// Parse the input words stream into corresponding statement
public Statement parseStatement(TokenScanner scanner) {
String token = scanner.nextToken();
if (token.equals("PRINT")) {
return new PrintStmt(scanner);
}
if (token.equals("LET")) {
return new LetStmt(scanner);
}
if (token.equals("INPUT")) {
return new InputStmt(scanner);
}
if (token.equals("GOTO")) {
return new GoToStmt(scanner);
}
if (token.equals("IF")) {
return new IfStmt(scanner);
}
if (token.equals("END")) {
return new EndStmt(scanner);
}
if (token.equals("REM")) {
return new RemStmt(scanner);
}
System.out.println("Not a legal statement");
return null;
}
public Expression readT(TokenScanner scanner) {
String token = scanner.nextToken();
if (token.charAt(0) >= 'A' && token.charAt(0) <= 'Z'
|| token.charAt(0) >= 'a' && token.charAt(0) <= 'z') {
return new IdentifierExp(token);
} else if (token.charAt(0) >= '0' && token.charAt(0) <= '9') {
return new ConstantExp(Integer.parseInt(token));
}
if (!token.equals("(")) {
System.out.println("Unexpected token");
}
Expression exp = readE(scanner, 0);
if (!scanner.nextToken().equals(")")) {
System.out.println("Unbalanced parentheses");
}
return exp;
}
public Expression readE(TokenScanner scanner, int prec) {
Expression exp = readT(scanner);
String token = null;
while (true) {
token = scanner.nextToken();
int tprec = precedence(token);
if (tprec <= prec) {
break;
}
Expression rhs = readE(scanner, tprec);
exp = new CompoundExp(token, exp, rhs);
}
if (token != null) {
scanner.saveToken(token);
}
return exp;
}