org.sc3d.apt.sss.v3
Class Calculator

java.lang.Object
  org.sc3d.apt.sss.v3.Parser
      org.sc3d.apt.sss.v3.Calculator

public class Calculator
extends Parser

This class is a worked example of how to use SSS in general, and the tools provided in this Java package in particular. It is not itself a particularly useful tool. To get the most out of this example, read its source code.

This class uses an SSS syntax to implement an interactive arbitrary precision integer calculator, using the java.math.BigInteger class as the back-end. If you really wanted to write a calculator, there are easier ways of doing it than using SSS. Having said that:

• this class only took a few hours to write (it's about 200 lines),
• it was trivial to debug and I'm confident it works correctly,
• it produces excellent error messages, and
• it is good, maintainable, flexible, future-proof code.

If you wanted to do something slightly more complicated, the balance tips firmly in favour of using something like SSS, and you might well want to imitate this code.

Nested Class Summary

static class	Calculator.Expression The data structure that represents an arithmetic expression.
static class	Calculator.Negation An Expression whose outermost operator is a negation.
static class	Calculator.Number An Expression that consists only of a number.
static class	Calculator.Operation An Expression whose outermost operator is an addition, subtraction, multiplication of division.

Field Summary

static Grammar

SUM The grammar of arithmetic expressions that this calculator accepts.

Fields inherited from class org.sc3d.apt.sss.v3.Parser

ndfa

Constructor Summary

Calculator()
Constructs a Calculator.

Method Summary

java.math.BigInteger	evaluate(Sentence expression) Evaluates an arithmetic expression.
static void	main(java.lang.String[] args) Repeatedly accepts an arithmetic expression from the console and evaluates it.
java.lang.Object	postProcess(Tree raw) Returns an Expression, given a Tree representing a parse-tree obeying 'Grammar.SUM'.
Calculator.Expression	postProcessExpression(Tree.Production raw) Converts a Tree representing the parse-tree of an 'atom', 'product' or 'sum' into an Expression.

Methods inherited from class org.sc3d.apt.sss.v3.Parser

interactiveTest, parse, parseTokens

Methods inherited from class java.lang.Object

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

Field Detail

SUM

public static final Grammar SUM

The grammar of arithmetic expressions that this calculator accepts. Here is an SSS grammar specification for this Grammar:

 sign ::= {Minus {"-"}}
 atom ::= {Number {sign* NUMBER} Bracket {sign* Round(sum)}}
 multiplicand ::= {Multiply {"*" atom} Divide {"/" atom}}
 product ::= {Product {atom multiplicand*}}
 summand ::= {Add {"+" product} Subtract {"-" product}}
 sum ::= {Sum {product summand*}}
 ROOT sum
 

Constructor Detail

Calculator

public Calculator()

Constructs a Calculator.

Method Detail

evaluate

public java.math.BigInteger evaluate(Sentence expression)

Evaluates an arithmetic expression. If the expression is not syntactically correct, this method attaches appropriate error messages to 'expression' and returns 'null'.

postProcessExpression

public Calculator.Expression postProcessExpression(Tree.Production raw)

Converts a Tree representing the parse-tree of an 'atom', 'product' or 'sum' into an Expression. If the Tree contains errors, this method annotates the Tokens it contains with error messages and returns 'null'.

postProcess

public java.lang.Object postProcess(Tree raw)

Returns an Expression, given a Tree representing a parse-tree obeying 'Grammar.SUM'. If 'raw' contains errors, this method annotates the Tokens it contains with error messages and returns 'null'.

Overrides:

postProcess in class Parser

Parameters:

raw - the non-null parse-tree.

Returns:

a grammar-specific data structure, or 'null' if there were errors.

main

public static void main(java.lang.String[] args)
                 throws java.io.IOException

Repeatedly accepts an arithmetic expression from the console and evaluates it.

Throws:

java.io.IOException