In computer programming, an indent style is a convention governing the indentation of blocks of code to convey the program's structure. This article largely addresses the C programming language and its descendants, but can be (and frequently is) applied to most other programming languages (especially those in the curly bracket family). Indent style is just one aspect of programming style. Computer programming (often shortened to programming or coding) is the process of writing, testing, and maintaining the source code of computer programs. ... An indentation can mean two things: To make notches in something or form deep recesses in a coastline for instance. ... In computer programming, a statement block is a section of code which is grouped together, much like a paragraph; such blocks consist of one, or more, statements. ... C is a general-purpose, block structured, procedural, imperative computer programming language developed in 1972 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system. ... Curly brace or bracket programming languages are those which use balanced brackets ({ and }, also known as brace brackets or simply braces) to make blocks in their syntax or formal grammar, mainly due to being C-influenced. ... Programming style refers to a set of rules or guidelines used when writing the source code for a computer program. ...

Indentation is not a requirement of most programming languages. Rather, programmers indent to better convey the structure of their program to human readers. In particular, indentation is used to show the relationship between control flow constructs such as conditions or loops and code contained within and outside them. However, some programming languages (such as Python and Occam) use the indentation to determine the structure instead of using braces or keywords. In computer science control flow (or alternatively, flow of control) refers to the order in which the individual statements or instructions of an imperative program are performed or executed. ... Python is a high-level programming language first released by Guido van Rossum in 1991. ... Occam is a parallel programming language that builds on Communicating Sequential Processes (CSP) and shares many of their features. ...

The size of the indent is usually independent of the style. Many early programs used tab characters for indentation, for simplicity and to save on source file size. Unix editors generally view tabs as equivalent to eight characters, while Macintosh environments would set them to four, creating confusion when code was transferred back and forth. Modern programming editors are now often able to set arbitrary indentation sizes, and will insert the appropriate combination of spaces and tabs. They can also help with the cross-platform tab confusion by being configured to insert only spaces. This article is about the use of the term Tab in computing. ... Filiation of Unix and Unix-like systems Unix (officially trademarked as UNIX®) is a computer operating system originally developed in the 1960s and 1970s by a group of AT&T employees at Bell Labs including Ken Thompson, Dennis Ritchie and Douglas McIlroy. ... The first Macintosh computer, introduced in 1984, upgraded to a 512K Fat Mac. The Macintosh or Mac, is a line of personal computers designed, developed, manufactured, and marketed by Apple Computer. ...

There are a number of computer programs that automatically correct indent styles as well as the length of tabs. A famous one among them is indent, a program included with many Unix-like operating systems. These programs work best for those who use an indent style close to that considered "proper" by their programmers; those who use other styles will more likely become frustrated. Tab (also spelled TaB) is a diet cola. ... indent is a Unix utility that reformats C and C++ code in a user-defined coding style. ... Diagram of the relationships between several Unix-like systems A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification. ... // An operating system (OS) is a set of computer programs that manage the hardware and software resources of a computer. ...

K&R style

The K&R style, so-called because it was used in Kernighan and Ritchie's book The C Programming Language, is commonly used in C. It is less common for Objective C, C++, C#, and others. It keeps the first opening brace on the same line as control statement, indents the statements within the braces, and puts the closing brace on the same indentation level as the control statement (on a line of its own). Functions, however, are braced distinctly from statements; an opening function brace is placed on the line following the declaration, at the same indentation level as the declaration. This is because in the original C language argument types needed to be declared on the subsequent line, whereas, when no arguments were necessary, the opening brace would appear in the same line with the function declaration. The C Programming Language, Brian Kernighan and Dennis Ritchie, one of the most read and trusted books on C. The C Programming Language (also known as K&R or the white book) is a famous computer science book which has been influential in the application and development of the C... Brian Wilson Kernighan (IPA pronunciation: , the g is silent), (born 1942 in Toronto, Ontario, Canada) is a computer scientist who worked at Bell Labs alongside Unix creators Ken Thompson and Dennis Ritchie and contributed greatly to Unix and its school of thought. ... Dennis Ritchie Dennis MacAlistair Ritchie (born September 9, 1941) is a computer scientist notable for his influence on ALTRAN, B, BCPL, C, Multics, and Unix. ... The C Programming Language, second edition, by Brian Kernighan and Dennis Ritchie, widely regarded to be the authoritative reference on C. The C Programming Language (sometimes referred to as K&R) is a well-known computer science book written by Brian Kernighan and Dennis Ritchie, the latter of whom originally...

 int main(argc, argv) int argc; char **argv; { ... while (x == y) { something(); somethingelse(); if (some_error) do_correct(); else continue_as_usual(); } finalthing(); ... }  

Advocates of this style sometimes refer to it as "The One True Brace Style" (abbreviated as 1TBS or OTBS) because of the precedent set by C (although advocates of other styles have been known to use similarly strong language). The source code of the UNIX kernel and Linux kernel is written in this style. Filiation of Unix and Unix-like systems Unix (officially trademarked as UNIX®) is a computer operating system originally developed in the 1960s and 1970s by a group of AT&T employees at Bell Labs including Ken Thompson, Dennis Ritchie and Douglas McIlroy. ... A kernel connects the application software to the hardware of a computer. ... The Linux kernel is a Unix-like operating system kernel. ...

Advantages of this style are that the beginning brace does not require an extra line by itself; and the ending brace lines up with the statement it conceptually belongs to. One disadvantage of this style is that the ending brace of a block takes up an entire line by itself, which can be partially resolved in if/else blocks and do/while blocks:

 if (x < 0) { printf("Negative"); negative(x); } else { printf("Positive"); positive(x); } 

This style makes it difficult to scan any source code for the opening brace of a block; however, it is otherwise as easy to find the beginning of the block by locating first line where the block 'pulls left' (where the indentation level decreases).

The motivation of this style is apparently to conserve screen real estate (and keep more code visible at once) by avoiding the need to dedicate an entire line to a single character (at the time this style was originated, a typical terminal had only 24 lines visible). This is most visibly demonstrated in the preceding example, as a chain of if...else if...[etc.]...else statements will display more code in a given number of lines than a style that places braces on new lines.

While Java is often written in BSD/Allman or other styles, a significant body of Java code uses the K&R style, largely because Sun's original style guides (see here and here) used K&R, and as a result most of the standard source code for the Java API is written in K&R. It is also a popular indent style for ActionScript, along with the BSD/Allman style. The C Programming Language, Brian Kernighan and Dennis Ritchie, one of the most read and trusted books on C. The C Programming Language (also known as K&R or the white book) is a famous computer science book which has been influential in the application and development of the C... The C Programming Language, Brian Kernighan and Dennis Ritchie, one of the most read and trusted books on C. The C Programming Language (also known as K&R or the white book) is a famous computer science book which has been influential in the application and development of the C... The C Programming Language, Brian Kernighan and Dennis Ritchie, one of the most read and trusted books on C. The C Programming Language (also known as K&R or the white book) is a famous computer science book which has been influential in the application and development of the C... ActionScript is a scripting language based on ECMAScript, primarily used to develop software for the Adobe Flash . ...

It should be noted that The C Programming Language does not explicitly specify this style, though it is followed consistently throughout the book. Of note from the book: The C Programming Language, second edition, by Brian Kernighan and Dennis Ritchie, widely regarded to be the authoritative reference on C. The C Programming Language (sometimes referred to as K&R) is a well-known computer science book written by Brian Kernighan and Dennis Ritchie, the latter of whom originally...

The position of braces is less important, although people hold passionate beliefs. We have chosen one of several popular styles. Pick a style that suits you, then use it consistently.

This might serve as a caution to those who may be overly partisan regarding indentation style in general, especially considering the source(s) -- one of the authors, Dennis Ritchie, designed and developed the C language itself, (and UNIX development used the style, Kernighan having been a primary contributor), yet their book remains nonpartisan about indentation style. Dennis Ritchie Dennis MacAlistair Ritchie (born September 9, 1941) is a computer scientist notable for his influence on ALTRAN, B, BCPL, C, Multics, and Unix. ... C is a general-purpose, block structured, procedural, imperative computer programming language developed in 1972 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system. ... Filiation of Unix and Unix-like systems Unix (officially trademarked as UNIX®) is a computer operating system originally developed in the 1960s and 1970s by a group of AT&T employees at Bell Labs including Ken Thompson, Dennis Ritchie and Douglas McIlroy. ... Brian Wilson Kernighan (IPA pronunciation: , the g is silent), (born 1942 in Toronto, Ontario, Canada) is a computer scientist who worked at Bell Labs alongside Unix creators Ken Thompson and Dennis Ritchie and contributed greatly to Unix and its school of thought. ...

BSD/Allman style

The BSD/Allman style is common, and is named after Eric Allman. It puts the brace associated with a control statement on the next line, indented to the same level as the control statement. Statements within the braces are indented to the next level. Berkeley Software Distribution (BSD, sometimes called Berkeley Unix) is the Unix derivative distributed by the University of California, Berkeley, starting in the 1970s. ... Eric Allman (born 1959) is a computer programmer. ...

 while (x == y) { something(); somethingelse(); } finalthing(); 

This style is similar to the standard indentation used by the Pascal programming language and Transact-SQL, where the braces are equivalent to the "begin" and "end" keywords. Pascal is an imperative computer programming language, developed in 1970 by Niklaus Wirth as a language particularly suitable for structured programming. ... Transact-SQL (TSQL) is an enhanced version of the SQL relational database language. ...

Advantages of this style are that the indented code is clearly set apart from the containing statement by lines that are almost completely whitespace, improving readability and the ending brace lines up in the same column as the beginning brace, making easy to find the matching brace. For information on the programming language Whitespace, see Whitespace programming language. ...

A disadvantage of this style is that each of the enclosing braces occupies an entire line by itself without adding any actual code. This once was an important consideration when programs were usually edited on terminals that displayed only 24 lines, but is less significant with larger resolutions.

The motivation of this style is probably to promote code readability through visually separating blocks from their control statements, deeming screen real estate a secondary concern.

This style is used by default in Microsoft Visual Studio 2005 and Apple's Xcode. Microsoft Visual Studio is Microsofts flagship software development product for computer programmers. ... Xcode is Apple Computers IDE for developing applications and other software for Mac OS X. It is shipped free with Mac OS X. First introduced on October 24, 2003 along with the release of Mac OS X v10. ...

BSD/KNF style

Also known as Kernel Normal Form style, this is currently the form of most of the code used in the Berkeley Software Distribution operating systems. Although mostly intended for kernel code, it is widely used as well in userland code. Kernel normal form, or KNF, is the normal form used in the development of code for the BSD operating systems. ... Berkeley Software Distribution (BSD, sometimes called Berkeley Unix) is the Unix derivative distributed by the University of California, Berkeley, starting in the 1970s. ... For information on the company called UserLand, see UserLand Software. ...

The hard tabulator (ts in vi) is kept at 8 columns, while a soft tabulator is often defined as a helper as well (sw in vi), and set at 4. The correct title of this article is vi. ...

The hard tabulators are used to indent code blocks, while a soft tabulator (4 spaces) of additional indent is used for all continuing lines which must be split over multiple lines.

Moreover, function calls do not use a space before the parenthesis, although C language native statements such as if, while, do, switch and return do (in the case where return is used with parens).

Here follows a few samples:

 while (x == y) { something(); somethingelse(); } finalthing(); 

 if (data != NULL && res > 0) { if (!JS_DefineProperty(cx, o, "data", STRING_TO_JSVAL( JS_NewStringCopyN(cx, data, res)), NULL, NULL, JSPROP_ENUMERATE)) { QUEUE_EXCEPTION("Internal error!"); goto err; } PQfreemem(data); } else { if (!JS_DefineProperty(cx, o, "data", OBJECT_TO_JSVAL(NULL), NULL, NULL, JSPROP_ENUMERATE)) { QUEUE_EXCEPTION("Internal error!"); goto err; } } 

 static JSBool pgresult_constructor(JSContext *cx, JSObject *obj, uintN argc, jsval *argv, jsval *rval) { QUEUE_EXCEPTION("PGresult class not user-instanciable"); return JS_FALSE; } 

Whitesmiths style

The Whitesmiths style is relatively uncommon today compared to the prior three. It was originally used in the documentation for the first commercial C compiler, the Whitesmiths Compiler. It was also popular in the early days of Windows, since it was used in three influential Windows programming books, Programmer's Guide to Windows by Durant, Carlson & Yao, Programming Windows by Petzold, and Windows 3.0 Power Programming Techniques by Norton & Yao. Symbian Ltd continues to advocate this as the recommended bracing style for Symbian C++ mobile phone applications. Whitesmiths was the first commercial C programming language compiler. ... Charles Petzold is a technical author on Microsoft Windows applications. ... Peter Norton Peter Norton (born November 14, 1943) is an American software publisher and philanthropist. ... Symbian Ltd. ... Symbian OS is an operating system with associated libraries, user interface frameworks and reference implementations of common tools, produced by Symbian. ...

This style puts the brace associated with a control statement on the next line, indented. Statements within the braces are indented to the same level as the braces.

 while (x == y) { something(); somethingelse(); } finalthing(); 

There's no clear consensus on whether to indent the body of a function in the same manner as the statements within it:

 void MyFunc () { while (x == y) { something(); somethingelse(); } finalthing(); } 

 void MyFunc () { while (x == y) { something(); somethingelse(); } finalthing(); } 

The advantages of this style are similar to those of the BSD/Allman style in that blocks are clearly set apart from control statements. However with Whitesmiths style, the block is still visually connected to its control statement instead of looking like an unrelated block of code surrounded by whitespace. Another advantage is that the alignment of the braces with the block emphasizes the fact that the entire block is conceptually (as well as programmatically) a single compound statement. Furthermore, indenting the braces emphasizes that they are subordinate to the control statement.

A disadvantage of this style could be that the braces do not stand out as well. However this is largely a matter of opinion, because the braces occupy an entire line to themselves even if they are indented to the same level as the block.

Another disadvantage could be that the ending brace no longer lines up with the statement it conceptually belongs to, although others argue that the closing brace belongs to the opening brace and not to the control statement.

An example:

 if (data != NULL && res > 0) { if (!JS_DefineProperty(cx, o, "data", STRING_TO_JSVAL(JS_NewStringCopyN(cx, data, res)), NULL, NULL, JSPROP_ENUMERATE)) { QUEUE_EXCEPTION("Internal error!"); goto err; } PQfreemem(data); } else { if (!JS_DefineProperty(cx, o, "data", OBJECT_TO_JSVAL(NULL), NULL, NULL, JSPROP_ENUMERATE)) { QUEUE_EXCEPTION("Internal error!"); goto err; } } 

GNU style

Like the BSD/Allman and Whitesmiths styles, GNU style puts braces on a line by themselves. The braces are indented by 2 spaces, and the contained code is indented by a further 2 spaces. Popularised by Richard Stallman, the layout may be influenced by his background of writing Lisp code. Although not directly related to indentation, GNU coding style also includes a space before the bracketed list of arguments to a function. GNU (pronounced ) is a computer operating system composed entirely of free software. ... Richard Matthew Stallman (often abbreviated as RMS) (born March 16, 1953) is a software freedom activist, hacker, and software developer. ... Lisp is a family of computer programming languages with a long history and a distinctive fully-parenthesized syntax. ...

 while (x == y) { something (); somethingelse (); } finalthing (); 

This style combines the advantages of BSD/Allman and Whitesmiths, thereby removing the possible Whitesmiths disadvantage of braces not standing out from the block.

The GNU Emacs text editor and the GNU systems' indent command will reformat code according to this style by default. It is mandated by nearly all maintainers of GNU project software, but is rarely used outside of the GNU community. Another disadvantage is that the ending brace no longer lines up with the statement it conceptually belongs to. GNU Emacs is one of the two most popular versions of Emacs (see also XEmacs). ... indent is a Unix utility that reformats C and C++ code in a user-defined coding style. ... GNU (pronounced ) is a computer operating system composed entirely of free software. ...

Those who do not use GNU Emacs, or similarly extensible/customisable editors, may find that the automatic indenting settings of their editor are unhelpful for this style. However, many editors defaulting to KNF style cope well with the GNU style when the tab width is set to 2 spaces; likewise, GNU Emacs adapts well to KNF style just by setting the tab width to 8 spaces. In both cases, automatic reformatting will destroy the original spacing, but automatic line indentation will work correctly.

Link: GNU Formatting

Pico style

The style used most commonly in the Pico programming language by its designers is different from the aforementioned styles. The lack of return statements and the fact that semicolons are used in Pico as statement separators, instead of terminators, leads to the following syntax: Pico is a programming language developed at the PROG lab at the Dutch-speaking Free University of Brussels (Vrije Universiteit Brussel, VUB). ...

 stuff(n): { x: 3 * n; y: doStuff(x); y + x } 

The advantages and disadvantages are similar to those of saving screen real estate with K&R style. One additional advantage is that the beginning and closing braces are consistent in application (both share space with a line of code), as opposed to K&R style where one brace shares space with a line of code and one brace has a line to itself.

Banner style

The banner style makes visual scanning easier for some, since the "headers" of any block are the only thing exdented at that level (the theory being that the closing control of the previous block interferes with the header of the next block in the K&R and BSD/Allman styles). In this style, which is to Whitesmiths as K&R is to BSD/Allman, the closing control is indented as the last item in the list (and thus appropriately loses salience).

 function1 () { dostuff do more stuff } function2 () { etc } 

or, in a markup language...

 <table> <tr> <td> lots of stuff... more stuff </td> <td> alternate for short lines </td> <td> etc. </td> </tr> </table> <table> <tr ... etc> </table> 

A programmer may even go as far as to insert closing brackets in the last line of a block. This style makes indentation the only way of distinguishing blocks of code, however has the advantage of containing no uninformative lines.

 for(i = 0; i < 10; i++) { if(i % 2 == 0) { doSomething(i); } else { doSomethingElse(i); } } 

Other considerations

Losing track of blocks

In certain situations, there is a risk of losing track of block boundaries. This is often seen in large sections of code containing many compound statements nested to many levels of indentation - by the time the programmer scrolls to the bottom of a huge set of nested statements, he may have lost track of which control statements go where.

Programmers who rely on counting the opening braces may have difficulty with indentation styles such as K&R, where the beginning brace is not visually separated from its control statement. Programmers who rely more on indentation will gain more from styles that are vertically compact, such as K&R, because the blocks are shorter. In computer science control flow (or alternatively, flow of control) refers to the order in which the individual statements or instructions of an imperative program are performed or executed. ...

To avoid losing track of control statements such as for, one can use a large indent, such as an 8-unit wide hard tab, along with breaking up large functions into smaller and more readable functions. Linux is done this way, as well as using the K&R style. It has been suggested that Foreach be merged into this article or section. ...

Another way is to use inline comments added after the closing brace:

 for ( int i = 0 ; i < total ; i++ ) { foo(bar); } //for ( i ) 

 if (x < 0) { bar(foo); } //if (x < 0) 

However, there is the major disadvantage in this of having to maintain the same code in two places.

Another solution is implemented in a folding editor, which lets the developer hide or reveal blocks of code by their indentation level or by their compound statement structure. It has been suggested that this article or section be merged into code folding. ...

See also

• Indentation
• Programming style
• Kernel Normal Form

An indentation can mean two things: To make notches in something or form deep recesses in a coastline for instance. ... Programming style refers to a set of rules or guidelines used when writing the source code for a computer program. ... Kernel normal form, or KNF, is the normal form used in the development of code for the BSD operating systems. ...

References

• Jargon File article on indent style