Template:If empty/doc: Difference between revisions

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It may contain usage information, categories and other content that is not part of the original template page.

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Template:Used in system

This template uses Lua:

Module:If empty (sandbox)

This template is used inside other templates. It takes any number of parameters and returns the first found to be defined and non-empty. If none is found, nothing is returned. By writing {{if empty|a|b|c}} instead of {{#if:a|a|{{#if:b|b|c}}}}, expression a and b do not have to be repeated and evaluated twice. The template provides a fallback order, similar to a try catch based "It is easier to ask for forgiveness than permission" (EAFP) programming style.

Typical usage is like this:

{{If empty |{{{logo|}}} |{{{image|}}} |{{{picture|}}} |default.svg}}

— Return {{{logo|}}} if it has a value; else {{{image|}}} if it has a value; else {{{picture|}}} if it has a value; else return "default.svg".

This returns the first of the parameters logo, image and picture that is defined and non-empty, otherwise "default.svg".

Rationale

The MediaWiki parameter default function doesn't return the default value for empty parameters. That is, {{{logo|default.svg}}} does not return "default.svg" if the template was called like this: {{template|logo=}}.

The usual workaround for a single parameter is:

{{#if:{{{logo|}}} |{{{logo}}} |default.svg}}

— If {{{logo}}} has a value, return it, else return "default.svg".

But this becomes complex when several parameters are to be checked:

{{#if:{{{logo|}}} |{{{logo}}} |{{#if:{{{image|}}} |{{{image}}} |{{#if:{{{picture|}}} |{{{picture}}} |default.svg}} }} }}

— If {{{logo}}} has a value, return it; else if {{{image}}} has a value, return that; else if {{{picture}}} has a value, return that; else return "default.svg".

In these cases, {{if empty}} produces the simpler syntax (as above):

{{if empty |{{{logo|}}} |{{{image|}}} |{{{picture|}}} |default.svg}}

Piping

Parameters used with {{if empty}} must be piped – i.e. include the vertical bar (pipe) symbol ("Template:Thin space|Template:Thin space") as a trailing character – so that empty or undefined parameters aren't treated as text and returned incorrectly. Hence, for example, {{{logo|}}}, {{{image|}}} and {{{picture|}}} rather than Template:Param, Template:Param and Template:Param in the above examples.

@@ Line 28: / Line 28: @@
 ====Piping====
 Parameters used with {{tlf|if empty}} must be piped&nbsp;– i.e. include the vertical bar (pipe) symbol {{nowrap|("{{thin space}}&#124;{{thin space}}")}} as a trailing character&nbsp;– so that empty or undefined parameters aren't treated as text and returned incorrectly. Hence, for example, <code><nowiki>{{{logo|}}}</nowiki></code>, <code><nowiki>{{{image|}}}</nowiki></code> and <code><nowiki>{{{picture|}}}</nowiki></code> rather than {{param|logo}}, {{param|image}} and {{param|picture}} in the above examples.
-===Examples===
-{{Aligned table |class=wikitable
- |cols=3 |col1style=padding-right:1.0em; |col2style=padding-left:0.5em;padding-right:1.0em; |col3style=padding-left:0.5em;
- |row1style=background:whitesmoke;padding-left:0.5em;font-weight:bold;
- | Code | Result |
- | {{tlc|if empty}} | {{if empty}} | Returns an empty string.
- | {{tlc|if empty|one}} | {{if empty|one}} | Returns the first parameter that is defined and not empty.
- | {{tlc|if empty|one|two|three|four}} | {{if empty|one|two|three|four}} |
- | <code><nowiki>{{if empty||two}}</nowiki></code> | {{if empty||two}} | The first parameter is empty/undefined, so is passed over.
- | <code><nowiki>{{if empty||two||four}}</nowiki></code> | {{if empty||two||four}} |
- | <code><nowiki>{{if empty||||||||||ten}}</nowiki></code> | {{if empty||||||||||ten}} | Unlike in previous versions, this template is no longer limited by 9 parameters.
- | {{tlc|if empty{{!}}}} | {{if empty|}} | The only parameter is empty or undefined, so the template returns an empty string.
- | {{tlc|if empty|{{!}}{{!}}{{!}}{{!}}}} | {{if empty||||}} | Returns an empty string.
- | <code><nowiki>{{if empty|{{{1|}}}|{{{2|}}}|three}}</nowiki></code> | {{if empty|{{{1|}}}|{{{2|}}}|three}} | Neither 1 nor 2 are defined, so "three" is returned.
- | <code><nowiki>{{if empty|{{{1}}}|{{{2}}}|three}}</nowiki></code> | {{if empty|{{{1}}}|{{{2}}}|three}} | No pipe-characters following the names of the first two parameters ("1" and "2"), so the first is returned as text ("{{{1}}}").
- | <code><nowiki>{{if empty|{{{logo|}}}|two}}</nowiki></code> | {{if empty|{{{logo|}}}|two}} |
- | <code><nowiki>{{if empty|{{{logo}}}|two}}</nowiki></code> | {{if empty|{{{logo}}}|two}} | No pipe-character following the parameter name "logo", so the text "{{{logo}}}" is returned.
- | <code><nowiki>{{if empty|p=q}}</nowiki></code> | {{if empty|p=q}} | The template identifies the parameters it receives as parameters 1 to 9, not using names such as "p", etc.
-}}
-===See also===
-* {{tl|if all}}
-* {{tl|if either}}
-* Help:Parameter default
-* {{tl|Category see also if exists 2}}
 <includeonly>{{Sandbox other|