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19. Indentation

Indent current line "appropriately" in a mode-dependent fashion.
Perform RET followed by TAB (newline-and-indent).
Merge two lines (delete-indentation). This would cancel out the effect of LFD.
Split line at point; text on the line after point becomes a new line indented to the same column that it now starts in (split-line).
Move (forward or back) to the first non-blank character on the current line (back-to-indentation).
Indent several lines to same column (indent-region).
Shift block of lines rigidly right or left (indent-rigidly).
Indent from point to the next prespecified tab stop column (tab-to-tab-stop).
M-x indent-relative
Indent from point to under an indentation point in the previous line.

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 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, TAB aligns a line according to its depth in parentheses. No matter where in the line you are when you type TAB, it aligns the line as a whole. In C mode, TAB implements a subtle and sophisticated indentation style that knows about many aspects of C syntax.

In Text mode, TAB runs the command tab-to-tab-stop, which indents to the next tab stop column. You can set the tab stops with M-x edit-tab-stops.

19.1 Indentation Commands and Techniques  Various commands and techniques for indentation.
19.2 Tab Stops  You can set arbitrary "tab stops" and then indent to the next tab stop when you want to.
19.3 Tabs vs. Spaces  You can request indentation using just spaces.

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19.1 Indentation Commands and Techniques

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

To move over the indentation on a line, type Meta-m (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 C-a C-o TAB. To make an indented line after the current line, use C-e LFD.

C-M-o (split-line) moves the text from point to the end of the line vertically down, so that the current line becomes two lines. 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, C-M-o resembles C-o.

To join two lines cleanly, use the Meta-^ (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 Meta-\ (delete-horizontal-space), which deletes all spaces and tabs around the cursor.

There are also commands for changing the indentation of several lines at once. Control-Meta-\ (indent-region) gives each line which begins in the region the "usual" indentation by invoking 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. C-x TAB (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.

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, tab-to-tab-stop is run (see next section).

indent-relative is the definition of TAB in Indented Text mode. See section 20. Commands for Human Languages.

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19.2 Tab Stops

For typing in tables, you can use Text mode's definition of TAB, 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 M-i anyway.

You can arbitrarily set the tab stops used by M-i. They are stored as a list of column-numbers in increasing order in the variable tab-stop-list.

The convenient way to set the tab stops is using 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 C-c C-c to make those new tab stops take effect. In the tab stop buffer, C-c C-c runs the function edit-tab-stops-note-changes rather than the default save-buffer. 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 tab-stop-list local in one buffer edit-tab-stops in that buffer edits only the local settings.

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

        :       :       :       :       :       :
0         1         2         3         4
To install changes, type C-c C-c

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 tab-to-tab-stop have nothing to do with displaying tab characters in the buffer. See section 11.4 Variables Controlling Display, for more information on that.

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19.3 Tabs vs. Spaces

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 indent-tabs-mode to 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. See section 27.3.4 Local Variables.

There are also commands to convert tabs to spaces or vice versa, always preserving the columns of all non-blank text. 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. M-x untabify changes all tabs in the region to corresponding numbers of spaces.

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