Help:Parameter default: Difference between revisions

{{h:h|editor toc}}

In non-substituted [[Special:MyLanguage/Help:Template|template]] [[Special:ExpandTemplates|expansion]], an expression <code><nowiki>{{{p|q}}}</nowiki></code> inside the template is expanded to <code>{{{p}}}</code> if that is defined, and else to the [[w:Default argument|default argument]] <code>q</code>. Thus the default can be specified independently for every occurrence of the parameter.

These results <code>{{{p}}}</code> and <code>q</code> can be end results, but they can also be in an expression for a parameter name (inside a pair of triple braces or in a template call or a switch), a parameter value (in the call of a template or parser function), or name of a template, parser function or variable.

Similarly, on [[Special:MyLanguage/Help:Substitution|substitution]] of a template, an expression <code><nowiki>{{{p|q}}}</nowiki></code> inside the template, where p is a parameter name (or an expression which is simultaneously substituted and results in a parameter name) and q is any wikitext (with restrictions regarding pipes and triple braces) is processed as follows:

*if <code>p</code> is defined, <code><nowiki>{{{p|q}}}</nowiki></code> is changed to the wikitext of the value of p (or the wikitext to which that is changed if there is simultaneous substitution inside that wikitext)

*if <code>p</code> is undefined, <code><nowiki>{{{p|q}}}</nowiki></code> is changed to <code>q</code>, possibly with simultaneous substitutions inside the wikitext for <code>q</code>.

==Notes==

In the case of multiple pipes, anything from the second pipe is ignored: <code><nowiki>{{{a|b|c|d}}}</nowiki></code> is equivalent with <code><nowiki>{{{a|b}}}</nowiki></code>. The default part can only contain "<code>|</code>" as part of full template, parser function, parameter, link, or image syntax within it, within nowiki-tags, and as content of a template, as in [[mw:Special:MyLanguage/Help:Magic words#Other|Template:!]].

The expression for the parameter name can also contain "<code>|</code>", as part of full template, parser function, or parameter syntax within it.

Examples, in some cases using {{timc|3x}}<br />and {{timc|t2}}:

*<code><nowiki>{{{a|b|c|d}}}</nowiki></code> gives <code>{{{a|b|c|d}}}</code> - second pipe onward is ignored

*<code><nowiki>{{{a|{{3x|b}}}}}</nowiki></code> gives <code>{{{a|{{3x|b}}}}}</code>

*<code><nowiki>{{{a|{{{b|c}}}}}}</nowiki></code> gives <code>{{{a|{{{b|c}}}}}}</code>

*<code><nowiki>{{{a|[[b|c]]}}}</nowiki></code> gives <code>{{{a|[[b|c]]}}}</code>

*<code><nowiki>{{{{{3x|a}}|b}}}</nowiki></code> gives <code>{{{{{3x|a}}|b}}}</code> - parameter aaa is undefined

*<code><nowiki>{{{{{{a|b}}}|c}}}</nowiki></code> gives <code>{{{{{{a|b}}}|c}}}</code> - parameter b is undefined

*<code><nowiki>{{{a|<nowiki>b|c</nowiki><nowiki><</nowiki>/nowiki>}}}</code> gives <code>{{{a|<nowiki>b|c</nowiki>}}}</code> - works fine for rendering text, but is not suitable for putting parameters b and c in a template call (there is no function for removing nowiki tags):

*:<code><nowiki>{{t2|{{{a|<nowiki>b|c</nowiki><nowiki><</nowiki>/nowiki>}}}}}</code> gives <code>{{t2|{{{a|<nowiki>b|c</nowiki>}}}}}</code>

==Repetition==

A "for-loop" is achieved using

{{tim|fors}}, containing:

----

<pre>{{fors/aux

|v@=

|c={{{call}}}

|pv={{{pv|1}}}

|s={{{sep|}}}

|pc1={{{pc1|=}}}

|pc2={{{pc2|=}}}

|pc3={{{pc3|=}}}

|pc4={{{pc4|=}}}|

1={{{1|@}}}|2={{{2|@}}}|3={{{3|@}}}

}}</pre>

---- with {{tim|fors/aux}}, containing:

----

<pre>{{{v{{{1}}}|{{{{{c}}}|{{{pc1}}}|{{{pc2}}}|{{{pc3}}}|{{{pc4}}}|{{{pv}}}={{{1}}}}}}}}

{{{v{{{2}}}|{{{s}}}{{{{{c}}}|{{{pc1}}}|{{{pc2}}}|{{{pc3}}}|{{{pc4}}}|{{{pv}}}={{{2}}}}}}}}

{{{v{{{3}}}|{{{s}}}{{{{{c}}}|{{{pc1}}}|{{{pc2}}}|{{{pc3}}}|{{{pc4}}}|{{{pv}}}={{{3}}}}}}}}</pre>

----

In short form the latter consists of components

<code>[ v[''i''] | [s] {{[c]|[pc1]|[pc2]|[pc3]|[pc4]|[pv]=[''i'']}} ]</code> (''i'' = 1, 2, 3, for ''i'' = 1 without [s])

or in terms of the parameters of the first template:

<code>[v[''i''|@] | [sep|] {{[call]|[pc1|]|[pc2|]|[pc3|]|[pc4|]|[pv|1]=[''i'']}} ]</code>

This is indeed of the above-mentioned form <code>[a[b|c]|f([b])]</code>, with <code>a=v</code>, <code>b=''i''</code>, <code>c=@</code>, and

<code>f(x) = [sep|] {{[call]|[pc1|]|[pc2|]|[pc3|]|[pc4|]|[pv|1]=x}}</code>

The assumptions mentioned above apply for d equal to null, and provided that no <code>[''i'']</code> is equal to <code>"@"</code>.

===Variations===

Since <code>v@=null</code> we can also take make the concatenation the outer operation:

<code>[v[''i''|@] | [sep|]] [v[''i''|@] | {{[call]|[pc1|]|[pc2|]|[pc3|]|[pc4|]|[pv|1]=[''i'']}} ]</code>

== Last-but technique ==

Sometimes the last or last but <code>''x''</code> parameter value may be needed, but the numbers of supplied parameters is unknown. The following is a technique to get last "assigned" parameter value without using a template:<br>

<code>[4|[3|[2|[1|*]]]]</code>.

Then last but one can be achieved like this: ("<code>ifu</code>" is preassigned with empty)

<pre>

[[if[4|u]|3]|

[[if[3|u]|2]|

[[if[2|u]|1]|

[[if[1|u]|0]|*] //this line is actually useless, can be replaced by "*".

]

]

]

</pre>

I.e., to replace "4", "3", "2", "1" with D₄, D₃, D₂, D₁. And D<i>_x</i>=[if[<i>x</i>|u]|<i>x-1</i>]. Like this, you can get "last but <i>x</i>" parameter value.<br>

Examples using {{tim|lastbut0}} and {{tim|lastbut1}}:

*<code><nowiki>{{lastbut0}}</nowiki></code> gives {{lastbut0}}.

*<code><nowiki>{{lastbut0|a}}</nowiki></code> gives {{lastbut0|a}}.

*<code><nowiki>{{lastbut0|a|b|c|d|e|f|g|h|i|j}}</nowiki></code> gives {{lastbut0|a|b|c|d|e|f|g|h|i|j}}.

*<code><nowiki>{{lastbut1|ifu=}}</nowiki></code> gives {{lastbut1|ifu=}}.

*<code><nowiki>{{lastbut1|ifu=|a}}</nowiki></code> gives {{lastbut1|ifu=|a}}.

*<code><nowiki>{{lastbut1|ifu=|a|b}}</nowiki></code> gives {{lastbut1|ifu=|a|b}}.

*<code><nowiki>{{lastbut1|ifu=|a|b|c|d|e|f|g|h|i|j}}</nowiki></code> gives {{lastbut1|ifu=|a|b|c|d|e|f|g|h|i|j}}.

*<code><nowiki>{{lastbut0|53=53|81=81|28=28}}</nowiki></code> gives {{lastbut0|53=53|81=81|28=28}} (finds maximum of a list of integers in the range 1 - 100)

See also: {{timcnl|max}}

==History==

Parameter defaults were introduced before [[mw:Extension:ParserFunction|parser functions]], and were therefore originally used as a way of creating conditional statements. This was superseded by the <code>#if</code> parser function, and later by [[mw:Extension:Scribunto|Scribunto]].

{{h:f|enname=Parameter default}}

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