Many transformers in the Filters and Joins category can carry out tests and redirect data according to the results:

Although the Tester transformer is the most used of this category, there are many other transformers such as the TestFilter, GeometryFilter, AttributeFilter, SpatialFilter, and Sampler.

The Tester

The Tester and TestFilter are the two key transformers for conditional filtering. They test the values on attribute values.

The Tester transformer is generally for single tests that produce a Yes/No result.

For example, here we wish to decide whether to send out snow plows to a particular road based on whether the value of the Snowfall attribute is greater than 150 mm (approximately 6 inches):

If snowfall is greater than 150 (mm), the road feature will pass the test and snow plows will be sent.

Each clause in the Tester is an individual test that allows a Passed/Failed result. For example, each of the following criteria might be separate tests:

However, the Tester allows the combination of multiple tests, where a user can combine any number of clauses using an AND and OR statement. So instead of individual tests, I might ask:

The Tester also allows the mixing of AND and OR statements using what is called a Composite Test. For example:

But - however complex the test becomes - it still results in a single Yes/No (binary) result; features will either pass or fail this set of tests.

It's also worth remembering that we aren't restricted to simple tests of equality (A=B); in the above, there are also "greater than" and "less than" tests. That's because there are many different operators available for use in a test clause.

You can type values into the Logic column, as well as select them from the dropdown list. That means you aren't limited to the basic, single parentheses that the dropdown field would lead you to believe!

In FME 2019.1 and newer, the overall logic of the conditional clauses is displayed in a text box below the tests:

The numbers in the expression refer to the row number in the test clauses table. So "1 AND 2" means the test clauses in row 1 and row 2 (the AND means both clauses have to pass for the overall feature to pass).

You can adjust the test composition by directly editing the composite expression. Simply click the Edit button and type in a new expression. Experienced users will find this faster that using the Logic column dropdown options.

The Logic column and the Composite Expression field are mirror images of each other. Any changes to either are reflected in the other. For example, in the above if I changed AND to OR in the logic column (second row) then the composite expression would change to match. Conversely, if I changed the composite expression to 1 AND ( 2 OR 3 ) then the test clauses would change to match, including deleting the now unused 4th row.

If a feature cache is present for the Tester or TestFilter, you can access a list of cached values to fill in your tests. This "data-aware" transformer dialog is available in the Tester, TestFilter, AttributeManager, and AttributeExposer. Let us know on the Ideas Page if you'd like to see it expand elsewhere.

The list of operators available in the Tester transformer (or in many of the other locations that make use of the Tester dialog) looks like this:

Besides the usual operators, there are also some based on a SQL WHERE clause. These include:

...plus there are other tests that check for the existence of attributes and values:

"Attribute has a value" is the opposite of the three other tests; for example, this attribute is not Null, AND it is not an empty string, AND it is not missing. Incidentally, "missing" means the attribute does not exist at all on the feature being tested.

"Contains Regex" means only part of the string needs to match. For example...
Attribute Value:  abcd Search String:    ^ab Contains Regex:  Passed
For example, the entire string doesn't need to match.

TestFilter

The TestFilter allows a number of conditions to be tested, each of which can have a number of test clauses. Each condition is given an output port, with an additional output port for features that fail all of the test conditions.

Notice that there are multiple conditions, and an output port for each. Each condition/port combination is equivalent to a single Tester transformer; hence the TestFilter is a good way to combine multiple Tester transformers into one.

The TestFilter output ports can be given custom names, rather than a simple passed/failed, which is another advantage to this transformer over the Tester.

The TestFilter has the full set of operators available with the Tester such as equals, greater than, less than, and so forth. Each condition is tested in turn.

Features that pass are output through the matching output port. Features that fail are sent on to the next condition in the list. Therefore it’s very important to get the conditions in the correct order.

The TestFilter is very good for filtering a feature by a set of cascading conditions, for example here are a set of tests to again determine whether to send out a snow plow:

Has there been more than 150 mm of snowfall?

Has there been more than 100 mm of snowfall AND is this an Arterial road?

Is the temperature less than zero degrees Celsius AND was sand last applied more than 24 hours ago?

It’s a set of cascading tests because if there has been more than 150 mm of snow, the plows are sent out anyway; you don’t need to test any other criteria. So the test order can be very important. If every test is a fail, then the plows are not sent out. If using the above snowfall example, you were using three Tester transformers, you could save space on the workspace canvas and replace that setup with just a single TestFilter.

Learning Objectives

Video

Transformers that Filter

The Tester

TestFilter