org.apache.felix.gogo.runtime

Class Expression

java.lang.Object

org.apache.felix.gogo.runtime.Expression

public class Expression
extends Object

EvalEx - Java Expression Evaluator

Introduction

EvalEx is a handy expression evaluator for Java, that allows to evaluate simple mathematical and boolean expressions.
Key Features:

• Uses BigDecimal for calculation and result
• Single class implementation, very compact
• No dependencies to external libraries
• Precision and rounding mode can be set
• Supports variables
• Standard boolean and mathematical operators
• Standard basic mathematical and boolean functions
• Custom functions and operators can be added at runtime

Examples

  BigDecimal result = null;

  Expression expression = new Expression("1+1/3");
  result = expression.eval():
  expression.setPrecision(2);
  result = expression.eval():

  result = new Expression("(3.4 + -4.1)/2").eval();

  result = new Expression("SQRT(a^2 + b^2").with("a","2.4").and("b","9.253").eval();

  BigDecimal a = new BigDecimal("2.4");
  BigDecimal b = new BigDecimal("9.235");
  result = new Expression("SQRT(a^2 + b^2").with("a",a).and("b",b).eval();

  result = new Expression("2.4/PI").setPrecision(128).setRoundingMode(RoundingMode.UP).eval();

  result = new Expression("random() > 0.5").eval();

  result = new Expression("not(x < 7 || sqrt(max(x,9)) <= 3))").with("x","22.9").eval();
 

Supported Operators

Mathematical Operators
Operator	Description
+	Additive operator
-	Subtraction operator
*	Multiplication operator
/	Division operator
%	Remainder operator (Modulo)
^	Power operator

Boolean Operators*
Operator	Description
=	Equals
==	Equals
!=	Not equals
<>	Not equals
<	Less than
<=	Less than or equal to
>	Greater than
>=	Greater than or equal to
&&	Boolean and
||	Boolean or

*Boolean operators result always in a BigDecimal value of 1 or 0 (zero). Any non-zero value is treated as a _true_ value. Boolean _not_ is implemented by a function.

Supported Functions

Function*	Description
NOT(expression)	Boolean negation, 1 (means true) if the expression is not zero
IF(condition,value_if_true,value_if_false)	Returns one value if the condition evaluates to true or the other if it evaluates to false
RANDOM()	Produces a random number between 0 and 1
MIN(e1,e2)	Returns the smaller of both expressions
MAX(e1,e2)	Returns the bigger of both expressions
ABS(expression)	Returns the absolute (non-negative) value of the expression
ROUND(expression,precision)	Rounds a value to a certain number of digits, uses the current rounding mode
FLOOR(expression)	Rounds the value down to the nearest integer
CEILING(expression)	Rounds the value up to the nearest integer
LOG(expression)	Returns the natural logarithm (base e) of an expression
SQRT(expression)	Returns the square root of an expression
SIN(expression)	Returns the trigonometric sine of an angle (in degrees)
COS(expression)	Returns the trigonometric cosine of an angle (in degrees)
TAN(expression)	Returns the trigonometric tangens of an angle (in degrees)
SINH(expression)	Returns the hyperbolic sine of a value
COSH(expression)	Returns the hyperbolic cosine of a value
TANH(expression)	Returns the hyperbolic tangens of a value
RAD(expression)	Converts an angle measured in degrees to an approximately equivalent angle measured in radians
DEG(expression)	Converts an angle measured in radians to an approximately equivalent angle measured in degrees

*Functions names are case insensitive.

Supported Constants

Constant	Description
PI	The value of PI, exact to 100 digits
TRUE	The value one
FALSE	The value zero

Add Custom Operators

Custom operators can be added easily, simply create an instance of `Expression.Operator` and add it to the expression. Parameters are the operator string, its precedence and if it is left associative. The operators `eval()` method will be called with the BigDecimal values of the operands. All existing operators can also be overridden.
For example, add an operator `x >> n`, that moves the decimal point of _x_ _n_ digits to the right:

 Expression e = new Expression("2.1234 >> 2");

 e.addOperator(e.new Operator(">>", 30, true) {
      @Override
     public BigDecimal eval(BigDecimal v1, BigDecimal v2) {
         return v1.movePointRight(v2.toBigInteger().intValue());
     }
 });

 e.eval(); // returns 212.34
 

Add Custom Functions

Adding custom functions is as easy as adding custom operators. Create an instance of `Expression.Function`and add it to the expression. Parameters are the function name and the count of required parameters. The functions `eval()` method will be called with a list of the BigDecimal parameters. All existing functions can also be overridden.
For example, add a function `average(a,b,c)`, that will calculate the average value of a, b and c:

 Expression e = new Expression("2 * average(12,4,8)");

 e.addFunction(e.new Function("average", 3) {
      @Override
     public BigDecimal eval(List<BigDecimal> parameters) {
         BigDecimal sum = parameters.get(0).add(parameters.get(1)).add(parameters.get(2));
         return sum.divide(new BigDecimal(3));
     }
 });

 e.eval(); // returns 16
 

The software is licensed under the MIT Open Source license (see LICENSE file).

• The *power of* operator (^) implementation was copied from [Stack Overflow](http://stackoverflow.com/questions/3579779/how-to-do-a-fractional-power-on-bigdecimal-in-java) Thanks to Gene Marin
• The SQRT() function implementation was taken from the book [The Java Programmers Guide To numerical Computing](http://www.amazon.de/Java-Number-Cruncher-Programmers-Numerical/dp/0130460419) (Ronald Mak, 2002)

Author:

Udo Klimaschewski (http://about.me/udo.klimaschewski)

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
class	Expression.Assignment Marker class for assignment operators.
class	Expression.Comma
class	Expression.Comparator
class	Expression.Constant
class	Expression.ExpressionException The expression evaluators exception class.
class	Expression.Function Abstract definition of a supported expression function.
class	Expression.LeftParen
class	Expression.Operator Abstract definition of a supported operator.
class	Expression.Variable

Field Summary

Fields

Modifier and Type	Field and Description
static BigDecimal	PI Definition of PI as a constant, can be used in expressions as variable.

Constructor Summary

Constructors

Constructor and Description
Expression(String expression) Creates a new expression instance from an expression string.

Method Summary

Modifier and Type	Method and Description
Expression	addConstant(String name, Object value) Sets a constant value.
Expression.Function	addFunction(Expression.Function function) Adds a function to the list of supported functions
Expression.Operator	addOperator(Expression.Operator operator) Adds an operator to the list of supported operators.
Object	eval() Evaluates the expression.
Object	eval(Map<String,Object> variables) Evaluates the expression.
Iterator<String>	getExpressionTokenizer() Get an iterator for this expression, allows iterating over an expression token by token.
Expression	setPrecision(int precision) Sets the precision for expression evaluation.
Expression	setRoundingMode(RoundingMode roundingMode) Sets the rounding mode for expression evaluation.
String	toRPN() Get a string representation of the RPN (Reverse Polish Notation) for this expression.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

PI

public static final BigDecimal PI

Definition of PI as a constant, can be used in expressions as variable.

Constructor Detail

Expression

public Expression(String expression)

Creates a new expression instance from an expression string.

Parameters:

expression - The expression. E.g. "2.4*sin(3)/(2-4)" or "sin(y)>0 & max(z, 3)>3"

Method Detail

eval

public Object eval()

Evaluates the expression.

Returns:

The result of the expression.

eval

public Object eval(Map<String,Object> variables)

Evaluates the expression.

Parameters:

variables - the variables

Returns:

The result of the expression.

setPrecision

public Expression setPrecision(int precision)

Sets the precision for expression evaluation.

Parameters:

precision - The new precision.

Returns:

The expression, allows to chain methods.

setRoundingMode

public Expression setRoundingMode(RoundingMode roundingMode)

Sets the rounding mode for expression evaluation.

Parameters:

roundingMode - The new rounding mode.

Returns:

The expression, allows to chain methods.

addOperator

public Expression.Operator addOperator(Expression.Operator operator)

Adds an operator to the list of supported operators.

Parameters:

operator - The operator to add.

Returns:

The previous operator with that name, or null if there was none.

addFunction

public Expression.Function addFunction(Expression.Function function)

Adds a function to the list of supported functions

Parameters:

function - The function to add.

Returns:

The previous operator with that name, or null if there was none.

addConstant

public Expression addConstant(String name,
                              Object value)

Sets a constant value.

Parameters:

name - The constant name.

value - The constant value.

Returns:

The expression, allows to chain methods.

getExpressionTokenizer

public Iterator<String> getExpressionTokenizer()

Get an iterator for this expression, allows iterating over an expression token by token.

Returns:

A new iterator instance for this expression.

toRPN

public String toRPN()

Get a string representation of the RPN (Reverse Polish Notation) for this expression.

Returns:

A string with the RPN representation for this expression.