模板:If empty/doc

这是{{If empty}}模板。

• 2014年12月23日更新：此模板过去最多只可使用9个参数。此限制已移除。

此模板用于其他模板内。此模板将读取任何参数并将第一个有参数的定义输出。如果并无发现，不作输出。典型用法如下：

{{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}}	one
{{if empty|one|two|three|four}}	one
{{if empty||two}}	two	The first parameter is empty/undefined, so is passed over.
{{if empty||two|three|four}}	two
{{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 returns an empty string.
{{if empty||||}}		Returns an empty string.
{{if empty|{{{1|}}}|{{{2|}}}|three}}	three
{{if empty|{{{1}}}|{{{2}}}|three}}	{{{1}}}	No pipe-characters following the names of the first two parameters ("1" and "2"), so the first of these 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}}}" returned.
{{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

• {{if all}}
• {{if either}}
• Help:Parameter default