Editing How To Use Variables

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This function 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. 

*This function requires the [http://www.mediawiki.org/wiki/Extension:VariablesExtension Variables] Extension be installed. 

== 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 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>

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