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Parser.java
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Parser.java
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class GrammerException extends Exception{
}
public class Parser{
Token tokens[]={
// new Token(TokenType.NUM,"3"),
// new Token(TokenType.MULOP, "*"),
// new Token(TokenType.LEFT_PAR,"("),
// new Token(TokenType.NUM, "4"),
// new Token(TokenType.RIGHT_PAR, ")"),
// new Token(TokenType.NEWLINE, "\n"),
new Token(TokenType.INT,"INT"),
new Token(TokenType.ID,"x"),
new Token(TokenType.SEMICOLON,";"),
new Token(TokenType.DOUBLE,"DOUBLE"),
new Token(TokenType.ID,"y"),
new Token(TokenType.SEMICOLON,";"),
new Token(TokenType.ID,"x"),
new Token(TokenType.EQUAL,"="),
new Token(TokenType.NUM, "10"),
new Token(TokenType.SEMICOLON,";"),
new Token(TokenType.ID,"y"),
new Token(TokenType.EQUAL,"="),
new Token(TokenType.NUM,"3.14"),
new Token(TokenType.SEMICOLON,";"),
new Token(TokenType.INT,"INT"),
new Token(TokenType.ID,"z"),
new Token(TokenType.SEMICOLON,";"),
new Token(TokenType.ID,"z"),
new Token(TokenType.EQUAL,"="),
new Token(TokenType.ID,"x"),
new Token(TokenType.MULOP,"*"),
new Token(TokenType.LEFT_PAR,"("),
new Token(TokenType.ID,"y"),
new Token(TokenType.ADDOP,"+"),
new Token(TokenType.NUM,"2"),
new Token(TokenType.RIGHT_PAR,")"),
new Token(TokenType.SEMICOLON,";"),
new Token(TokenType.ID, "x"),
new Token(TokenType.EQUAL, "="),
new Token(TokenType.ID, "y"),
new Token(TokenType.EQUAL, "="),
new Token(TokenType.NUM, "10"),
new Token(TokenType.SEMICOLON, ";")
};
int token_pos=0;
void print_grammer(String left,String right){
System.out.println(left+" -> "+right);
}
Token getNextToken(){
if (token_pos<tokens.length){
return tokens[token_pos++];
}
return null;
}
Token peekNextToken(){
if (token_pos<tokens.length){
return tokens[token_pos];
}
return null;
}
void p() throws GrammerException{
// Token next_token=peekNextToken();
// if(next_token==null){
// print_grammer("P", "empty");
// }else{
// stmnts();
// }
print_grammer("P", "Stmmts");
stmnts();
}
void stmnts() throws GrammerException{
Token next_token=peekNextToken();
if(next_token==null){
print_grammer("Stmnts", "empty");
}else{
print_grammer("Stmnts", "Stmnt Stmnts");
stmnt();
stmnts();
}
}
void stmnt() throws GrammerException{
Token next_token=peekNextToken();
if(next_token.tokenType==TokenType.INT || next_token.tokenType==TokenType.DOUBLE){
declaration();
match_terminal(TokenType.SEMICOLON);
}
else if(next_token.tokenType==TokenType.NUM || next_token.tokenType==TokenType.LEFT_PAR){
e();
match_terminal(TokenType.SEMICOLON);
}else{
int current_token_pos=token_pos;
try{
assignment();
match_terminal(TokenType.SEMICOLON);
}catch(GrammerException ge){
token_pos=current_token_pos;
e();
match_terminal(TokenType.SEMICOLON);
}
}
}
void assignment() throws GrammerException{
int current_token_pos=token_pos;
try{
variable();
match_terminal(TokenType.EQUAL);
assignment();
}catch(GrammerException ge){
token_pos=current_token_pos;
variable();
match_terminal(TokenType.EQUAL);
e();
}
}
void declaration() throws GrammerException{
print_grammer("Declaration", "Type ID");
type();
match_terminal(TokenType.ID);
}
void type() throws GrammerException{
Token next_token=peekNextToken();
if(next_token.tokenType==TokenType.INT){
print_grammer("Type", "INT");
match_terminal(TokenType.INT);
}else{
print_grammer("Type", "DOUBLE");
match_terminal(TokenType.DOUBLE);
}
}
void e() throws GrammerException{
print_grammer("E", "TE\'");
t();
e_bar();
}
void e_bar()throws GrammerException{
Token next_token=peekNextToken();
// if(next_token.tokenType==TokenType.NEWLINE || next_token.tokenType==TokenType.RIGHT_PAR){
// print_grammer("E\'", "empty");
// }else{
// print_grammer("E\'", "+TE\'");
// match_terminal(TokenType.ADDOP);
// t();
// e_bar();
// }
if(next_token.tokenType == TokenType.ADDOP){
print_grammer("E\'", "+TE\'");
match_terminal(TokenType.ADDOP);
t();
e_bar();
}
else{
print_grammer("E\'", "empty");
}
}
void t()throws GrammerException{
print_grammer("T", "FT\'");
f();
t_bar();
}
void t_bar() throws GrammerException{
Token next_token=peekNextToken();
// if(next_token.tokenType==TokenType.ADDOP || next_token.tokenType==TokenType.RIGHT_PAR || next_token.tokenType==TokenType.NEWLINE){
// print_grammer("T\'", "empty");
// }
// else{
// print_grammer("T\'", "*FT\'");
// match_terminal(TokenType.MULOP);
// f();
// t_bar();
// }
if(next_token.tokenType == TokenType.MULOP){
print_grammer("T\'", "*FT\'");
match_terminal(TokenType.MULOP);
f();
t_bar();
}else{
print_grammer("T\'", "empty");
}
}
void f() throws GrammerException{
if (peekNextToken().tokenType==TokenType.NUM){
print_grammer("F", "NUMBER");
match_terminal(TokenType.NUM);
}else if(peekNextToken().tokenType==TokenType.LEFT_PAR){
print_grammer("F", "(E)");
match_terminal(TokenType.LEFT_PAR);
e();
match_terminal(TokenType.RIGHT_PAR);
}
else{
print_grammer("F", "Variable");
variable();
}
}
void variable() throws GrammerException{
print_grammer("Variable", "ID");
match_terminal(TokenType.ID);
}
void match_terminal(TokenType expected_token)throws GrammerException{
if (expected_token!=getNextToken().tokenType)
throw new GrammerException();
// print_grammer(expected_token.toString(), expected_token.toString());
}
public void parse(){
try{
p();
}catch(GrammerException g){
System.out.println("parse error");
}
}
public static void main(String[] args) {
Parser parser=new Parser();
parser.parse();
}
}