The %IF construct gives TWiki the power to include content in topics based on the value of simple expressions.

=%<nop>IF{"CONDITION" then="THEN" else="ELSE"}%=

In the example above, if CONDITION evaluates to TRUE, then THEN will be included in the topic; otherwise ELSE will be included.

What can be included in the THEN and ELSE parameters is obviously limited by standard TWiki syntax for parameters. The best way to use this feature is to define two TWikiVariables, one for the THEN case and the other for the ELSE case. FOr example:
<verbatim>
<!--
	* Set LI = Fred is here!
	* Set LO = Fred is there!
-->
%IF{"context authenticated and defined WIKINAME and $ WIKINAME='FredBloggs'" then="%LI%" else="%LO%"}%
</verbatim>

The basic syntax of a condition is as follows:
<verbatim>
expr ::= '(' expr ')' ;
expr ::= andexpr | andexpr 'or' expr ;
andexpr ::= notexpr | notexpr 'and' andexpr ;
notexpr ::= basexpr | 'not' baseexpr ;
basexpr ::= atom | uop atom | atom bop basexpr ;
uop ::= 'context' | 'defined' | '$' ;
bop ::= '=' | '!=' | '>' | '<' | '>=' | '<=' ;
atom ::= context identifier, TWiki variable name, single-quoted string
</verbatim>
| *Operators* ||
| and | True if both sides are true |
| or | True if one or other side is true |
| not | negate the following expression |
| <nop>=, !<nop>= | String comparison |
| &lt;, &gt;, &lt;=, &gt;= | Number comparison (there is no explicit numeric =) |
| context | True if the current context is set (see below) |
| defined | True if a preference variable or url parameter of this name is defined. |
| $ | expands a URL parameter or TWikiVariable. Plugins handlers *are not called*. Built-in variables and user-defined preferences are supported. You can pass a limited subset of parameters to TWiki variables by enclosing the variable name in single quotes; for example, =$ 'VARIABLE{value}'=. The quoted string may *not* contain quotes (' or "). |
examples:
<verbatim>
%IF{"defined WIKINAME" then="WIKINAME is defined" else="WIKINAME is not defined"}%

You are %IF{ "$ WIKINAME='TWikiGuest' and not defined OPEN_DAY" then="not" }% allowed to
%IF{ "context view" then="view" else="edit"}% this TWiki today.

%IF{ "defined search" then="Search: %URLPARAM{search}%" else="No search passed in"}%

url param t is %IF{ "0 < $ t and $ t < 1000" then="in" else="out of"}% range.

%IF{ "$'URLPARAM{scope}'='text'" then="Plain text search"}% 
</verbatim>
Given
	* Set OPEN_DAY = yes
this expands to:

You are %IF{ "$ WIKINAME='TWikiGuest' and not defined OPEN_DAY" then="not" }% allowed to
%IF{ "context view" then="view" else="edit"}% this TWiki today.

Context identifiers are used in TWiki to label various stages of the rendering process. They are especially useful for [[TWikiSkins][skin]] authors to find out where they are in the rendering process. The following context identifiers are available:
| *id* | *context* |
| absolute_urls | Set if absolute URLs are required |
| authenticated | a user is authenticated |
| body_text  | when the body text is being processed in a view (useful in plugins handlers) |
| can_login | current environment supports login |
| changes | in changes script (see TWikiScripts) |
| command_line | the running script was run from the command line, and not from CGI |
| diff | in rdiff script (see TWikiScripts) |
| edit | in edit script (see TWikiScripts) |
| footer_text | when the footer text is being processed in a view (useful in plugins handlers) |
| header_text | when the header text is being processed in a view (useful in plugins handlers) |
| i18n_enabled | when user interface I18N support is enabled (i.e., user can choose the language for UI |
| inactive | if active links such as 'edit' and 'attach' should be disabled |
| mirror | if this is a mirror |
| new_topic | if the topic doesn't already exist |
| oops | in oops script (see TWikiScripts) |
| preview | in preview script (see TWikiScripts) |
| rss | if this is an rss skin rendering |
| save | in save script (see TWikiScripts) |
| search | in search script (see TWikiScripts) |
| view | in view script (see TWikiScripts) |
| rest | in rest script (see TWikiScripts) |

In addition there is an identifier for each enabled plugin; for example, if =Gallous<nop>BreeksPlugin is installed *and enabled*, then the context ID =Gallous<nop>BreeksPluginEnabled= will be set.

The %IF statements is kept deliberately simple. In particular, note that there is no way to conditionally execute a Set statement. If you need more sophisticated control over formatting, then consider using the SpreadSheetPlugin.
