View source for Help:Truth value

A template parameter is considered to be '''"true"''' if its value contains '''at least one [[Help:Whitespace|non-whitespace]] character'''.

For example, the template call {{tc|TV 2|link{{=}}y}} sets the parameter named {{tp|link}} equal to the true value {{tpv|y}} to indicate that the template should provide a link (see [[Template:TV 2|the template's documentation]] for an explanation in the context of that particular template). Note that the fact that the letter "y" happens to connote "yes" is irrelevant: even values such as {{code|0}} and {{code|FALSE}} are true, because they are strings containing non-whitespace. (But, of course, saying {{tc|TV 2|link{{=}}FALSE}} when you want a link is a bit silly.)

By contrast, none of the calls {{tc|TV 2|link{{=}}}}, {{tc|TV 2|link{{=}}{{sp}}}}, and {{tc|TV 2}} set {{tp|link}} to a true value.

The situation is the same for so-called "unnamed" or "positional" parameters. The template calls {{tl|all-of|1}}, {{tc|all-of|none}}, and {{tc|all-of|1{{=}}two}} are passing true values in the first positional parameter (the values {{code|1}}, {{code|none}}, and {{code|two}}, respectively), whereas the calls {{tc|all-of}}, {{tc|all-of{{!}}}}, {{tc|all-of|{{sp}}}}, and {{tc|all-of|1{{=}}{{sp}}}} are not. (As seen in these examples, the name {{tp|1}} can be used to explicitly refer to the first positional parameter. The last example is setting the first positional parameter equal to whitespace. Well… okay, technically it's setting it equal to nothing. See [[Template:Test-params]] for why.)

== Parser functions ==

There are two MediaWiki [[mw:Help:Parser functions|parser functions]] that determine the '''truth value''' of their input, but they do it in very different ways:

* '''{{tc|#if:}}''' selects between two alternatives based on whether a given '''string''' contains '''at least one non-whitespace character''', a true value; if the string contains only whitespace or no characters at all, it is considered false<!-- but see this caveat -->
* '''{{tc|#ifexpr}}''' selects between two alternatives based on whether a '''numerical and/or logical expression''' evaluates to 1 (true) or 0 (false); for the purpose of interpreting input, the Boolean functions that can be used in the expression ({{code|and}}, {{code|or}}, {{code|not}}, and so forth) treat 0 as false and any other value as true.

So far, {{tc|#ifexpr:}} has not seen any use on this wiki, so the first interpretation of truth is always the relevant one here. For further information about how {{tc|#if:}} works, see the documentation of [[Template:Test-if]] and {{mw|Help:Extension:ParserFunctions|the relevant help at MediaWiki.org}}. For more about template parameters, see [[Template:Test-params]].

=== Testing for definedness rather than truth ===

In some situations, you want a template to do something when a certain parameter was present ({{aka|"defined"}}) in the template call, regardless of whether that parameter was given a true or false value.  In that case, you can do something like this:

: <code><nowiki>{{#ifeq:{{{1|+}}}|{{{1|-}}}|…parameter 1 was defined…|…parameter 1 was not defined…}}</nowiki></code>

[[Category:MediaWiki help]]