ExpressionEvaluator

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:

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.
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:

• fme_expression_warnings{}.attrName

• fme_expression_warnings{}.message

• fme_expression_warnings{}.transformerName

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.

Configuration

Parameters

General

Evaluation Mode

Choose Create New Attribute to evaluate the specified expression and have the calculated results put in the specified attribute.

Choose Overwrite Existing Attributes to evaluate the specified expression once per selected attribute. The result of the evaluation will be put back into the attribute.

Attributes To Overwrite

This parameter allows the same expression to be evaluated, once per selected attribute. For each attribute specified, the result of the expression will be stored into that attribute.

If the expression needs the value of the current attribute, retrieve it using the function @CurrentAttribute(). For example, if the incoming feature has attributes X and Y, and we wish to increment each by one, select both attributes, and set the expression to @CurrentAttribute() + 1. This will set the attribute X to X+1 and Y to Y+1.

Warning The order of evaluation is not guaranteed. If overwriting attr, @Value(attr) should not be used.

Advanced: Attribute Value Handling

Substitute Missing, Null and Empty By

This parameter specifies the desired behavior when the specified attribute does not exist, has a null value, or has an empty string value. Such attributes can be thought of as unresolved attributes.

No Substitution: The ExpressionEvaluator will preserve the current unresolved value when evaluating the expression. For example, @Value(nullAttr) would resolve to NULL.
Default Value: The ExpressionEvaluator will replace the unresolved value with the value specified in the Default Value parameter. For example, @Value(nullAttr) would resolve to 0 if the Default Value is set to 0.

Default Value

This parameter specifies the value used to resolve unresolved attributes when the Substitute Missing, Null and Empty by parameter is set to Default Value.

Arithmetic Expression

An expression consists of a combination of operators and operands, functions and arguments, and parentheses. White spaces may be used between the operands, operators, and parentheses, and are ignored by the expression processor.

Operands and arguments may be specified in the following ways:

As a constant numeric value, either integer or floating-point.
As the value of an FME feature attribute, using the @Value() or @CurrentAttribute() notation. The attribute's value is used as the operand or argument.
As an FME feature or mathematical function, such as @Area() or @sin(1). The function is evaluated and the result used as the operand or argument.

Numeric values may be specified in decimal, such as 123 or 12.3, in octal if the first two characters of the operand are 0o (zero followed by the letter o), or in hexadecimal if the first two characters of the operand are 0x.

Floating-point numbers may be specified in any of the ways accepted by an ANSI-compliant C compiler, except that "f", "F", "l", and "L" suffixes are not permitted. For example, all of the following are valid floating-point numbers: 2.1, 3., 6e4, 7.91e+16. If no numeric interpretation is possible, then an operand is considered to be null.

For more information on the elements of the editor GUI, see the Arithmetic Editor.

FME Feature Functions

For more information, see FME Feature Functions.

String Functions

For more information, see String Functions.

Math Functions

For more information, see Math Functions.

Math Operators

For more information, see Math Operators.

Example

FME Support Center 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

Transformer parameters can be set by directly entering values, using expressions, or referencing other elements in the workspace such as attribute values or user parameters. Various editors and context menus are available to assist. To see what is available, click beside the applicable parameter.

How to Set Parameter Values

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.

Using the Text Editor

The Text Editor provides a convenient way to construct text strings (including regular expressions) from various data sources, such as attributes, parameters, and constants, where the result is used directly inside a parameter.

Text Editor

Using the Arithmetic Editor

The Arithmetic Editor provides a convenient way to construct math expressions from various data sources, such as attributes, parameters, and feature functions, where the result is used directly inside a parameter.

Arithmetic Editor

Conditional Values

Set values depending on one or more test conditions that either pass or fail.

Parameter Condition Definition Dialog

Content

Expressions and strings can include a number of functions, characters, parameters, and more.

When setting values - whether entered directly in a parameter or constructed using one of the editors - strings and expressions containing String, Math, Date/Time or FME Feature Functions will have those functions evaluated. Therefore, the names of these functions (in the form @<function_name>) should not be used as literal string values.

Content Types

String Functions	These functions manipulate and format strings.
Special Characters	A set of control characters is available in the Text Editor.
Math Functions	Math functions are available in both editors.
Date/Time Functions	Date and time functions are available in the Text Editor.
Math Operators	These operators are available in the Arithmetic Editor.
FME Feature Functions	These return primarily feature-specific values.
FME Parameters	FME and workspace-specific parameters may be used.
Creating and Modifying User Parameters	Create your own editable parameters.

Dialog Options - Tables

Table Tools

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

Row Reordering

Enabled once you have clicked on a row item. Choices include:

Add a row
Remove a row
Move current row up one
Move current row down one
Move current row to top
Move current row to bottom

Cut, Copy, and Paste

Enabled once you have clicked on a row item. Choices include:

Cut a row - delete and copy to clipboard
Copy a row to the clipboard
Paste a row from the clipboard

Cut, copy, and paste may be used within a transformer, or between transformers.

Filter

Start typing a string, and the matrix will only display rows matching those characters. Searches all columns. This only affects the display of attributes within the transformer - it does not alter which attributes are output.

Import

Import populates the table with a set of new attributes read from a dataset. Specific application varies between transformers.

Reset/Refresh

Generally resets the table to its initial state, and may provide additional options to remove invalid entries. Behavior varies between transformers.

Note: Not all tools are available in all transformers.

For more information, see Transformer Parameter Menu Options.

Technical History

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

arithmetic

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