一个简单的java递归下降语法分析器例子

news/2024/9/25 0:28:30/

import parser.Parser;
import parser.RecursiveDescentParser;import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;public class Main {public static void main(String[] args) {// 关键词List<String> keyList = new ArrayList<>(Arrays.asList("int","String"));// 关键词数List<Integer> keyNum = new ArrayList<>(Arrays.asList(1,2));// 运算符和界符List<String> symbolList = new ArrayList<>(Arrays.asList("+","-"));// 运算符和界符的数List<Integer> symbolNum = new ArrayList<>(Arrays.asList(10,11));// 从文件取出的字符String letter;// 将字符转为单词String words;String test ="int main() { int i,j; String a,b;} 123 ccd";//        Parser parser = new Parser();
//        parser.analysis(test,keyList);// 测试语法分析，递归下降分析String input = "5+10*2";RecursiveDescentParser recursiveDescentParser = new RecursiveDescentParser(input);int rs = recursiveDescentParser.parseExpression();System.out.println("Result: " + rs);}
}

package parser;/*** 递归下降解析* 文法规则：* expression = term + expression|term* term = = factor * term|factor* factor = (expression)|number* number = [0-9]+**/
public class RecursiveDescentParser {private String input;private int position;public RecursiveDescentParser(String input){this.input = input;this.position = 0;}public int parseExpression(){int term = parseTerm();if (position < input.length() && Character.toString(input.charAt(position)).equals("+")) {position++;int expression =parseExpression();return term+expression;}return term;}public int parseTerm(){int factor = parsefactor();if (position < input.length() && Character.toString(input.charAt(position)).equals("*")) {position++;int term =parseTerm();return factor*term;}return factor;}public int parsefactor(){if (position < input.length() && Character.toString(input.charAt(position)).equals("(")){position++;int expression =parseExpression();if (position < input.length() && Character.toString(input.charAt(position)).equals(")")){position++;return expression;}}else{StringBuilder number = new StringBuilder();while (position < input.length() && Character.isDigit(input.charAt(position))){number.append(input.charAt(position));position++;System.out.println("number: " + number);}return Integer.parseInt(number.toString());}throw new RuntimeException("错误输入");}
}

测试结果