<filter> Examples

Identify the variable to filter in inputvariable and the variable to receive the filtered output in outputvariable. Define the filter as an XPath 2.0 expression in filterexpr. This is a relative expression within the input variable.

The XPath predicate defines the comparison criteria for the filter. See A Brief Introduction to XPath 2.0 for basic information about XPath predicates.

The filter expression uses XPath predicates to compare some value in the input variable to a literal value, as shown above. The path in the filter expression to find the input variable node to compare must include any namespaces used, as shown in this example:

<filter inputvariable="$result" filterexpr="//ns:list[ns:rating]"
outputvariable="$rate"/>

You can also use wildcards to match any namespace as shown in this example:

<filter inputvariable="$result" filterexpr="//*:list[*:rating]"
outputvariable="$rate"/>

You can also filter based on any variable defined in the mashup or any Presto attribute. See Using Presto Attributes in Mashups for more information on Presto attributes and their use in mashups.

You reference variables or Presto attributes using $variable-name. For example:

When results contain repeating items, you can filter based on the cardinal order or position of items. You can select one specific item by number or select a contiguous group using the XPath function position(). For example:

<filter inputvariable="$staff" outputvariable="$result"
filterexpr="/personnel/person[3]"/>

<filter inputvariable="$staff" outputvariable="$result"
filterexpr="/personnel/person[position() = (1 to 3)]"/>

Using XPath Functions or Other Comparison Operations in Filtering

In addition to the = operator, you can use any valid XPath operator in the filtering expression. This includes >, >=, <, <= and != (not equals). For example:

<filter inputvariable="$result" outputvariable="$highRated"
filterexpr="//*:list[*:rating > 5]" />

Note:

You must use XML escaping with XPath operators that use the < or > characters, as this example shows.

You can also use two XPath functions, contains(string-to-search,string-to-match) and matches(string-to-search,pattern-to-match,[flags]), to handle substring comparisons. Both of these functions look for a string anywhere in the node you specify.

The contains() function does case sensitive comparisons. The example shown below selects all record nodes from the employees variable whose fname child contains "So" exactly. This would match names such as Sophy, Sonja or OnSo, but would not match Jefferson:

<filter inputvariable="$employees" outputvariable="$result"
filterexpr='/records/record[contains(fname,"So")]' />

The matches() function can do either case-sensitive or case-insensitive searches. To make the match case-insensitive, pass i in the flags parameter. You can also use regular expressions in the second parameter to do wildcard matching or handle specific patterns.

The example shown below selects all record nodes from the employees variable whose fname child contains "so" in any case. This would match names such as Sophy, Sonja and Jefferson

<filter inputvariable="$employees" outputvariable="$result"
filterexpr='/records/record[matches(fname,"so","i")]' />

In addition to these two built-in functions, you can use any other built-in XPath function in a filter expression. XPath contains a wide variety of string, number and date functions to allow you to work with data. See XPath 2.0 functions for more information.

You can also create you own XPath functions and add them to the Presto Server to perform specific data transformations to handle specific comparisons that are unique to your organization. See Defining Custom XPath Functions for more information.

You can combine criteria in a filter expression to handle more complicated filtering. Use the keywords and or or in the expression. This example filters a variable based on any one of several criteria:

Filtering expressions contain both a path to a node and a predicate that defines a comparison to use as the filter. You can make the value of that comparison dynamic using variables or Presto parameters (see Filtering Comparisons Using Variables or Presto Attributes).

You can also make the path portion of filtering expressions dynamic using input parameters and dynamic mashup expressions. Dynamic mashup expressions enclose references to variables in braces ( { } ) to ensure that they are evaluated before the XPath expression itself is evaluated.

This example invokes a Presto mashable information source and filters the results based on a path and field identified in input parameters:

<input name="selectedItem" type="string"/>
<input name="comparisonField" type="string"/>

<invoke service="YahooHotJobsRss" operation="getFeed"
filterexpr="{$selectedItem}[matches({$comparisonField},'Ruby')]"
outputvariable="$result"/>

See Dynamic Mashup Expressions for more examples and information.

There are several sample mashups for Presto that use the <filter> statement. Some of particular interest include:

DynamicHotJobFilters (dynamicfilter.emml) or DynamicDataFiltering (dynamicfilter2.emml) for examples of dynamic filtering

MergeUniqueItems (unique.emml) for a sample of using <filter> to find unique values

See Mashup Samples for a list of Presto mashup samples and where to find them.