Template:If empty/sandbox
 | This is the template sandbox page for Template:If empty (diff). See also the companion subpage for test cases. |
 | This template is transcluded in one or more cascade-protected pages, so only administrators can edit it. |
 | This template is used in system messages, and on approximately 3,890,000 pages, or roughly 6% of all pages. Changes to it can cause immediate changes to the Wikipedia user interface. To avoid major disruption and server load, any changes should be tested in the template's /sandbox or /testcases subpages, or in your own user subpage. The tested changes can be added to this page in a single edit. Please discuss changes on the talk page before implementing them. |
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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 (" | ") 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 {{{logo}}}, {{{image}}} and {{{picture}}} in the above examples.
Examples
| Code | Result | |
{{if empty}} | Returns an empty string. | |
{{if empty|one}} | one | Returns the first parameter that is defined and not empty. |
{{if empty|one|two|three|four}} | one | |
{{if empty||two}} | two | The first parameter is empty/undefined, so is passed over. |
{{if empty||two||four}} | two | |
{{if empty||||||||||ten}} | ten | Unlike in previous versions, this template is no longer limited by 9 parameters. |
{{if empty|}} | The only parameter is empty or undefined, so the template returns an empty string. | |
{{if empty|||||}} | Returns an empty string. | |
{{if empty|{{{1|}}}|{{{2|}}}|three}} | three | Neither 1 nor 2 are defined, so "three" is returned. |
{{if empty|{{{1}}}|{{{2}}}|three}} | {{{1}}} | No pipe-characters following the names of the first two parameters ("1" and "2"), so the first is returned as text ("{{{1}}}"). |
{{if empty|{{{logo|}}}|two}} | two | |
{{if empty|{{{logo}}}|two}} | {{{logo}}} | No pipe-character following the parameter name "logo", so the text "{{{logo}}}" is returned. |
{{if empty|p=q}} | The template identifies the parameters it receives as parameters 1 to 9, not using names such as "p", etc. |