of-buffer
@findex goto-char
@findex goto-line
@findex move-to-window-line
@table @kbd
@item C-a
Move to the beginning of the line (@code{beginning-of-line}).
@item C-e
Move to the end of the line (@code{end-of-line}).
@item C-f
Move forward one character (@code{forward-char}).
@item C-b
Move backward one character (@code{backward-char}).
@item M-f
Move forward one word (@code{forward-word}).
@item M-b
Move backward one word (@code{backward-word}).
@item C-n
Move down one line, vertically (@code{next-line}).  This command
attempts to keep the horizontal position unchanged, so if you start in
the middle of one line, you end in the middle of the next.  When on
the last line of text, @kbd{C-n} creates a new line and moves onto it.
@item C-p
Move up one line, vertically (@code{previous-line}).
@item M-r
Move point to left margin, vertically centered in the window
(@code{move-to-window-line}).  Text does not move on the screen.

A numeric argument says which screen line to place point on.  It counts
screen lines down from the top of the window (zero for the top line).  A
negative argument counts lines from the bottom (@minus{}1 for the bottom
line).
@item M-<
Move to the top of the buffer (@code{beginning-of-buffer}).  With
numeric argument @var{n}, move to @var{n}/10 of the way from the top.
@xref{Arguments}, for more information on numeric arguments.@refill
@item M->
Move to the end of the buffer (@code{end-of-buffer}).
@item M-x goto-char
Read a number @var{n} and move point to buffer position @var{n}.
Position 1 is the beginning of the buffer.
@item M-x goto-line
Read a number @var{n} and move point to line number @var{n}.  Line 1
is the beginning of the buffer.
@item C-x C-n
@findex set-goal-column
@kindex C-x C-n
Use the current column of point as the @dfn{semipermanent goal column} for
@kbd{C-n} and @kbd{C-p} (@code{set-goal-column}).  Henceforth, those
commands always move to this column in each line moved into, or as
close as possible given the contents of the line.  This goal column remains
in effect until canceled.
@item C-u C-x C-n
Cancel the goal column.  Henceforth, @kbd{C-n} and @kbd{C-p} once
again try to stick to a fixed horizontal position, as usual.
@end table

@vindex track-eol
  If you set the variable @code{track-eol} to a non-@code{nil} value,
then @kbd{C-n} and @kbd{C-p} when at the end of the starting line move
to the end of another line.  Normally, @code{track-eol} is @code{nil}.
@xref{Variables}, for how to set variables such as @code{track-eol}.

@vindex next-line-add-newlines
  Normally, @kbd{C-n} on the last line of a buffer appends a newline to
it.  If the variable @code{next-line-add-newlines} is @code{nil}, then
@kbd{C-n} gets an error instead (like @kbd{C-p} on the first line).

@node Erasing   
@section Erasing Text

@table @kbd
@item @key{DEL}
Delete the character before point (@code{delete-backward-char}).
@item C-d
Delete the character after point (@code{delete-char}).
@item C-k
Kill to the end of the line (@code{kill-line}).
@item M-d
Kill forward to the end of the next word (@code{kill-word}).
@item M-@key{DEL}
Kill back to the beginning of the previous word
(@code{backward-kill-word}).
@end table

@cindex killing characters and lines
@cindex deleting characters and lines
@cindex erasing characters and lines
  You already know about the @key{DEL} key which deletes the character
before point (that is, before the cursor).  Another key, @kbd{Control-d}
(@kbd{C-d} for short), deletes the character after point (that is, the
character that the cursor is on).  This shifts the rest of the text on
the line to the left.  If you type @kbd{C-d} at the end of a line, it
joins together that line and the next line.

  To erase a larger amount of text, use the @kbd{C-k} key, which kills a
line at a time.  If you type @kbd{C-k} at the beginning or middle of a
line, it kills all the text up to the end of the line.  If you type
@kbd{C-k} at the end of a line, it joins that line and the next line.

  @xref{Killing}, for more flexible ways of killing text.

@node Undo
@section Undoing Changes
@cindex undo
@cindex changes, undoing

  You can undo all the recent changes in the buffer text, up to a
certain point.  Each buffer records changes individually, and the undo
command always applies to the current buffer.  Usually each editing
command makes a separate entry in the undo records, but some commands
such as @code{query-replace} make many entries, and very simple commands
such as self-inserting characters are often grouped to make undoing less
tedious.

@table @kbd
@item C-x u
Undo one batch of changes---usually, one command worth (@code{undo}).
@item C-_
The same.
@item C-u C-x u
Undo one batch of changes in the region.
@end table

@kindex C-x u
@kindex C-_
@findex undo
  The command @kbd{C-x u} or @kbd{C-_} is how you undo.  The first time
you give this command, it undoes the last change.  Point moves back to
where it was before the command that made the change.

  Consecutive repetitions of @kbd{C-_} or @kbd{C-x u} undo earlier and
earlier changes, back to the limit of the undo information available.
If all recorded changes have already been undone, the undo command
prints an error message and does nothing.

  Any command other than an undo command breaks the sequence of undo
commands.  Starting from that moment, the previous undo commands become
ordinary changes that you can undo.  Thus, to redo changes you have
undone, type @kbd{C-f} or any other command that will harmlessly break
the sequence of undoing, then type more undo commands.

@cindex selective undo
@kindex C-u C-x u
  Ordinary undo applies to all changes made in the current buffer.  You
can also perform @dfn{selective undo}, limited to the current region.
To do this, specify the region you want, then run the @code{undo}
command with a prefix argument (the value does not matter): @kbd{C-u C-x
u} or @kbd{C-u C-_}.  This undoes the most recent change in the region.
To undo further changes in the same region, repeat the @code{undo}
command (no prefix argument is needed).  In Transient Mark mode, any use
of @code{undo} when there is an active region performs selective undo;
you do not need a prefix argument.

  If you notice that a buffer has been modified accidentally, the
easiest way to recover is to type @kbd{C-_} repeatedly until the stars
disappear from the front of the mode line.  At this time, all the
modifications you made have been canceled.  Whenever an undo command
makes the stars disappear from the mode line, it means that the buffer
contents are the same as they were when the file was last read in or
saved.

  If you do not remember whether you changed the buffer deliberately,
type @kbd{C-_} once.  When you see the last change you made undone, you
will see whether it was an intentional change.  If it was an accident,
leave it undone.  If it was deliberate, redo the change as described
above.

  Not all buffers record undo information.  Buffers whose names start with
spaces don't; these buffers are used internally by Emacs and its extensions
to hold text that users don't normally look at or edit.

  You cannot undo mere cursor motion; only changes in the buffer
contents save undo information.  However, some cursor motion commands
set the mark, so if you use these commands from time to time, you can
move back to the neighborhoods you have moved through by popping the
mark ring (@pxref{Mark Ring}).

@vindex undo-limit
@vindex undo-strong-limit
@cindex undo limit
  When the undo information for a buffer becomes too large, Emacs
discards the oldest undo information from time to time (during garbage
collection).  You can specify how much undo information to keep by
setting two variables: @code{undo-limit} and @code{undo-strong-limit}.
Their values are expressed in units of bytes of space.

  The variable @code{undo-limit} sets a soft limit: Emacs keeps undo
data for enough commands to reach this size, and perhaps exceed it, but
does not keep data for any earlier commands beyond that.  Its default
value is 20000.  The variable @code{undo-strong-limit} sets a stricter
limit: the command which pushes the size past this amount is itself
forgotten.  Its default value is 30000.

  Regardless of the values of those variables, the most recent change is
never discarded, so there is no danger that garbage collection occurring
right after an unintentional large change might prevent you from undoing
it.

  The reason the @code{undo} command has two keys, @kbd{C-x u} and
@kbd{C-_}, set up to run it is that it is worthy of a single-character
key, but on some keyboards it is not obvious how to type @kbd{C-_}.
@kbd{C-x u} is an alternative you can type straightforwardly on any
terminal.

@node Basic Files
@section Files

  The commands described above are sufficient for creating and altering
text in an Emacs buffer; the more advanced Emacs commands just make
things easier.  But to keep any text permanently you must put it in a
@dfn{file}.  Files are named units of text which are stored by the
operating system for you to retrieve later by name.  To look at or use
the contents of a file in any way, including editing the file with
Emacs, you must specify the file name.

  Consider a file named @file{/usr/rms/foo.c}.  In Emacs, to begin editing
this file, type

@example
C-x C-f /usr/rms/foo.c @key{RET}
@end example

@noindent
Here the file name is given as an @dfn{argument} to the command @kbd{C-x
C-f} (@code{find-file}).  That command uses the @dfn{minibuffer} to
read the argument, and you type @key{RET} to terminate the argument
(@pxref{Minibuffer}).@refill

  Emacs obeys the command by @dfn{visiting} the file: creating a buffer,
copying the contents of the file into the buffer, and then displaying
the buffer for you to edit.  If you alter the text, you can @dfn{save}
the new text in the file by typing @kbd{C-x C-s} (@code{save-buffer}).
This makes the changes permanent by copying the altered buffer contents
back into the file @file{/usr/rms/foo.c}.  Until you save, the changes
exist only inside Emacs, and the file @file{foo.c} is unaltered.

  To create a file, just visit the file with @kbd{C-x C-f} as if it
already existed.  This creates an empty buffer in which you can insert
the text you want to put in the file.  The file is actually created when
you save this buffer with @kbd{C-x C-s}.

  Of course, there is a lot more to learn about using files.  @xref{Files}.

@node Basic Help
@section Help

@cindex getting help with keys
  If you forget what a key does, you can find out with the Help
character, which is @kbd{C-h} (or @key{F1}, which is an alias for
@kbd{C-h}).  Type @kbd{C-h k} followed by the key you want to know
about; for example, @kbd{C-h k C-n} tells you all about what @kbd{C-n}
does.  @kbd{C-h} is a prefix key; @kbd{C-h k} is just one of its
subcommands (the command @code{describe-key}).  The other subcommands of
@kbd{C-h} provide different kinds of help.  Type @kbd{C-h} twice to get
a description of all the help facilities.  @xref{Help}.@refill

@node Blank Lines
@section Blank Lines

@cindex inserting blank lines
@cindex deleting blank lines
  Here are special commands and techniques for putting in and taking out
blank lines.

@c widecommands
@table @kbd
@item C-o
Insert one or more blank lines after the cursor (@code{open-line}).
@item C-x C-o
Delete all but one of many consecutive blank lines
(@code{delete-blank-lines}).
@end table

@kindex C-o
@kindex C-x C-o
@cindex blank lines
@findex open-line
@findex delete-blank-lines
  When you want to insert a new line of text before an existing line, you
can do it by typing the new line of text, followed by @key{RET}.
However, it may be easier to see what you are doing if you first make a
blank line and then insert the desired text into it.  This is easy to do
using the key @kbd{C-o} (@code{open-line}), which inserts a newline
after point but leaves point in front of the newline.  After @kbd{C-o},
type the text for the new line.  @kbd{C-o F O O} has the same effect as
@w{@kbd{F O O @key{RET}}}, except for the final location of point.

  You can make several blank lines by typing @kbd{C-o} several times, or
by giving it a numeric argument to tell it how many blank lines to make.
@xref{Arguments}, for how.  If you have a fill prefix, then @kbd{C-o}
command inserts the fill prefix on the new line, when you use it at the
beginning of a line.  @xref{Fill Prefix}.

  The easy way to get rid of extra blank lines is with the command
@kbd{C-x C-o} (@code{delete-blank-lines}).  @kbd{C-x C-o} in a run of
several blank lines deletes all but one of them.  @kbd{C-x C-o} on a
solitary blank line deletes that blank line.  When point is on a
nonblank line, @kbd{C-x C-o} deletes any blank lines following that
nonblank line.

@node Continuation Lines
@section Continuation Lines

@cindex continuation line
@cindex wrapping
@cindex line wrapping
  If you add too many characters to one line without breaking it with
@key{RET}, the line will grow to occupy two (or more) lines on the screen,
with a @samp{\} at the extreme right margin of all but the last of them.
The @samp{\} says that the following screen line is not really a distinct
line in the text, but just the @dfn{continuation} of a line too long to fit
the screen.  Continuation is also called @dfn{line wrapping}.

  Sometimes it is nice to have Emacs insert newlines automatically when
a line gets too long.  Continuation on the screen does not do that.  Use
Auto Fill mode (@pxref{Filling}) if that's what you want.

@vindex truncate-lines
@findex toggle-truncate-lines
@cindex truncation
  As an alternative to continuation, Emacs can display long lines by
@dfn{truncation}.  This means that all the characters that do not fit in
the width of the screen or window do not appear at all.  They remain in
the buffer, temporarily invisible.  @samp{$} is used in the last column
instead of @samp{\} to inform you that truncation is in effect.

  Truncation instead of continuation happens whenever horizontal
scrolling is in use, and optionally in all side-by-side windows
(@pxref{Windows}).  You can enable truncation for a particular buffer by
setting the variable @code{truncate-lines} to non-@code{nil} in that
buffer.  (@xref{Variables}.)  Altering the value of
@code{truncate-lines} makes it local to the current buffer; until that
time, the default value is in effect.  The default is initially
@code{nil}.  @xref{Locals}.

  The command @kbd{M-x toggle-truncate-lines} toggles the display
between continuation and truncation.

  @xref{Display Vars}, for additional variables that affect how text is
displayed.

@node Position Info
@section Cursor Position Information

  Here are commands to get information about the size and position of
parts of the buffer, and to count lines.

@table @kbd
@item M-x what-page
Print page number of point, and line number within page.
@item M-x what-line
Print line number of point in the buffer.
@item M-x line-number-mode
@itemx M-x column-number-mode
Toggle automatic display of current line number or column number.
@xref{Optional Mode Line}.
@item M-=
Print number of lines in the current region (@code{count-lines-region}).
@xref{Mark}, for information about the region.
@item C-x =
Print character code of character after point, character position of
point, and column of point (@code{what-cursor-position}).
@item M-x hl-line-mode
Highlighting the current line.
@end table

@findex what-page
@findex what-line
@cindex line number commands
@cindex location of point
@cindex cursor location
@cindex point location
  There are two commands for working with line numbers.  @kbd{M-x
what-line} computes the current line number and displays it in the echo
area.  To go to a given line by number, use @kbd{M-x goto-line}; it
prompts you for the number.  These line numbers count from one at the
beginning of the buffer.

  You can also see the current line number in the mode line; @xref{Mode
Line}.  If you narrow the buffer, then the line number in the mode line
is relative to the accessible portion (@pxref{Narrowing}).  By contrast,
@code{what-line} shows both the line number relative to the narrowed
region and the line number relative to the whole buffer.

  By contrast, @kbd{M-x what-page} counts pages from the beginning of
the file, and counts lines within the page, printing both numbers.
@xref{Pages}.

@kindex M-=
@findex count-lines-region
  While on this subject, we might as well mention @kbd{M-=} (@code{count-lines-region}),
which prints the number of lines in the region (@pxref{Mark}).
@xref{Pages}, for the command @kbd{C-x l} which counts the lines in the
current page.

@kindex C-x =
@findex what-cursor-position
  The command @kbd{C-x =} (@code{what-cursor-position}) can be used to find out
the column that the cursor is in, and other miscellaneous information about
point.  It prints a line in the echo area that looks like this:

@smallexample
Char: c (0143, 99, 0x63)  point=21044 of 26883(78%)  column 53 
@end smallexample

@noindent
(In fact, this is the output produced when point is before the
@samp{column} in the example.)

  The four values after @samp{Char:} describe the character that follows
point, first by showing it and then by giving its character code in
octal, decimal and hex.  For a non-ASCII multibyte character, these are
followed by @samp{ext} and the character's representation, in hex, in
the buffer's coding system, if that coding system encodes the character
safely and with a single byte (@pxref{Coding Systems}).  If the
character's encoding is longer than one byte, Emacs shows @samp{ext ...}.

  @samp{point=} is followed by the position of point expressed as a character
count.  The front of the buffer counts as position 1, one character later
as 2, and so on.  The next, larger, number is the total number of characters
in the buffer.  Afterward in parentheses comes the position expressed as a
percentage of the total size.

  @samp{column} is followed by the horizontal position of point, in
columns from the left edge of the window.

  If the buffer has been narrowed, making some of the text at the
beginning and the end temporarily inaccessible, @kbd{C-x =} prints
additional text describing the currently accessible range.  For example, it
might display this:

@smallexample
Char: C (0103, 67, 0x43)  point=252 of 889(28%) <231 - 599>  column 0 
@end smallexample

@noindent
where the two extra numbers give the smallest and largest character
position that point is allowed to assume.  The characters between those
two positions are the accessible ones.  @xref{Narrowing}.

  If point is at the end of the buffer (or the end of the accessible
part), the @w{@kbd{C-x =}} output does not describe a character after
point.  The output might look like this:

@smallexample
point=26957 of 26956(100%)  column 0 
@end smallexample

  @w{@kbd{C-u C-x =}} displays additional information about a character,
in place of the buffer coordinates and column: the character set name
and the codes that identify the character within that character set;
ASCII characters are identified as belonging to the @code{ASCII}
character set.  In addition, the full character encoding, even if it
takes more than a single byte, is shown after @samp{ext}.  Here's an
example for a Latin-1 character A with a grave accent in a buffer whose
coding system is iso-2022-7bit@footnote{On terminals that support
Latin-1 characters, the character shown after @samp{Char:} is displayed
as the actual glyph of A with grave accent.}:

@example
Char: @`A (04300, 2240, 0x8c0, ext ESC , A @@) (latin-iso8859-1 64)
@end example

@findex hl-line-mode
@findex blink-cursor-mode
@cindex cursor, locating visually
@cindex cursor, blinking
@kbd{M-x hl-line-mode} turns on a global minor mode which highlights the
line about point in the selected window (on terminals which support
highlighting).  Some people find this convenient.  If you find the
cursor difficult to spot, you might try changing its color by
customizing the @code{cursor} face or rely on (the default)
@code{blink-cursor-mode}.  Cursor color and blinking can be conrolled
via the @code{cursor} Custom group.

@node Arguments
@section Numeric Arguments
@cindex numeric arguments
@cindex prefix arguments
@cindex arguments, numeric
@cindex arguments, prefix

  In mathematics and computer usage, the word @dfn{argument} means
``data provided to a function or operation.''  You can give any Emacs
command a @dfn{numeric argument} (also called a @dfn{prefix argument}).
Some commands interpret the argument as a repetition count.  For
example, @kbd{C-f} with an argument of ten moves forward ten characters
instead of one.  With these commands, no argument is equivalent to an
argument of one.  Negative arguments tell most such commands to move or
act in the opposite direction.

@kindex M-1
@kindex M-@t{-}
@findex digit-argument
@findex negative-argument
  If your terminal keyboard has a @key{META} key, the easiest way to
specify a numeric argument is to type digits and/or a minus sign while
holding down the @key{META} key.  For example,
@example
M-5 C-n
@end example
@noindent
would move down five lines.  The characters @kbd{Meta-1}, @kbd{Meta-2},
and so on, as well as @kbd{Meta--}, do this because they are keys bound
to commands (@code{digit-argument} and @code{negative-argument}) that
are defined to contribute to an argument for the next command.  Digits
and @kbd{-} modified with Control, or Control and Meta, also specify
numeric arguments.

@kindex C-u
@findex universal-argument
  Another way of specifying an argument is to use the @kbd{C-u}
(@code{universal-argument}) command followed by the digits of the
argument.  With @kbd{C-u}, you can type the argument digits without
holding down modifier keys; @kbd{C-u} works on all terminals.  To type a
negative argument, type a minus sign after @kbd{C-u}.  Just a minus sign
without digits normally means @minus{}1.

  @kbd{C-u} followed by a character which is neither a digit nor a minus
sign has the special meaning of ``multiply by four.''  It multiplies the
argument for the next command by four.  @kbd{C-u} twice multiplies it by
sixteen.  Thus, @kbd{C-u C-u C-f} moves forward sixteen characters.  This
is a good way to move forward ``fast,'' since it moves about 1/5 of a line
in the usual size screen.  Other useful combinations are @kbd{C-u C-n},
@kbd{C-u C-u C-n} (move down a good fraction of a screen), @kbd{C-u C-u
C-o} (make ``a lot'' of blank lines), and @kbd{C-u C-k} (kill four
lines).@refill

  Some commands care only about whether there is an argument, and not about
its value.  For example, the command @kbd{M-q} (@code{fill-paragraph}) with
no argument fills text; with an argument, it justifies the text as well.
(@xref{Filling}, for more information on @kbd{M-q}.)  Plain @kbd{C-u} is a
handy way of providing an argument for such commands.

  Some commands use the value of the argument as a repeat count, but do
something peculiar when there is no argument.  For example, the command
@kbd{C-k} (@code{kill-line}) with argument @var{n} kills @var{n} lines,
including their terminating newlines.  But @kbd{C-k} with no argument is
special: it kills the text up to the next newline, or, if point is right at
the end of the line, it kills the newline itself.  Thus, two @kbd{C-k}
commands with no arguments can kill a nonblank line, just like @kbd{C-k}
with an argument of one.  (@xref{Killing}, for more information on
@kbd{C-k}.)@refill

  A few commands treat a plain @kbd{C-u} differently from an ordinary
argument.  A few others may treat an argument of just a minus sign
differently from an argument of @minus{}1.  These unusual cases are
described when they come up; they are always for reasons of convenience
of use of the individual command.

  You can use a numeric argument to insert multiple copies of a
character.  This is straightforward unless the character is a digit; for
example, @kbd{C-u 6 4 a} inserts 64 copies of the character @samp{a}.
But this does not work for inserting digits; @kbd{C-u 6 4 1} specifies
an argument of 641, rather than inserting anything.  To separate the
digit to insert from the argument, type another @kbd{C-u}; for example,
@kbd{C-u 6 4 C-u 1} does insert 64 copies of the character @samp{1}.

  We use the term ``prefix argument'' as well as ``numeric argument'' to
emphasize that you type the argument before the command, and to
distinguish these arguments from minibuffer arguments that come after
the command.

@node Repeating
@section Repeating a Command
@cindex repeating a command

@kindex C-x z
@findex repeat
  The command @kbd{C-x z} (@code{repeat}) provides another way to repeat
an Emacs command many times.  This command repeats the previous Emacs
command, whatever that was.  Repeating a command uses the same arguments
that were used before; it does not read new arguments each time.

  To repeat the command more than once, type additional @kbd{z}'s: each
@kbd{z} repeats the command one more time.  Repetition ends when you
type a character other than @kbd{z}, or press a mouse button.

  For example, suppose you type @kbd{C-u 2 0 C-d} to delete 20
characters.  You can repeat that command (including its argument) three
additional times, to delete a total of 80 characters, by typing @kbd{C-x
z z z}.  The first @kbd{C-x z} repeats the command once, and each
subsequent @kbd{z} repeats it once again.