Custom Functions

This may be best explained by an example. Let's assume you want to add a function "half" to divide a number by two (for demonstration purposes).

1. Create a class that extends PostfixMathCommand (in the org.nfunk.jep.function package). In this example we will name it "Half"
2. In the constructor set the number of arguments to be taken. In our case this is one. Do this by writing "numberOfParameters = 1;"
   If you want to allow any number of parameters, initialize the numberOfParameters value to -1. It is highly recommended to study the Sum.java code as an example of a function that accepts any number of parameters.
3. Implement the run(Stack inStack) method. The parameters are passed in a Stack object. This same stack is used as the output for the function. So we first need to pop the parameter off the stack, calculate the result, and finally pop the result back on the stack.

   public void run(Stack inStack) throws ParseException {
   
      // check the stack
      checkStack(inStack);
      
      // get the parameter from the stack
      Object param = inStack.pop();
   
      // check whether the argument is of the right type
      if (param instanceof Double) {
         // calculate the result
         double r = ((Double)param).doubleValue() / 2;
         // push the result on the inStack
         inStack.push(new Double(r)); 
      } else {
         throw new ParseException("Invalid parameter type");
      }
   }

4. In your main program or applet that is using the parser, add the new function. Do this by writing
   

   parser.addFunction("half", new Half());

Source files

• CustFunc.java (a testing class)
• Half.java (the Half function class)