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Created page with "{{wikipedia|Template:If_empty/doc}} 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..."
{{wikipedia|Template:If_empty/doc}}
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 <code><nowiki>{{if empty|a|b|c}}</nowiki></code> instead of <code><nowiki>{{#if:a|a|{{#if:b|b|c}}}}</nowiki></code>, expression a and b do not have to be repeated and evaluated twice. The template provides a [[wikipedia:fallback|fallback]] order, similar to a [[wikipedia:try catch|try catch]] based "It is easier to ask for forgiveness than permission" ([[wikipedia:Time-of-check to time-of-use#Preventing TOCTOU|EAFP]]) programming style.
Typical usage is like this:
: <code><nowiki>{{If empty |{{{logo|}}} |{{{image|}}} |{{{picture|}}} |default.svg}}</nowiki></code>
: — <nowiki>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".</nowiki>
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, <code><nowiki>{{{logo|default.svg}}}</nowiki></code> does not return "default.svg" if the template was called like this: <code><nowiki>{{template|logo=}}</nowiki></code>.
The usual workaround for a single parameter is:
: <code><nowiki>{{#if:{{{logo|}}} |{{{logo}}} |default.svg}}</nowiki></code>
: — <nowiki>If {{{logo}}} has a value, return it, else return "default.svg".</nowiki>
But this becomes complex when several parameters are to be checked:
: <code><nowiki>{{#if:{{{logo|}}} |{{{logo}}} |{{#if:{{{image|}}} |{{{image}}} |{{#if:{{{picture|}}} |{{{picture}}} |default.svg}} }} }}</nowiki></code>
: — <nowiki>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".</nowiki>
In these cases, <nowiki>{{if empty}}</nowiki> produces the simpler syntax (as above):
: <code><nowiki>{{if empty |{{{logo|}}} |{{{image|}}} |{{{picture|}}} |default.svg}}</nowiki></code>
====Piping====
Parameters used with <nowiki>{{if empty}}</nowiki> must be [[wikipedia:Pipe (Unix)|pipe]]d – i.e. include the [[wikipedia:Vertical bar|vertical bar (pipe) symbol]] ("|") as a trailing character – 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 <code><nowiki>{{logo}}</nowiki></code>, <code><nowiki>{{image}}</nowiki></code> and <code><nowiki>{{picture}}</nowiki></code> in the above examples.
===Examples===
{| class="wikitable sortable" style="text-align:center;"
! Code
! Result
!
|-
| <code><nowiki>{{if empty}}</nowiki></code> || {{if empty}} || Returns an empty string.
|-
| <code><nowiki>{{if empty|one}}</nowiki></code> || {{if empty|one}} || Returns the first parameter that is defined and not empty.
|-
| <code><nowiki>{{if empty|one|two}}</nowiki></code> || {{if empty|one|two}} ||
|-
| <code><nowiki>{{if empty|one|two|three|four}}</nowiki></code> || {{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|three|four}}</nowiki></code> || {{if empty||two|three|four}} ||
|-
| <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.
|-
| <code><nowiki>{{if empty{{!}}}}</nowiki></code> || {{if empty|}} || The only parameter is empty or undefined, so returns an empty string.
|-
| <code><nowiki>{{if empty|{{!}}{{!}}{{!}}{{!}}}}</nowiki></code> || {{if empty||||}} || Returns an empty string.
|-
| <code><nowiki>{{if empty|{{{1|}}}|{{{2|}}}|three}}</nowiki></code> || {{if empty|{{{1|}}}|{{{2|}}}|three}} ||
|-
| <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 of these 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}}}" 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.
|}
<includeonly>{{Sandbox other|
| <!-- ADD TEMPLATE CATEGORIES BELOW THIS LINE, PLEASE: -->
[[Category:If-then-else templates]]
[[Category:Parameter selection templates]]
}}</includeonly>
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 <code><nowiki>{{if empty|a|b|c}}</nowiki></code> instead of <code><nowiki>{{#if:a|a|{{#if:b|b|c}}}}</nowiki></code>, expression a and b do not have to be repeated and evaluated twice. The template provides a [[wikipedia:fallback|fallback]] order, similar to a [[wikipedia:try catch|try catch]] based "It is easier to ask for forgiveness than permission" ([[wikipedia:Time-of-check to time-of-use#Preventing TOCTOU|EAFP]]) programming style.
Typical usage is like this:
: <code><nowiki>{{If empty |{{{logo|}}} |{{{image|}}} |{{{picture|}}} |default.svg}}</nowiki></code>
: — <nowiki>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".</nowiki>
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, <code><nowiki>{{{logo|default.svg}}}</nowiki></code> does not return "default.svg" if the template was called like this: <code><nowiki>{{template|logo=}}</nowiki></code>.
The usual workaround for a single parameter is:
: <code><nowiki>{{#if:{{{logo|}}} |{{{logo}}} |default.svg}}</nowiki></code>
: — <nowiki>If {{{logo}}} has a value, return it, else return "default.svg".</nowiki>
But this becomes complex when several parameters are to be checked:
: <code><nowiki>{{#if:{{{logo|}}} |{{{logo}}} |{{#if:{{{image|}}} |{{{image}}} |{{#if:{{{picture|}}} |{{{picture}}} |default.svg}} }} }}</nowiki></code>
: — <nowiki>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".</nowiki>
In these cases, <nowiki>{{if empty}}</nowiki> produces the simpler syntax (as above):
: <code><nowiki>{{if empty |{{{logo|}}} |{{{image|}}} |{{{picture|}}} |default.svg}}</nowiki></code>
====Piping====
Parameters used with <nowiki>{{if empty}}</nowiki> must be [[wikipedia:Pipe (Unix)|pipe]]d – i.e. include the [[wikipedia:Vertical bar|vertical bar (pipe) symbol]] ("|") as a trailing character – 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 <code><nowiki>{{logo}}</nowiki></code>, <code><nowiki>{{image}}</nowiki></code> and <code><nowiki>{{picture}}</nowiki></code> in the above examples.
===Examples===
{| class="wikitable sortable" style="text-align:center;"
! Code
! Result
!
|-
| <code><nowiki>{{if empty}}</nowiki></code> || {{if empty}} || Returns an empty string.
|-
| <code><nowiki>{{if empty|one}}</nowiki></code> || {{if empty|one}} || Returns the first parameter that is defined and not empty.
|-
| <code><nowiki>{{if empty|one|two}}</nowiki></code> || {{if empty|one|two}} ||
|-
| <code><nowiki>{{if empty|one|two|three|four}}</nowiki></code> || {{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|three|four}}</nowiki></code> || {{if empty||two|three|four}} ||
|-
| <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.
|-
| <code><nowiki>{{if empty{{!}}}}</nowiki></code> || {{if empty|}} || The only parameter is empty or undefined, so returns an empty string.
|-
| <code><nowiki>{{if empty|{{!}}{{!}}{{!}}{{!}}}}</nowiki></code> || {{if empty||||}} || Returns an empty string.
|-
| <code><nowiki>{{if empty|{{{1|}}}|{{{2|}}}|three}}</nowiki></code> || {{if empty|{{{1|}}}|{{{2|}}}|three}} ||
|-
| <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 of these 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}}}" 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.
|}
<includeonly>{{Sandbox other|
| <!-- ADD TEMPLATE CATEGORIES BELOW THIS LINE, PLEASE: -->
[[Category:If-then-else templates]]
[[Category:Parameter selection templates]]
}}</includeonly>
