实验一+表达式翻译器

1、实验一 表达式翻译器一、实验目的构造一个中缀表达式到后缀表达式的翻译器，初步了解递归下降语法分析原理及语法制导翻译的过程。二、实验内容1. 表达式文法及其翻译模式定义如下：expr expr + term print(+) | expr - term print(-) | termterm term * factor print(*) | term / factor print(/) | factorfactor ( expr ) | idprint(id.lexeme)| numprint(num.value)2说明：1）num为无符号整数；2）在后缀形式输出时，为了能区分每个单词，单词之间

2、用空格间隔。例如，输入a+18*b输出：a 18 b * +3）考虑简单的报错，包括：id名非法、num非法、非法字符、缺少操作数、括号不匹配等。三、算法思想 主要使用递归下降法,为了避免左递归,要进行消除左递归1、根据表达式文法,建立三个非终止符的函数四、源代码import java.io.IOException;import java.io.PushbackInputStream;public class Postfix /* * param args * throws IOException */public static void main(String args) throws IO

3、Exception / TODO Auto-generated method stubExpression expression = new Expression();expression.tran();class Expression static int lookahead;PushbackInputStream pushbackInputStream = new PushbackInputStream(new PushbackInputStream(System.in);public Expression() throws IOException lookahead = pushback

4、InputStream.read();public void tran() throws IOException expr();if (lookahead != 13) throw new Error(char) lookahead + syntax error);public void expr() throws IOException term();expr_rest();public void expr_rest() throws IOException if (lookahead = +) match(+);term();System.out.print(+ );expr_rest()

5、; else if (lookahead = -) match(-);term();System.out.print(- );expr_rest(); else public void term() throws IOException factor();term_rest();public void term_rest() throws IOException if (lookahead = *) match(*);factor();System.out.print(* );term_rest(); else if (lookahead = /) match(/);factor();Syst

6、em.out.print(/ );term_rest(); else public void factor() throws IOException int num = 0;if (Character.isDigit(char) lookahead) while (Character.isDigit(char) lookahead) num = num * 10 + lookahead - 0;match(lookahead);if (lookahead != + & lookahead != - & lookahead != *& lookahead != / & lookahead !=

7、13 & lookahead != ) throw new Error(num非法);System.out.print(num + ); else if (lookahead = a & lookahead = A & lookahead = a & lookahead = A & lookahead = 1 & lookahead = 9)| (lookahead = _) str = str + (char) lookahead;match(lookahead);if (lookahead != + & lookahead != - & lookahead != *& lookahead != / & lookahead != 13 & lookahead != ) throw new Error(id名非法);System.out.print(str + ); else if (char) lookahead = () match(lookahead);expr();if (!match() throw new Error(括号不匹配); else if (char) lookahead = ) throw new Error(括号不匹配); else throw new Error(char) lookahead + syntax error);public boo