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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)) | 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