
@node Indentation, Text, Major Modes, Top
@chapter Indentation
@cindex indentation

@c WideCommands
@table @kbd
@item @key{TAB}
Indent current line ``appropriately'' in a mode-dependent fashion.
@item @key{LFD}
Perform @key{RET} followed by @key{TAB} (@code{newline-and-indent}).
@item M-^
Merge two lines (@code{delete-indentation}).  This would cancel out
the effect of @key{LFD}.
@item C-M-o
Split line at point; text on the line after point becomes a new line
indented to the same column that it now starts in (@code{split-line}).
@item M-m
Move (forward or back) to the first non-blank character on the current
line (@code{back-to-indentation}).
@item C-M-\
Indent several lines to same column (@code{indent-region}).
@item C-x @key{TAB}
Shift block of lines rigidly right or left (@code{indent-rigidly}).
@item M-i
Indent from point to the next prespecified tab stop column
(@code{tab-to-tab-stop}).
@item M-x indent-relative
Indent from point to under an indentation point in the previous line.
@end table

@kindex TAB
@cindex indentation
  Most programming languages have some indentation convention.  For Lisp
code, lines are indented according to their nesting in parentheses.  The
same general idea is used for C code, though details differ.

   Use the @key{TAB} command to indent a line whatever the language.
Each major mode defines this command to perform indentation appropriate
for the particular language.  In Lisp mode, @key{TAB} aligns a line
according to its depth in parentheses.  No matter where in the line you
are when you type @key{TAB}, it aligns the line as a whole.  In C mode,
@key{TAB} implements a subtle and sophisticated indentation style that
knows about many aspects of C syntax.

@kindex TAB
  In Text mode, @key{TAB} runs the command @code{tab-to-tab-stop}, which
indents to the next tab stop column.  You can set the tab stops with
@kbd{M-x edit-tab-stops}.

@menu
* Indentation Commands:: Various commands and techniques for indentation.
* Tab Stops::            You can set arbitrary "tab stops" and then
                         indent to the next tab stop when you want to.
* Just Spaces::          You can request indentation using just spaces.
@end menu

@node Indentation Commands, Tab Stops, Indentation, Indentation
@section Indentation Commands and Techniques
@c ??? Explain what Emacs has instead of space-indent-flag.

  If you just want to insert a tab character in the buffer, you can type
@kbd{C-q @key{TAB}}.

@kindex M-m
@findex back-to-indentation
  To move over the indentation on a line, type @kbd{Meta-m}
(@code{back-to-indentation}).  This command, given anywhere on a line,
positions point at the first non-blank character on the line.

  To insert an indented line before the current line, type @kbd{C-a C-o
@key{TAB}}.  To make an indented line after the current line, use
@kbd{C-e @key{LFD}}.

@kindex C-M-o
@findex split-line
  @kbd{C-M-o} (@code{split-line}) moves the text from point to the end of
the line vertically down, so that the current line becomes two lines.
@kbd{C-M-o} first moves point forward over any spaces and tabs.  Then it
inserts after point a newline and enough indentation to reach the same
column point is on.  Point remains before the inserted newline; in this
regard, @kbd{C-M-o} resembles @kbd{C-o}.

@kindex M-\
@kindex M-^
@findex delete-horizontal-space
@findex delete-indentation
  To join two lines cleanly, use the @kbd{Meta-^}
(@code{delete-indentation}) command to delete the indentation at the
front of the current line, and the line boundary as well.  Empty spaces
are replaced by a single space, or by no space if at the beginning of a
line, before a close parenthesis, or after an open parenthesis.  
To delete just the indentation of a line, go to the beginning of the 
line and use @kbd{Meta-\} (@code{delete-horizontal-space}), which 
deletes all spaces and tabs around the cursor.

@kindex C-M-\
@kindex C-x TAB
@findex indent-region
@findex indent-rigidly
  There are also commands for changing the indentation of several lines at
once.  @kbd{Control-Meta-\} (@code{indent-region}) gives each line which
begins in the region the ``usual'' indentation by invoking @key{TAB} at the
beginning of the line.  A numeric argument specifies the column to indent
to.  Each line is shifted left or right so that its first non-blank
character appears in that column.  @kbd{C-x @key{TAB}}
(@code{indent-rigidly}) moves all the lines in the region right by its
argument (left, for negative arguments).  The whole group of lines moves
rigidly sideways, which is how the command gets its name.@refill

@findex indent-relative
  @kbd{M-x indent-relative} indents at point based on the previous line
(actually, the last non-empty line.)  It inserts whitespace at point, moving
point, until it is underneath an indentation point in the previous line.
An indentation point is the end of a sequence of whitespace or the end of
the line.  If point is farther right than any indentation point in the
previous line, the whitespace before point is deleted and the first
indentation point then applicable is used.  If no indentation point is
applicable even then, @code{tab-to-tab-stop} is run (see next section).

  @code{indent-relative} is the definition of @key{TAB} in Indented Text
mode.  @xref{Text}.

@node Tab Stops, Just Spaces, Indentation Commands, Indentation
@section Tab Stops

@kindex M-i
@findex tab-to-tab-stop
  For typing in tables, you can use Text mode's definition of @key{TAB},
@code{tab-to-tab-stop}.  This command inserts indentation before point,
enough to reach the next tab stop column.  Even if you are not in Text mode,
this function is associated with @kbd{M-i} anyway.

@findex edit-tab-stops
@findex edit-tab-stops-note-changes
@kindex C-c C-c (Edit Tab Stops)
@vindex tab-stop-list
  You can arbitrarily set the tab stops used by @kbd{M-i}.  They are
stored as a list of column-numbers in increasing order in the variable
@code{tab-stop-list}.

  The convenient way to set the tab stops is using @kbd{M-x edit-tab-stops},
which creates and selects a buffer containing a description of the tab stop
settings.  You can edit this buffer to specify different tab stops, and
then type @kbd{C-c C-c} to make those new tab stops take effect.  In the
tab stop buffer, @kbd{C-c C-c} runs the function
@code{edit-tab-stops-note-changes} rather than the default
@code{save-buffer}.  @code{edit-tab-stops} records which buffer was current
when you invoked it, and stores the tab stops in that buffer.  Normally
all buffers share the same tab stops and changing them in one buffer
affects all.  If you make @code{tab-stop-list} local in one
buffer @code{edit-tab-stops} in that buffer edits only the local
settings.

  Below is the text representing ordinary tab stops every eight columns:

@example
        :       :       :       :       :       :
0         1         2         3         4
0123456789012345678901234567890123456789012345678
To install changes, type C-c C-c
@end example

  The first line contains a colon at each tab stop.  The remaining lines
help you see where the colons are and tell you what to do.

  Note that the tab stops that control @code{tab-to-tab-stop} have nothing
to do with displaying tab characters in the buffer.  @xref{Display Vars},
for more information on that.

@node Just Spaces,, Tab Stops, Indentation
@section Tabs vs. Spaces

@vindex indent-tabs-mode
  Emacs normally uses both tabs and spaces to indent lines.  If you prefer,
all indentation can be made from spaces only.  To request this, set
@code{indent-tabs-mode} to @code{nil}.  This is a per-buffer variable;
altering the variable affects only the current buffer, but there is a
default value which you can change as well.  @xref{Locals}.

@findex tabify
@findex untabify
  There are also commands to convert tabs to spaces or vice versa, always
preserving the columns of all non-blank text.  @kbd{M-x tabify} scans the
region for sequences of spaces, and converts sequences of at least three
spaces to tabs if that is possible without changing indentation.  @kbd{M-x
untabify} changes all tabs in the region to corresponding numbers of spaces.
