__FORCETOC__ In the written form of many languages, indentation describes empty space (white space) used before or around text to signify an important aspect of the text such as: Beginning of a paragraph Hierarchy subordinate concept Quotation
Many computer languages use block indentation to demarcate blocks of source code.
Indentation is essentially the same regardless of whether the writing system is left-to-right (e.g. Latin and Cyrillic) or right-to-left (e.g. Hebrew and Arabic) when considering line beginning and end. For example, indenting at the beginning of a line means on the left for a left-to-right script and on the right for right-to-left script.
Indent is both a noun and a verb. The verb is the act of formatting text to be indented whereas the noun refers to the resulting empty space.
Types There are three main types of indentation: first-line, hanging and block.
Each example below is in a box that represents the page boundary and uses the common typesetting lorem ipsum content. The width of indentation here is in units of em spaces.
For first-line indentation the first line of a paragraph is indented,
A first-line indentation of 2 em:
For hanging indentation all but the first line of a paragraph is indented.
A hanging indentation of 2 em:
For block indentation the entire paragraph is indented relative to the preceding paragraph.
A block indentation of 2 em on the left:
Block indentation on both sides is commonly used for a quotation, a block quotation, here shown with 2 em on the left and right but since it is not right-aligned, the empty space on the right varies based on line length:
Here's the same as the previous but justified (left and right aligned):
Indentation in programming
In computer programming, indentation describes formatting source code with whitespace to the left of code text often to visually show that a sequence of code lines is syntactically a code block. Typically, the lines of a block are aligned with an amount of white space that indicates the block's depth in the hierarchical structure of the code. Each inner level of the hierarchy is indented by a multiple of this indentation width.
White space in code is typically stored as whitespace characters.
For a free-form language, indentation is exclusively for the programmer since a code processor (i.e. compiler, interpreter) ignores whitespace characters. Code can have inconsistent or even no indentation, but in general is formatted with somewhat consistent indentation.
Some languages rely on indentation to demarcate block structure, often via the off-side rule. Due to this syntax requirement, the code must have a level of consistency that is not required in free-form language code.
The neologisms outdent, unindent and dedent describe the opposite of indentation aligning code text of a line to the left of the previous line.
Variations
Common variations in the implementation of indentation include: how much to indent a block at each level of the code hierarchy, usually measured in spaces, and whether to store whitespace characters as space or tab characters. Although there are common practices, consensus is not universal. These variations are driven by factors that may include but are not limited to: language syntax, organizational mandate and personal preference.
Common practices
The following table identifies notable practices with respect to code indentation.
Language Free-form? Note Bash Fritz Mehner's style guide suggests 2, 4, or 8 spaces and uses 2 in all examples
Google uses 2 spaces C The Linux kernel uses 1 tab
NASA uses 4 spaces.
Clinton Staley advocates 3 spaces C++ WebKit recommends 4 spaces Google uses 2 spaces C# Microsoft convention: 4 spaces CSS Drupal, GitHub, and Google use 2 spaces This is an HTML-rendered copy of the official version, which is maintained in XML format
WordPress uses tabs Dart dart format uses 2 spaces Delphi Delphi style guide: 2 spaces F# F# style guide: 4 spaces Go Effective Go: 1 tab Haskell Google uses 2 spaces HTML Google uses 2 spaces
HTML Tidy defaults to 2 spaces Java Oracle uses 4 spaces
Android uses 4 spaces
Most Eclipse IDE components use tabs JavaScript Douglas Crockford advocates 4 spaces
GitHub and Google use 2 spaces
jQuery uses tabs
Firefox's built-in jsbeautifier defaults to 2 spaces
prettyprinter in Google Chrome and Internet Explorer use 4 spaces Julia Julia style guide: 4 spaces Kotlin Kotlin style guide: 4 spaces Lua Lua style guide: 2 spaces Perl Larry Wall prefers 4 spaces PHP Drupal use 2 spaces
PEAR and Zend use 4 spaces
CodeIgniter and WordPress use tabs
PSR-2 specifies 4 spaces PowerShell Unofficial PowerShell Best Practices and Style Guide: 4 spaces Python PEP 8: 4 spaces Ruby Ruby style guide: 2 spaces Rust Rust style guide: 4 spaces Scala Scala style guide: 2 spaces Swift Google uses 2 spaces Tcl Tcl style guide: 4 spaces Visual Basic Microsoft convention: 4 spaces XML Google uses 2 spaces
Elastic tabstops
In 2006, a new method of indentation was proposed, called elastic tabstops. 111 comments.
References