hectormethod

import java.util.Scanner;

public class PeekableScanner
{
    private Scanner scan;
    private String next;

    public PeekableScanner( String source )
    {
        scan = new Scanner( source );

hectormethod

When we try to parse a phrase that contains a syntax error, we need some way to respond to the error. A convenient way of doing this is to throw an exception. I'll use an exception class called ParseError, defined as follows:

/**
 * An object of type ParseError represents a syntax error found in 
 * the user's input.
 */
private static class ParseError extends Exception {
   ParseError(String message) {
      super(message);
   }

hectormethod

import java.io.BufferedReader;
import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;

/* The trick is to recognize what the 'terminal' nodes, or literal characters are, what would come from scanning the character stream. In this case, 

hectormethod

 
/**
 *Created on Oct 21, 2009
 *http://www.java-forums.org/new-java/22195-help-bnf-grammar-program-using-recursive-descent-parsing.html
 */
 

/**

hectormethod

 
/**
 *Created on Oct 21, 2009
 *http://www.java-forums.org/new-java/22195-help-bnf-grammar-program-using-recursive-descent-parsing.html
 */
 

/**

hectormethod

 
/**
 *Created on Oct 21, 2009
 *http://www.java-forums.org/new-java/22195-help-bnf-grammar-program-using-recursive-descent-parsing.html
 */
 

/**

hectormethod

# TreePrinter
# Prints text representation of parse tree for given statement
# Items in the same vertical column are on the same tree level
# Items one column to the right are children of the item

class ParseTreeDisplay
	class Item
		def initialize(node)
			@node = node
			@remaining = node.get_children().length

hectormethod

require 'Token.rb'
require 'Tokenizer.rb'
require 'Node.rb'
require 'TreePrinter.rb'

# Parser class
class Parser
  def initialize(tokenizer)
    @tokenizer = tokenizer

hectormethod

syntax:

[Stmts [Stmt [ id [x]] [ assign [:=]] [Expr]] [op [; ]] [Stmts]]

hectormethod

x := 1;
z := 0

--------


x11 := (2 + 3) * 5;
z__ := x11 + 2