aud.example.expr.ExpressionParser Class Reference

Simple expression parser. More...

Collaboration diagram for aud.example.expr.ExpressionParser:

Public Member Functions
ExpressionTree	parse (String input)
	parse input More...
void	setVerbose (boolean b)
	set verbose mode (report state to System.err) More...

Static Public Member Functions
static void	main (String[] args)
	test and example for usage More...

Protected Member Functions
String	errorNear ()
	helper: generate a simple error message More...
int	lookahead ()
	helper: "lookahead" is the usual phrasing More...
void	nextToken ()
	helper: consume current token and advance to next token More...
String	matchedText ()
	helper: get match corresponding to lookahead More...
void	expect (int tokenid, String text)
	helper: check token (without calling nextToken!) More...
void	verbose (int level, String message)
	helper: print out information More...
ExpressionTree	expression (int level)
	parse expression More...
ExpressionTree	sum (int level)
	parse sum More...
ExpressionTree	product (int level)
	parse product More...
ExpressionTree	factor (int level)
	parse factor More...

Detailed Description

Simple expression parser.

Build ExpressionTree from (infix) string representation of a term.

Implements a top-down recusive descent parser based on the following rules in BNF form (w/o <> to ease HTML formatting of this page)

expression ::= sum

sum        ::= product |
               product '+' sum | product '-' sum

product    ::= factor |
               factor '*' product | factor '/' product

factor     ::= number | symbol | '(' expression ')' |
               '+' factor | '-' factor

<number> and <symbol> are identified during the lexcical analysis. They correspond to AtomicExpression instances, which are used to build the tree. The same applies to operators '+',...

Notes

• The left-hand-sides of the BNF rules are "mapped" to (recursive) method calls, e.g. expression.

• The level argument is provided only for formatting verbose output.

• The parser uses a Tokenizer for lexcical scanning.

• The current token is used for "looking ahead" (lookahead): it is advanced (by nextToken only if this token is "consumed" by part of a BNF rule.

• The current implementation has a problem! Do you see what and why?

See also

ExpressionTree

Tokenizer

AtomicExpression

Definition at line 61 of file ExpressionParser.java.

Member Function Documentation

errorNear()

String aud.example.expr.ExpressionParser.errorNear ( )

protected

helper: generate a simple error message

Definition at line 85 of file ExpressionParser.java.

                               {
    return "near '"+scanner_.matchedText()+
      "'\nbefore '"+scanner_.remainder()+"'";
  }

Referenced by aud.example.expr.ExpressionParser.factor().

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expect()

void aud.example.expr.ExpressionParser.expect ( int  tokenid, String  text  )

protected

helper: check token (without calling nextToken!)

Exceptions

SyntaxError

if token does not match

Definition at line 112 of file ExpressionParser.java.

                                                 {
    if (lookahead()!=tokenid)
      throw new SyntaxError("expected '"+text+"' got '"+
                            scanner_.matchedText()+"'\nbefore '"+
                            scanner_.remainder());
  }

References aud.example.expr.ExpressionParser.lookahead().

Referenced by aud.example.expr.ExpressionParser.factor().

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expression()

ExpressionTree aud.example.expr.ExpressionParser.expression ( int  level )

protected

parse expression

expression ::= sum

Definition at line 134 of file ExpressionParser.java.

                                                 {
    verbose(level,"<expression>");
    return sum(level+1);
  }

References aud.example.expr.ExpressionParser.sum(), and aud.example.expr.ExpressionParser.verbose().

Referenced by aud.example.expr.ExpressionParser.factor().

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factor()

ExpressionTree aud.example.expr.ExpressionParser.factor ( int  level )

protected

parse factor

factor ::= number | symbol | '(' expression ')' |
           '+' factor | '-' factor

The last part of the rule correspong to unary plus and minus operators +x and -x. The implementation ignores unary plus, i.e., +x -> x.

Definition at line 199 of file ExpressionParser.java.

                                             {
    verbose(level,"<factor>");
 
    ExpressionTree tree=null;
 
    if (lookahead()==Tokenizer.NUMBER) {
      tree=new ExpressionTree
                (new Number(Double.parseDouble(matchedText())));
    }
    else if (lookahead()==Tokenizer.IDENTIFIER) {
      tree=new ExpressionTree(new Symbol(matchedText()));
    }
    else if (lookahead()==Tokenizer.LEFT_PAREN) {
      nextToken();
      tree=expression(level+1);
      expect(Tokenizer.RIGHT_PAREN,"closing parenthesis )");
    }
    else if (lookahead()==Tokenizer.PLUS) { // unary plus
      nextToken();
      return factor(level+1); // ignore: +x == x
     }
    else if (lookahead()==Tokenizer.MINUS) { // unary minus
      nextToken();
      return new ExpressionTree(new UnaryMinus(),factor(level+1),null);
    }
    else
      throw new SyntaxError(errorNear());
 
    nextToken();
    assert(tree!=null);
 
    return tree;
  }

References aud.example.expr.ExpressionParser.errorNear(), aud.example.expr.ExpressionParser.expect(), aud.example.expr.ExpressionParser.expression(), aud.example.expr.ExpressionParser.factor(), aud.example.expr.Tokenizer.IDENTIFIER, aud.example.expr.Tokenizer.LEFT_PAREN, aud.example.expr.ExpressionParser.lookahead(), aud.example.expr.ExpressionParser.matchedText(), aud.example.expr.Tokenizer.MINUS, aud.example.expr.ExpressionParser.nextToken(), aud.example.expr.Tokenizer.NUMBER, aud.example.expr.Tokenizer.PLUS, aud.example.expr.Tokenizer.RIGHT_PAREN, and aud.example.expr.ExpressionParser.verbose().

Referenced by aud.example.expr.ExpressionParser.factor(), and aud.example.expr.ExpressionParser.product().

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lookahead()

int aud.example.expr.ExpressionParser.lookahead ( )

protected

helper: "lookahead" is the usual phrasing

Definition at line 91 of file ExpressionParser.java.

                            {
    return scanner_.matchedTokenId();
  }

Referenced by aud.example.expr.ExpressionParser.expect(), aud.example.expr.ExpressionParser.factor(), aud.example.expr.ExpressionParser.product(), and aud.example.expr.ExpressionParser.sum().

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main()

static void aud.example.expr.ExpressionParser.main ( String[]  args )

static

test and example for usage

Definition at line 237 of file ExpressionParser.java.

                                         {
 
    String input=(args.length>0) ? args[0] : "1+2*(3+4)";
 
    System.out.println("input = '"+input+"'");
 
    ExpressionParser p=new ExpressionParser();
    p.setVerbose(true);
    ExpressionTree tree=p.parse(input);
 
    System.out.println("output = '"+tree+"'");
 
    System.out.println(tree.postorder().toString());
 
    System.out.println(tree.toTikZ());
  }

References aud.example.expr.ExpressionParser.parse(), aud.BinaryTree< T >.postorder(), aud.example.expr.ExpressionParser.setVerbose(), aud.BinaryTreeTraversal< T >.Traversal.toString(), and aud.BinaryTree< T >.toTikZ().

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matchedText()

String aud.example.expr.ExpressionParser.matchedText ( )

protected

helper: get match corresponding to lookahead

Returns

Tokenizer#matchedText

Definition at line 105 of file ExpressionParser.java.

                                 {
    return scanner_.matchedText();
  }

Referenced by aud.example.expr.ExpressionParser.factor().

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nextToken()

void aud.example.expr.ExpressionParser.nextToken ( )

protected

helper: consume current token and advance to next token

Definition at line 96 of file ExpressionParser.java.

                             {
    if (scanner_.next()==Tokenizer.NO_MATCH)
      throw new SyntaxError("unknown token: lexcial analysis failed at '"+
                            scanner_.remainder()+"'");
  }

Referenced by aud.example.expr.ExpressionParser.factor(), aud.example.expr.ExpressionParser.product(), and aud.example.expr.ExpressionParser.sum().

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parse()

ExpressionTree aud.example.expr.ExpressionParser.parse

(

String

input

)

parse input

Parameters

input

term

Exceptions

SyntaxError

on syntax error

Returns

extression tree that represents input term

Definition at line 71 of file ExpressionParser.java.

                                            {
    scanner_ = new Tokenizer(input);
    scanner_.next(); // use as "lookahead"
    ExpressionTree tree=expression(0);
    expect(Tokenizer.END_OF_INPUT,"<end of input>");
    return tree;
  }

Referenced by aud.example.expr.ExpressionParser.main(), and aud.example.expr.ExpressionTreeTraversal.main().

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product()

ExpressionTree aud.example.expr.ExpressionParser.product ( int  level )

protected

parse product

product ::= factor |
            factor '*' product | factor '/' product

Definition at line 170 of file ExpressionParser.java.

                                              {
    verbose(level,"<product>");
 
    ExpressionTree left=factor(level+1);
 
    if (lookahead()==Tokenizer.TIMES) {
      nextToken();
      ExpressionTree right=product(level+1);
      return new ExpressionTree(new Times(),left,right);
    }
    else if (lookahead()==Tokenizer.DIVIDE) {
      nextToken();
      ExpressionTree right=product(level+1);
      return new ExpressionTree(new Divide(),left,right);
    }
 
    return left;
  }

References aud.example.expr.Tokenizer.DIVIDE, aud.example.expr.ExpressionParser.factor(), aud.example.expr.ExpressionParser.lookahead(), aud.example.expr.ExpressionParser.nextToken(), aud.example.expr.ExpressionParser.product(), aud.example.expr.Tokenizer.TIMES, and aud.example.expr.ExpressionParser.verbose().

Referenced by aud.example.expr.ExpressionParser.product(), and aud.example.expr.ExpressionParser.sum().

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setVerbose()

void aud.example.expr.ExpressionParser.setVerbose

(

boolean

b

)

set verbose mode (report state to System.err)

Definition at line 80 of file ExpressionParser.java.

                                    {
    verbose_=b;
  }

Referenced by aud.example.expr.ExpressionParser.main().

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sum()

ExpressionTree aud.example.expr.ExpressionParser.sum ( int  level )

protected

parse sum

sum ::= product |
        product '+' sum | product '-' sum

Definition at line 145 of file ExpressionParser.java.

                                          {
    verbose(level,"<sum>");
 
    ExpressionTree left=product(level+1);
 
    if (lookahead()==Tokenizer.PLUS) {
      nextToken();
      ExpressionTree right=sum(level+1);
      return new ExpressionTree(new Plus(),left,right);
    }
    else if (lookahead()==Tokenizer.MINUS) {
      nextToken();
      ExpressionTree right=sum(level+1);
      return new ExpressionTree(new Minus(),left,right);
    }
 
    return left;
  }

References aud.example.expr.ExpressionParser.lookahead(), aud.example.expr.Tokenizer.MINUS, aud.example.expr.ExpressionParser.nextToken(), aud.example.expr.Tokenizer.PLUS, aud.example.expr.ExpressionParser.product(), aud.example.expr.ExpressionParser.sum(), and aud.example.expr.ExpressionParser.verbose().

Referenced by aud.example.expr.ExpressionParser.expression(), and aud.example.expr.ExpressionParser.sum().

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verbose()

void aud.example.expr.ExpressionParser.verbose ( int  level, String  message  )

protected

helper: print out information

Definition at line 120 of file ExpressionParser.java.

                                                   {
    if (verbose_) {
      System.err.println(Sys.indent(level)+message+
                         " ['"+scanner_.matchedText()+"','"+
                         scanner_.remainder()+"']");
    }
  }

Referenced by aud.example.expr.ExpressionParser.expression(), aud.example.expr.ExpressionParser.factor(), aud.example.expr.ExpressionParser.product(), and aud.example.expr.ExpressionParser.sum().

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The documentation for this class was generated from the following file:

• ExpressionParser.java