Question

The following grammar for a program has a problem with semicolons:

• Program ::= Block "."
• Block ::= "{" Statement (";" Statement )* "}"
• Statement ::= Assignment | IfStatement | Block
• Assignment ::= "="
• IfStatement ::= "if" "(" Assignment ")" Statement

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When an if statement has a block after it, the block sometimes must end with a semicolon sometimes but not all the time.

1. Write an example showing a required semicolon after a "}"
2. Write an example where a semicolon is not required after a "}"
3. How can the grammar be modified so that several semicolons can occur in a row with not statements.
4. Can the grammar be modified so that a semicolon is not required after a "}" that ends a Block? Rewrite the grammar to make a semicolon not be required after a "}".

Answer 1

The Main purpose of semicolen here is to separate independent statements.

Example : Statement 1 : statement 2 : statement 3;

semicolens are used where the comma and full stops can not be used and are inappropriate.

Javacc 1 .A program that creates parsers

2.The source code(input) is a definition file ◦ Grammar definition ◦ Inline Java code

3 The target(output) is java-based parser

BASICS of JavaCC need to know for better understanding of program

javacc_input ::= javacc_options

                 "PARSER_BEGIN" "(" identifier ")"

                 CompilationUnit

                 "PARSER_END" "(" identifier ")"

                 ( production )+

                 <EOF>

javacc_options ::= ( <IDENTIFIER> "{" ( option_binding )* "}" )?

option_binding ::= ( <IDENTIFIER> | "LOOKAHEAD" | "IGNORE_CASE" | "static" | "PARSER_BEGIN" )

                   "="

                   ( IntegerLiteral | BooleanLiteral | StringLiteral | StringList )

                   ";"

option_binding ::= ( <IDENTIFIER> | "LOOKAHEAD" | "IGNORE_CASE" | "static" | "PARSER_BEGIN" )

                   "="

                   ( IntegerLiteral | BooleanLiteral | StringLiteral | StringList )

                   ";"

StringList ::= "(" StringLiteral ( "," StringLiteral )* ")"

production ::= javacode_production

             | cppcode_production

             | regular_expr_production

             | token_manager_decls

             | bnf_production

javacode_production ::= "JAVACODE"

                        AccessModifier

                        ResultType

                        identifier

                        FormalParameters ( "throws" Name ( "," Name )* )?

                        Block

cppcode_production ::= "CPPCODE"

                       AccessModifier

                       ResultType

                       identifier

                       FormalParameters ( "throws" Name ( "," Name )* )?

                       Block

bnf_production ::= AccessModifier

                   ResultType

                   identifier

                   FormalParameters ( "throws" Name ( "," Name )* )?

                   ":"

                   Block

                   "{" expansion_choices "}"

AccessModifier ::= ( "public" | "protected" | "private" )?

regular_expr_production ::= ( "<" "*" ">"

                            | "<" <IDENTIFIER> ( "," <IDENTIFIER> )* ">"

                            )?                    

                            regexpr_kind ( "[" "IGNORE_CASE" "]" )? ":" "{"

                            regexpr_spec ( "|" regexpr_spec )* "}"

token_manager_decls ::= "TOKEN_MGR_DECLS" ":" ( ClassOrInterfaceBody )?

regexpr_kind ::= "TOKEN"

               | "SPECIAL_TOKEN"

               | "SKIP"

               | "MORE"

regexpr_spec ::= regular_expression

                 ( Block )?

                 ( ":" <IDENTIFIER> )?

expansion_choices ::= expansion ( "|" expansion )*

expansion ::= ( "LOOKAHEAD" "(" local_lookahead ")" )?

              ( expansion_unit )+

local_lookahead ::= ( IntegerLiteral )?

                    ( "," )?

                    ( expansion_choices )?

                    ( "," )?

                    ( "{" ( Expression )? "}" )?

expansion_unit ::= "LOOKAHEAD" "(" local_lookahead ")"

                 | Block

                 | "[" expansion_choices "]"

                 | "try" "{" expansion_choices "}"

                 ( "catch" "(" ( Name <IDENTIFIER> )? ")" Block )*

                 ( "finally" Block )?

                 | ( PrimaryExpression "=" )?

                 (

                   identifier ( TypeArguments )? Arguments

                 | regular_expression ( "." <IDENTIFIER> )?

                 )

                 | "(" expansion_choices ")"

                 ( "+" | "*" | "?" )?

regular_expression ::= StringLiteral

                     | <LANGLE: "<">

                       ( ( "#" )? identifier ":" )?

                       complex_regular_expression_choices <RANGLE: ">">

                     | "<" identifier ">"

                     | "<" "EOF" ">"

complex_regular_expression_choices ::= complex_regular_expression

                                       ( "|" complex_regular_expression )*

complex_regular_expression ::= ( complex_regular_expression_unit )+

complex_regular_expression_unit ::= StringLiteral

                                  | "<" identifier ">"

                                  | character_list

                                | "(" complex_regular_expression_choices ")"

                                  (

                                    "+" | "*" | "?" | "{"

                                    IntegerLiteral

                                    ( "," ( IntegerLiteral )? )?

                                    "}"

                                  )?

character_list ::= ( "~" )? "[" ( character_descriptor ( "," character_descriptor )* )? "]"

character_descriptor ::= StringLiteral ( "-" StringLiteral )?

identifier ::= <IDENTIFIER>

Program structure

CompilationUnit ::= ( PackageDeclaration )?

                    ( ImportDeclaration )*

                    ( TypeDeclaration )*

PackageDeclaration ::= Modifiers "package" Name ";"

ImportDeclaration ::= "import"

                      ( "static" )?

                      Name

                      ( "." "*" )?

                      ";"

Example Program :

PARSER_BEGIN(Example)

/**

* Simple brace matcher.

*/

public class Example{

/** Main entry point. */

public static void main(String args[]) throws ParseException {

    Example3 parser = new Example(System.in);

    parser.Input();

}

}

PARSER_END(Example)

SKIP :

{

" "

| "\t"

| "\n"

| "\r"

}

TOKEN :

{

<LBRACE: "{">

| <RBRACE: "}">

}

/** Root production. */

void Input() :

{ int count; }

{

count=MatchedBraces() <EOF>

{ System.out.println("The levels of nesting is " + count); }

}

/** Brace counting production. */

int MatchedBraces() :

{ int nested_count=0; }

{

<LBRACE> [ nested_count=MatchedBraces() ] <RBRACE>

{ return ++nested_count; }

}