Performs a mathematical calculation on an expression that consists of FME Feature Functions, String Functions, Math Functions, and Math Operators. The operands and function arguments consist of attributes on the input feature, constant literals, published and private parameters, as well as functions and operators. You can use the GUI interface to set up an expression, but you can also edit an expression manually. The results of the expression are stored inside attributes specified by user.

There are two main uses for ExpressionEvaluator:

  1. Apply one expression to one attribute
    • set Evaluation Mode to either Create New Attribute or Overwrite Existing Attributes, but only overwrite one attribute.
    • expression might use @Value() function, but doesn’t use @CurrentAttribute() function.
    • For example, if you want to add 1 to the values in a single attribute, use the expression @Value(attr) + 1, set Evaluation Mode to Overwrite Existing Attributes, and set Attributes to Overwrite to attr.
  2. Apply one expression to multiple attributes
    • set Evaluation Mode to Overwrite Existing Attributes and only select the attributes that will be overwritten with new values. Unselected attributes will keep their original values.
    • expression might use @CurrentAttribute() function, but typically won’t use @Value() function.
    • For example, if you want to add 1 to the values in a few attributes, use the expression @CurrentAttribute() + 1, set Evaluation Mode to Overwrite Existing Attributes, and select the attributes to apply this to.
Note  If you want to apply different expressions to different attributes, you can either use multiple ExpressionEvaluators, or use a single AttributeCreator.

The supported operators are a subset of the operators permitted in C expressions. They have the same meaning and precedence as the corresponding C operators with one notable exception: they support numeric nulls.

Expressions are expected to yield numeric results. FME supports numeric nulls, which permit expressions to return null as result. For example, the expression 8.2 + 6 returns 14.2, whereas the expression @Value(nullAttr) * 1 returns null.

Note   It is easy to build an invalid expression, so you may want to double check your expression, especially if you are using an @Value(attr) within the expression, as some attributes may have unexpected values. If an expression is invalid, then the corresponding result attribute will be set to null. When the result is null, the following attributes will be set to indicate what went wrong, and where:




Inside the ExpressionEvaluator, null, missing, and empty string attributes are all considered to be null. Thus, @Value(attr) * 1 will result in null if attr is not present on the feature or is an empty string.




FME Community has a good example of the ExpressionEvaluator.

Related Transformers

If you are setting more than one attribute, consider using the AttributeCreator, which contains the same functionality through the Arithmetic Editor.

Editing Transformer Parameters

Using a set of menu options, transformer parameters can be assigned by referencing other elements in the workspace. More advanced functions, such as an advanced editor and an arithmetic editor, are also available in some transformers. To access a menu of these options, click beside the applicable parameter. For more information, see Transformer Parameter Menu Options.

Defining Values

There are several ways to define a value for use in a Transformer. The simplest is to simply type in a value or string, which can include functions of various types such as attribute references, math and string functions, and workspace parameters. There are a number of tools and shortcuts that can assist in constructing values, generally available from the drop-down context menu adjacent to the value field.

Dialog Options - Tables

Transformers with table-style parameters have additional tools for populating and manipulating values.

Technical History

The mathematical operators in the ExpressionEvaluator are based on the Tool Command Language (Tcl) expr command.1


FME Community

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