Parsing and Evaluation

Parsing

To parse and evaluate an expression, you first create an expression object:

Expression expression = new Expression("1 + 2 / (4 * SQRT(4))");

Optionally, you can create a custom configuration and pass this in the constructor:

ExpressionConfiguration configuration = ExpressionConfiguration.builder()
    .decimalPlacesRounding(2)
    .build();

Expression expression = new Expression("1 + 2 / (4 * SQRT(4))", configuration);

Creating the expression object does not parse or validate the expression.

Parsing will be performed, when one of the following happens:

  • The expression is validated wih the validate() method.
  • The expression is evaluated with the evaluate() method.
  • An abstract syntax tree (AST) is created with the getAbstractSyntaxTree() method or any public method that uses it.

Once an expression was parsed, the resulting abstract syntax tree is cached inside the expression and wil be re-used on subsequent evaluations.

Evaluation

Evaluation is done by traversing the parsed abstract syntax tree. If it has not been generated before, it will be created and cached prior to the evaluation.

Expression expression = new Expression("1 + 2 / (4 * SQRT(4))");

EvaluationValue result = expression.evaluate();

System.out.println(result.getNumberValue()); // prints 1.25

The evaluation will return an EvaluationValue object. Depending on the expression, the EvaluationValue will be of one of the types:

  • NUMBER - If the expression resulted in a number.
  • STRING - If the expression resulted in a string.
  • BOOLEAN - If the expression resulted in a boolean value.
  • DATE_TIME - If the expression resulted in a date/time value.
  • DURATION - If the expression resulted in a duration value.
  • ARRAY - If the expression resulted in an array.
  • STRUCTURE - If the expression resulted in a structure.
  • BINARY - If the expression could not be converted to any of the previous types.

The EvaluationValue has methods to check and retrieve/convert the evaluation value.

See chapter Data Types for details on the different data types.

Validation

An expression can be validated with the validate() method. If there is a parsing problem, the method will throw a ParseException, which holds detailed information about the problem and its location.

Passing Variables

Variables can be used in an expression. Their value will be retrieved during evaluation by querying the defined data accessor.

by default, EvalEx uses the MapBasedDataAccessor, which stores the variable values in a case-insensitive map. Each expression will have an own instance of the accessor, so that different expression do not share the same data.

Prior to evaluation, the variable values can be set by setting them one by one, using the with() and and() methods. (Both methods are the same, the name difference is just for convenience).

Expression expression = new Expression("(a + b) * (a - b)");

EvaluationValue result = expression
    .with("a", 3.5)
    .and("b", 2.5)
    .evaluate();

System.out.println(result.getNumberValue()); // prints 6.00

Alternatively, the variable values can be set by defining a map with names and values and then passing it to the withValues() method:

Expression expression = new Expression("(a + b) * (a - b)");

Map<String, Double> values = new HashMap<>();
values.put("a", 3.5);
values.put("b", 2.5);

EvaluationValue result = expression.withValues(values).evaluate();

System.out.println(result.getNumberValue()); // prints 6.00

The data conversion of the passed values will automatically be performed through a customizable converter.

The map can also hold data of different types:

Expression expression = new Expression("a+b+c");

Map<String, Object> values = new HashMap<>();
values.put("a", true);
values.put("b", " : ");
values.put("c", 24.7);

EvaluationValue result = expression.withValues(values).evaluate();

System.out.println(result.getStringValue()); // prints "true : 24.7"

See chapter Data Types for details on the conversion.

Another option to have EvalEx use your data is to define a custom data accessor.

See chapter Data Access for details.

Null value Handling

Operations and Functions that can not handle null will throw either a NullPointerException or an EvaluationException when encountering null.

When passing null as variable values is a possibility the expression should be written to handle null itself, for example using the IF() function.

Exception Handling

In EvalEx, there are two general exceptions:

  • A ParseException is thrown, when the expression could not be parsed.
  • An EvaluationException is thrown, when the expression could be parsed, but not evaluated.

If possible, both exceptions report the following details:

  • Start Position of the error (character position, starting with 1).
  • End Position of the error (character position, starting with 1).
  • The Token string, usually the operator, function, variable or literal causing the error.
  • The error message.

Copyright © 2012-2022 Udo Klimaschewski