mathExpr - Tutorial - 1.3. How to define Variables and Functions

1.3. How to define Variables and Functions

You can also define new variables and even functions by calling the appropriate method:
- defineVariable(String,Object,Type)
- defineFunction(String,String[],String)

Define a Variable

All you need is

a String for the variable name,
a fitting Object as the value and
the Type of this value.

Example:

  ExpressionConfiguration config=new ExpressionConfiguration();

  //example 1:
  config.defineVariable("x", new Real(2.5), RealType.TYPE);

  //example 2:
  Complex[] complexArray=new Complex[5];
  for(int i=0; i<complexArray.length; i++)
    complexArray[i]=new Complex(i, i);
  config.defineVariable("v", new ComplexVector(complexArray), ComplexVectorType.TYPE);

  //now you can use these variables within any expression
  config.setExpression("x*v");
  ComplexVector cv=(ComplexVector)config.evaluateExpression();

Note: You can only define variables of the supported Types:

Note: It's very important to give the variable its correct Type (it doesn't work automatically). If not, a wrong Evaluator will be created and a ClassCastException will result. For example:

  ExpressionConfiguration config=new ExpressionConfiguration();

  //big mistake: variable type has to be ComplexType.TYPE
  config.defineVariable("x", new Complex(2,2), RealType.TYPE);
  config.setExpression("x+2");
  try
  {
    Object o=config.evaluateExpression();
  }
  catch(ClassCastException ex)
  {
    ex.printStackTrace();
  }
  //-> RealPlusComplexEvaluator will be created instead of the needed ComplexPlusComplexEvaluator
  //-> ClassCastException

Note: A variable can be defined independently from the chosen default type. For example: If the type is real, a variable can also be complex.

  ExpressionConfiguration config=new ExpressionConfiguration(RealType.TYPE);

  config.defineVariable("a", new Complex(1,1), ComplexType.TYPE);
  config.setExpression("2a"); //UML-Objektdiagram
  Complex c=(Complex)config.evaluateExpression(); //result: 2.0+2.0i

Define a Function

All you need is

a String for the function name,
a String array which contains all parameter names and
another String representing the function definition as an expression.

Example:

  ExpressionConfiguration config=new ExpressionConfiguration();

  //example 1: define f(x,y)=x*y
  config.defineFunction("f", new String[] {"x","y"}, "x*y");

  //example 2: define g(z)=z+f(z,z)
  config.defineFunction("g", new String[] {"z"}, "z+f(z,z)");

  //now you can use these functions within any expression
  config.setExpression("f(3,4)+g(2i)");
  Complex cv=(Complex)config.evaluateExpression();

Note: A function must be defined before you can call it in an expression whereas a variable can also be defined after the expression was setted (see: "Definition-Handling in mathExpr"). But in any case definitions must be closed before evaluation, otherwise it will result an UnknownDefinitionException. Example:

  ExpressionConfiguration config=new ExpressionConfiguration();

  //possible:
  config.defineVariable("a", new Real(1), RealType.TYPE);
  config.setExpression("2a");

  //possible, too:
  config.setExpression("4+b");
  config.defineVariable("b", new Real(2), RealType.TYPE);

  //not possible:
  config.setExpression("f(5)");
  config.defineFunction("f", new String[] {"x"}, "x^2");

  //of course not possible:
  config.setExpression("newVar-newVar");
  Object result=config.evaluateExpression();
  config.defineVariable("newVar", new Real(3), RealType.TYPE);

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