Help:Parameter default

Notation: on this page the wikitext {{{a}}} is written as [a].

In a template, [a|b] is equal to [a] if that is defined, and else equal to b.

These results [a] and b can be end results, but they can also be parameter names, parameter values, template names, parts thereof, etc.

If the results are template names as part of template calls involving a parameter of the outer template which may be undefined, one can arrange that an inner template is called with a parameter depending on an undefined outer parameter only if the parameter concerned is not actually used in the inner template, e.g. dummy template Template:x0 containing "<noinclude>{{documentation}}</noinclude>" can be called as {{x0|[a]}} even if [a] is undefined, see e.g. m:Template:foreach/pass1 (backlinks edit).

Limitation

In the case of multiple default parts, only the first applies: [a|b|c|d] is equivalent with [a|b]. The default part can only contain "|" as part of full template, parameter or link syntax within it, and within nowiki-tags. The expression for the parameter name can also contain "|", as part of full template or parameter syntax within it.

Examples, using Template:x3 containing "<noinclude>{{Please leave this line alone (template sandbox heading)}}</noinclude>" and Template:t2 containing "[[w:List of people by name: {{{1}}}{{{2}}}|{{{1}}}{{{2}}}]]<noinclude>[[Category:Help:Templates templates]]</noinclude>":

{{{a|b|c|d}}} gives b
{{{a|{{x3|b}}}}} gives
{{{a|{{{b|c}}}}}} gives c
{{{a|[[b|c]]}}} gives c
{{{a|<nowiki>b|c</nowiki>}}} gives b|c - 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):
- {{t2|{{{a|<nowiki>b|c</nowiki>}}}}} gives [[w:List of people by name: b|c{{{2}}}|b|c{{{2}}}]]
{{{{{x3|a}}|b}}} gives b - parameter aaa is undefined
{{{{{{a|b}}}|c}}} gives c - parameter b is undefined

Expressions containing a parameter, with default

Consider the expression [a[b|c]|f([b])] occurring in a template, where f([b]) denotes an expression in [b].

Assume:

[a[b]] is undefined for all applicable [b] (all values of parameter b for which the template is called)
[ac]=d (the template is called with ac=d)

Then the expression gives f([b]) if [b] is defined, and otherwise d.

Note that the assumptions require that no applicable [b] has value c. If one wants to be able to use all letters and digits in [b], and also allow null (the empty string), then for c one can take e.g. "@".

Repetition

A "for-loop" is achieved using

m:Template:fors (backlinks edit), containing:

{{fors/aux
  |v@=
  |c={{{call}}}
  |pv={{{pv|1}}}
  |s={{{sep|}}}
  |pc1={{{pc1|=}}}
  |pc2={{{pc2|=}}}
  |pc3={{{pc3|=}}}
  |pc4={{{pc4|=}}}|
1={{{1|@}}}|2={{{2|@}}}|3={{{3|@}}}
}}

with m:Template:fors/aux (backlinks edit), containing:

{{{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}}}}}}}}

In short form the latter consists of components

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

or in terms of the parameters of the first template:

[v[i|@] | [sep|] {{[call]|[pc1|]|[pc2|]|[pc3|]|[pc4|]|[pv|1]=[i]}} ]

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

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

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

Variations

Since v@=null we can also take make the concatenation the outer operation: [v[i|@] | [sep|]] [v[i|@] | {{[call]|[pc1|]|[pc2|]|[pc3|]|[pc4|]|[pv|1]=[i]}} ]

Conditional statement

m:Template:if (backlinks edit) contains:

{{{else{{{test|}}}|{{{test{{{test|}}}|{{{then|}}}}}}}}}

or in short form:

[ else[test|] | [ test[test|] | [then|] ] ]

If [test] is defined this reduces to

[ else[test] | [ test[test] | [then|] ] ]

For test equal to null this reduces to [else|]; otherwise, if no parameter name starting with "test" or "else" applies except these themselves, then this reduces to [then|].

If [test] is undefined we get

[ else | [then|] ]

Server efficiency

Parameter default constructs are said to be more efficiently executed by the server than similar constructs using extra layers of templates.

Template:H:f