Help:Parser Functions

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Parser Functions are extensions to the MediaWiki software, mainly allowing IF statements.

Read more at http://meta.wikimedia.org/wiki/ParserFunctions


#expr

Expr simply computes a mathematical expression. It uses standard operators, including: + - * / = > < <= >= != <> as well as not, or, and, mod, round, and brackets ( ) can be used to group operations.

{{ #expr: expression }}

#if

The If statement shows something if the condition evaluates to anything other than zero, with the option of displaying alternative text.

{{ #if: <condition string> | <code if true> }}
{{ #if: <condition string> | <code if true> | <code if false> }}

Putting anything in the <condition string> will output the [code if true] section. If the <condition string> is empty, null zero or white space, the [code if false] section will output (if it exists). If the [code if false] section is not included, and the <condition string> is null/zero/whitespace nothing will output.

#ifexist

Ifexist returns one of two results based on whether or not a particular page exists.

{{ #ifexist: <page name> | <page exists> | <page does not exist>  }}

Standard naming convention applies: the page Vehicles is accessible from Vehicles or vehicles, but not VEHICLES. Interwiki links and special pages will output as <page does not exist>.

#ifexpr

The function #ifexpr evaluates a mathematical expression (like #expr) and returns one of two strings depending on the result.

{{ #ifexpr: <expression> | <then text> | <else text>  }}

For example, {{ #ifexpr: 10 > 9 | yes | no }} outputs yes.

if expression is valid

If both the <then text> and <else text> are missing from #ifexpr, nothing is output except error messages (such as division by zero). This can be use with #if to determine if an expression is valid, rather than attempting to output the result:

{{ #if: {{#ifexpr: 1=2 }} | invalid | valid }}

The above expression is "valid". The condition will not output any error is a valid equation, so the output will be blank (as opposed to an error message). The if statement sees no output, so reports the statement, which proves the expression is "valid".

If the expression was invalid (such as {{#ifexpr: 2/0 }}), then the #ifexpr would output an error message. The if statement would see positive output (not zero/null/whitespace) and so output the <code if true>. This proves the expression is "invalid".

#switch

The function #switch compares a single value against multiple others, returning a string if a match is found.

{{#switch: <comparison value>
 | <value1> = <result1>
 | <value2> = <result2>
 | ...
 | <valuen> = <resultn>
 | <default result>
 }}

If <comparison value> is <value1>, then <result1> will output. If it is <value2>, then <result2> will output. If it is none of these, then <default result> will output.

It is possible to have a fall-through, allowing multiple <values> to have one <result> entry:

{{#switch: <comparison value>
 | <value1>
 | <value2>
 | <value3> = <result1,2,3>
 | <default result>
 }}

If <comparison value> is <value1> or <value2> or <value3>, then <result1,2,3> will output.

String comparison is case-sensitive. To be made case-insensitive, use the Magic Word {{lc:}} to force everything to lowercase (or for upper case):

 {{ #switch: A | a=lower | A=UPPER  }} gives UPPER
 {{ #switch: {{lc:A}} | a=lower | A=UPPER  }} gives lower

#time

The function #time outputs the time, using a format string similar to that of PHP Date:

 {{ #time: format }}
 {{ #time: format | time }}

For example:

 {{ #time: l j F Y | 20070304 }} gives "Sunday 4 March 2007"
 {{ #time: l j F Y | -32 days }} gives "Sunday 6 April 2008" (32 days ago)
 {{ #time: H:i | +6 hours }} gives "20:17" (6 hours later than UTC)

#rel2abs

rel2abs converts a relative path to an absolute path:

 {{ #rel2abs: path }}
 {{ #rel2abs: path | base path }}
  • If standing in Help:Foo/bar and is calling {{ #rel2abs: ../baz }}, the result will be Help:Foo/baz
  • If standing in Help:Foo and is calling {{ #rel2abs: ../baz }}, the result will be baz

If you try to go up too many levels (an invalid relative path) an error will be output.

if relative path exists

This can be used in combination with #ifexist to determine whether a path exists:

 {{ #ifexist: {{#rel2abs: .. }} | '..' exist | '..' does not exist }} gives '..' does not exist
 {{ #ifexist: {{#rel2abs: . }} | '.' exist | '.' does not exist }} gives '.' exist


#titleparts

Titleparts returns a number of slash-separated segments of given a page name, starting from a specified segment.

{{ #titleparts: pagename | num | starting_from }}

This returns num slash-separated segments of pagename, starting from the specified starting segment.

 "{{#titleparts:Help:Link/a/b|0|}}" gives "Help:Link/a/b" 
 "{{#titleparts:Help:Link/a/b|2|}}" gives "Help:Link/a"
 "{{#titleparts:Help:Link/a/b|2|2}}" gives "a/b"
 "{{#titleparts:Help:Link/a/b|1|3}}" gives "b"
 "{{#titleparts:Help:Link/a/b|1|-1|}}" gives b


Notes

  • Applying "subst:" to a parser function works, provided that there is no space between "subst:" and "#".
  • The page to which #ifexist is applied (the page you're checking) is registered as link target of a link from the page with the #ifexist code on
  • div is not integer division and is redundant, use / (slash) for real divisions.
  • mod uses PHP's % operator, which is different from modulo-operators in all other programming languages, see also template:mod ( talk edit history links ) and 6068.
  • mod sometimes returns wrong results for the same input values, see 6356 and /MOD10000. Update: values less than 1E+12 are apparently not affected.
  • Under certain conditions round 0 results in -0 instead of 0. For an expression x using 0+(x) fixes this oddity.


Table Syntax

Currently wiki pipe table syntax doesn't work inside conditionals, but this can be worked around by using {{pipe}} to replace using | for the table syntax.