Conditionals - Precisely_EnterWorks - EnterWorks - 11.0

If / ElseIf / Else

The #if directive in Velocity allows for text to be included when the web page is generated, on the conditional that the if statement is true. For example:

#if( $foo )
	<strong>Velocity!</strong>
#end

The variable $foo is evaluated to determine whether it is true, which will happen under one of two circumstances: (i) $foo is a boolean (true/false) which has a true value, or (ii) the value is not null. Remember that the Velocity context only contains Objects, so when we say "boolean," it will be represented as a Boolean (the class). This is true even for methods that return boolean - the introspection infrastructure will return a Boolean of the same logical value.

The content between the #if and the #end statements becomes the output if the evaluation is true. In this case, if $foo is true, the output will be: "Velocity!" Conversely, if $foo has a null value, or if it is a boolean false, the statement evaluates as false, and there is no output.

An #elseif or #else element can be used with an #if element. Note that the Velocity Templating Engine will stop at the first expression that is found to be true. In the following example, suppose that $foo has a value of 15 and $bar has a value of 6.

#if( $foo < 10 )
	<strong>Go North</strong>
#elseif( $foo == 10 )
	<strong>Go East</strong>
#elseif( $bar == 6 )
	<strong>Go South</strong>
#else
	<strong>Go West</strong>
#end

In this example, $foo is greater than 10, so the first two comparisons fail. Next $bar is compared to 6, which is true, so the output is Go South.

Relational and Logical Operators

Velocity uses the equivalent operator to determine the relationships between variables. Here is a simple example to illustrate how the equivalent operator is used:

#set ($foo = "deoxyribonucleic acid")
#set ($bar = "ribonucleic acid")
#if ($foo == $bar)
	In this case it's clear they aren't equivalent. So... #else
	They are not equivalent and this will be the output. #end

Note that the semantics of == are slightly different than Java where == can only be used to test object equality. In Velocity the equivalent operator can be used to directly compare numbers, strings, or objects. When the objects are of different classes, the string representations are obtained by calling toString() for each object, then compared.

Velocity has logical AND, OR and NOT operators as well. Below are examples demonstrating the use of the logical AND, OR and NOT operators.

## logical AND
#if( $foo && $bar )
	<strong> This AND that</strong>
#end

The #if() directive will evaluate to true only if both $foo and $bar are true. If $foo is false, the expression will evaluate to false; $bar will not be evaluated. If $foo is true, the Velocity Templating Engine will then check the value of $bar; if $bar is true, then the entire expression is true and This AND that becomes the output. If $bar is false, then there will be no output as the entire expression is false.

Logical OR operators work the same way, except only one of the references needs to evaluate to true in order for the entire expression to be considered true. Consider the following example.

## logical OR
#if( $foo || $bar )
	<strong>This OR That</strong>
#end

If $foo is true, the Velocity Templating Engine has no need to look at $bar; whether $bar is true or false, the expression will be true, and This OR That will be output. If $foo is false, however, $bar must be checked. In this case, if $bar is also false, the expression evaluates to false and there is no output. On the other hand, if $bar is true, then the entire expression is true, and the output is This OR That

With logical NOT operators, there is only one argument:

##logical NOT
#if( !$foo )
	<strong>NOT that</strong>
#end

Here, the if $foo is true, then !$foo evaluates to false, and there is no output. If $foo is false, then !$foo evaluates to true and NOT that will be output. Be careful not to confuse this with the quiet reference $!foo which is something altogether different.

There are text versions of all logical operators, including eq, ne, and, or, not, gt, ge, lt, and le.

One more useful note: when you want to include text immediately following a #else directive you will need to use curly brackets immediately surrounding the directive to differentiate it from the following text. (Any directive can be delimited by curly brackets, although this is most useful for #else).

#if( $foo == $bar)it's true!#{else}it's not!#end</li>