Help:Parameter default

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. Note that "|" can be avoided in table syntax by applying HTML coding (element "<table>").

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
{{{{{x3|a}}|b}}} gives b - parameter aaa is undefined
{{{{{{a|b}}}|c}}} gives c - parameter b is undefined

{{{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}}}]]

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|] ]

Last-but technique

Somewhen maybe the last or last but x parameter value is needed, but number of inputted parameter is unknown. The following is a technique to get last "assigned" parameter value without using a template:
[4|[3|[2|[1|*]]]].

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

[[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 "*".
  ]
 ]
]

I.e., to replace "4", "3", "2", "1" with D₄, D₃, D₂, D₁. And D_x=[if[x|u]|x-1]. Like this, you can get "last but x" parameter value.
Examples using m:Template:lastbut0 (backlinks edit) and m:Template:lastbut1 (backlinks edit):

{{lastbut0}} gives Template:Lastbut0.
{{lastbut0|a}} gives Template:Lastbut0.
{{lastbut0|a|b|c|d|e|f|g|h|i|j}} gives Template:Lastbut0.
{{lastbut1|ifu=}} gives Template:Lastbut1.
{{lastbut1|ifu=|a}} gives Template:Lastbut1.
{{lastbut1|ifu=|a|b}} gives Template:Lastbut1.
{{lastbut1|ifu=|a|b|c|d|e|f|g|h|i|j}} gives Template:Lastbut1.
{{lastbut0|53=53|81=81|28=28}} gives Template:Lastbut0 (finds maximum of a list of integers in the range 1 - 100)