ExpressionParser2.java

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package aud.example.expr;
 
import aud.Queue;
 
public class ExpressionParser2 extends ExpressionParser {
 
 
 
  //
  // Here is the implementation of BNF rules as methods:
  //
 
  @Override protected ExpressionTree sum(int level) {
    verbose(level,"<sum>");
 
    // temporary storage
    Queue<ExpressionTree> expr = new Queue<ExpressionTree>();
    Queue<Integer>        op   = new Queue<Integer>();
 
    ExpressionTree tree=product(level+1);
 
    // parse iteratively
    while (lookahead()==Tokenizer.PLUS || lookahead()==Tokenizer.MINUS) {
      op.enqueue(lookahead());
      nextToken();
      expr.enqueue(product(level+1));
    }
 
    // setup tree
    while (!op.is_empty()) {
      if (op.dequeue()==Tokenizer.PLUS)
        tree=new ExpressionTree(new Plus(),tree,expr.dequeue());
      else
        tree=new ExpressionTree(new Minus(),tree,expr.dequeue());
    }
    return tree;
  }
 
  protected ExpressionTree product(int level) {
    verbose(level,"<product>");
 
    // temporary storage
    Queue<ExpressionTree> expr = new Queue<ExpressionTree>();
    Queue<Integer>        op   = new Queue<Integer>();
 
    ExpressionTree tree=factor(level+1);
 
    // parse iteratively
    while (lookahead()==Tokenizer.TIMES || lookahead()==Tokenizer.DIVIDE) {
      op.enqueue(lookahead());
      nextToken();
      expr.enqueue(factor(level+1));
    }
 
    // setup tree
    while (!op.is_empty()) {
      if (op.dequeue()==Tokenizer.TIMES)
        tree=new ExpressionTree(new Times(),tree,expr.dequeue());
      else
        tree=new ExpressionTree(new Divide(),tree,expr.dequeue());
    }
    return tree;
  }
 
 
  public static void main(String[] args) {
 
    String input=(args.length>0) ? args[0] : "a*b*c";
 
    System.out.println("input = '"+input+"'");
 
    ExpressionParser2 p=new ExpressionParser2();
    p.setVerbose(true);
    ExpressionTree tree=p.parse(input);
 
    System.out.println("output = '"+tree+"'");
 
    System.out.println(tree.postorder().toString());
 
    System.out.println(tree.toTikZ());
  }
}