CeNobiteElf: Created page with ''''Variables''' is a very simple MediaWiki extension that allows you to define a variable on a page, use it later in that same page, change its value, possibly to a value given b…'

2009-10-16T08:58:41Z

Created page with ''''Variables''' is a very simple MediaWiki extension that allows you to define a variable on a page, use it later in that same page, change its value, possibly to a value given b…'

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'''Variables''' is a very simple MediaWiki extension that allows you to define a variable on a page, use it later in that same page, change its value, possibly to a value given by an expression in terms of the old value, etc.

It's much like a template, only very lightweight and scoped to only a single page, so you can use many variables on a page without polluting the wiki with huge numbers of templates. Combine with the [[ParserFunctions]] extension for best results.

== Assigning a value to a variable ==

=== #vardefine ===

:<nowiki>{{#vardefine:</nowiki>''variablename''|''specifiedvalue''}}

assigns the value ''specifiedvalue'' to the (already existing or hereby introduced) variable ''variablename''.

=== #vardefineecho ===

:<nowiki>{{#vardefineecho:</nowiki>''variablename''|''specifiedvalue''}}

works exactly as <tt>#vardefine</tt>, but the affected value is printed.

== Retrieving the value of a variable (#var) ==

The value of the variable ''variablename'' is produced by
:<nowiki>{{#var:</nowiki>''variablename''}}

If undefined, this produces the empty string; it does not give an error message.

It's possible to define a value for the case that the variable is undefined or void:
:<nowiki>{{#var:</nowiki>''variablename'' <nowiki>|</nowiki>''defaultvalue''<nowiki>}}</nowiki>
This is equivalent to:
:<nowiki>{{#if:</nowiki> <nowiki>{{#var</nowiki>:''variablename''}} |<nowiki>{{#var</nowiki>:''variablename''}} |''defaultvalue''}}
but it's much shorter and better arranged.

The value can be used in parser functions, etc.

== #varexists ==

:<nowiki>{{#varexists:</nowiki>''variablename''}} returns ''1'' if the variable is already defined (also when the value is a void string). If the variable is not defined the return value is void.

==Examples==
: ''Note that the [[Extension:ParserFunctions|ParserFunctions extension]] must also be installed to use #expr''

Compute 2*a + b:
:<nowiki>{{#expr:2*{{#var:a}}+{{#var:b}}}}</nowiki>
Add one to n:
:<nowiki>{{#vardefine:n|{{#expr:{{#var:n}}+1}}}}</nowiki>

== Variables and conditional parser functions ==
This section only applies to MediaWiki versions below 1.12.

=== [[Extension:ParserFunctions|ParserFunctions]] ===

It must be noted that everything in conditional parser functions such as #ifexpr gets executed (though only one result gets displayed), regardless of the condition,
see [[m:ParserFunctions#Code_execution]]. This applies also to #vardefine. Thus:
<nowiki>{{#ifexpr:..|
{{#vardefine:a|b}}|
{{#vardefine:a|c}}
}}</nowiki>

first assigns b and then c (hence effectively just c) regardless of the condition, while

<nowiki>{{#vardefine:a | {{#ifexpr:..|b|c}} }}</nowiki>

assigns only the applicable value.

Similarly
<nowiki>{{#ifexpr:..| {{#vardefine:a|b}}|}}</nowiki>

assigns b regardless of the condition, while

<nowiki>{{#vardefine:a | {{#ifexpr:..|b|{{#var:a}} }} }}</nowiki>

conditionally assigns value b to variable a (a dummy assignment of the value of a to a is done if the condition is not fulfilled).

=== [[Extension:Control Structure Functions|Control Structure Functions]] ===

Another way around the aforementioned limitation is to use the [[Extension:Control Structure Functions|Control Structure Functions]] extension, which allows one to delay the parsing of wiki markup through the use of [[Extension:Control Structure Functions#Character Escapes|character escape sequences]]. So, the first above example becomes this:

<nowiki>{{#ifexpr:..|
\o#vardefine:a\pb\c|
\o#vardefine:a\pc\c
}}</nowiki>

Alternatively, the [[Extension:Character Escapes|Character Escapes]] extension can be used to automate the escape sequences:

<nowiki>{{#ifexpr:..|
<esc>{{#vardefine:a|b}}</esc>|
<esc>{{#vardefine:a|c}}</esc>
}}</nowiki>

The [[Extension:Control Structure Functions|Control Structure Functions]] extension also has loop functions that support character escape sequences. The wiki markup:

<nowiki>{{ #vardefine: i | 0 }}{{
#while: expr
| <esc>{{ #var: i }} < 3</esc>
| <esc>
* {{ #var: i }}{{ #vardefine: i | {{ #expr: {{ #var: i }} + 1 }} }}</esc>
}}</nowiki>

produces the following:

* 0
* 1
* 2

==Links==
For more info see the [http://www.mediawiki.org/wiki/Extension:VariablesExtension extension page].

Help:Variables - Revision history

CeNobiteElf: Created page with ''''Variables''' is a very simple MediaWiki extension that allows you to define a variable on a page, use it later in that same page, change its value, possibly to a value given b…'