实现词法分析器
实验内容要求
一、实验目的 加深对词法分析器的工作过程的理解;加强对词法分析方法的掌握;能够采用一种编程 语言实现简单的词法分析程序;能够使用自己编写的分析程序对简单的程序段进行词法分 析。
二、实验内容 自定义一种程序设计语言,或者选择已有的一种高级语言,编制它的词法分析程序。词 法分析程序的实现可以采用任何一种编程语言和编程工具。 从输入的源程序中,识别出各个具有独立意义的单词,即关键字、标识符、常数、运算 符、界符。并依次输出各个单词的内部编码及单词符号自身值。(遇到错误时可显示“Error”, 然后跳过错误部分继续显示)
三、实验要求: 1. 对单词的构词规则有明确的定义; 2. 编写的分析程序能够正确识别源程序中的单词符号; 3. 识别出的单词以<种别码,值>的形式保存在符号表中,正确设计和维护符号表; 4. 对于源程序中的词法错误,能够做出简单的错误处理,给出简单的错误提示,保 证顺利完成整个源程序的词法分析;
四、实验步骤 1. 定义目标语言的可用符号表和构词规则; 2. 依次读入源程序符号,对源程序进行单词切分和识别,直到源程序结束; 3. 对正确的单词,按照它的种别以<种别码,值>的形式保存在符号表中; 4. 对不正确的单词,做出错误处理。
实验方法
根据对应的状态转换图完成编码就可以了,内容我不详细多说,下面的图可以作为参照,实际上还需要自己去思考更多,我的代码还是存在一点小问题的,仅供大家参考。
import java.io.File;
import java.io.FileNotFoundException;
import java.util.*;
public class LexicalAnalyzer {
/*
* 1表示关键字
* 2表示标识符
* 3表示常数
* 4表示运算符
* 5表示界符
* 6表示字符串
* */
//关键字
static String []keyWord={"private","protected","public","abstract","class","extends","final","implements",
"interface","native","new","static","strictfp","break","continue","return","do","while","if","else","for",
"instanceof","switch","case","default","boolean","byte","char","double","float","int","long","short",
"String","null","true","false","void","this","goto"};
//运算符
static String []operation={"+","-","*","/","%","++","--","-=","*=","/=","&","|","^","~","<<",">>",">>>","==","!=",
">","<","=",">=","<=","&&","||","!","."};
//界符
static String []symbol={",",";",":","(",")","{","}"};
static ArrayList<String> keyWords=null;
static ArrayList<String> operations=null;
static ArrayList<String> symbols=null;
//指向当前所读到字符串的位置的指针
static int p,lines;
public static void main(String []args) throws FileNotFoundException {
init();
File file=new File("E:\\code\\bytest\\test11\\test2.txt");
lines=1;
try(Scanner input=new Scanner(file)) {
while (input.hasNextLine()){
String str=input.nextLine();
analyze(str);
lines++;
}
}
}
//初始化把数组转换为ArrayList
public static void init(){
keyWords=new ArrayList<>();
operations=new ArrayList<>();
symbols=new ArrayList<>();
Collections.addAll(keyWords, keyWord);
Collections.addAll(operations, operation);
Collections.addAll(symbols, symbol);
}
public static void analyze(String str){
p=0;
char ch;
str=str.trim();
for (;p<str.length();p++){
ch=str.charAt(p);
if (Character.isDigit(ch)){
digitCheck(str);
}else if (Character.isLetter(ch)||ch=='_'){
letterCheck(str);
}else if (ch=='"'){
stringCheck(str);
}
else if (ch==' '){
continue;
}else {
symbolCheck(str);
}
}
}
/*数字的识别
* 1、识别退出:
* 1.1、遇到空格符
* 1.2、遇到运算符或者界符
* 2、错误情况:
* 2.1、两个及以上小数点
* 2.2、掺杂字母
* */
public static void digitCheck(String str){
String token= String.valueOf(str.charAt(p++));
//判断数字的小数点是否有且是否大于1
int flag=0;
boolean err=false;
char ch;
for (;p<str.length();p++) {
ch = str.charAt(p);
if (ch==' '||(!Character.isLetterOrDigit(ch)&&ch!='.')) {
break;
}else if (err){
token+=ch;
}
else {
token+=ch;
if (ch == '.') {
if (flag == 1) {
err = true;
} else {
flag++;
}
}else if (Character.isLetter(ch)){
err=true;
}
}
}
if (token.charAt(token.length()-1)=='.'){
err=true;
}
if (err){
System.out.println(lines+"line"+": "+token+" is wrong");
}else {
System.out.println("("+3+","+token+")");
}
if (p!=str.length()-1||(p==str.length()-1&&!Character.isDigit(str.charAt(p)))){
p--;
}
}
//标识符,关键字的识别
public static void letterCheck(String str){
String token= String.valueOf(str.charAt(p++));
char ch;
for (;p<str.length();p++){
ch=str.charAt(p);
if (!Character.isLetterOrDigit(ch)&&ch!='_'){
break;
}else{
token+=ch;
}
}
if (keyWords.contains(token)){
System.out.println("("+1+","+token+")");
}else {
System.out.println("("+2+","+token+")");
}
if (p!=str.length()-1||(p==str.length()-1&&(!Character.isLetterOrDigit(str.charAt(p))&&str.charAt(p)!='_'))){
p--;
}
}
//符号的识别
public static void symbolCheck(String str){
String token= String.valueOf(str.charAt(p++));
char ch;
if (symbols.contains(token)){
System.out.println("("+5+","+token+")");
p--;
}else {
if (operations.contains(token)){
if (p<str.length()){
ch=str.charAt(p);
if (operations.contains(token+ch)){
token+=ch;
p++;
if (p<str.length()){
ch=str.charAt(p);
if (operations.contains(token+ch)){
token+=ch;
System.out.println("("+4+","+token+")");
}else{
p--;
System.out.println("("+4+","+token+")");
}
}else{
System.out.println("("+4+","+token+")");
}
}else {
p--;
System.out.println("("+4+","+token+")");
}
}
}else {
p--;
System.out.println(lines+"line"+": "+token+" is wrong");
}
}
}
//字符串检查
public static void stringCheck(String str){
String token= String.valueOf(str.charAt(p++));
char ch;
for (;p<str.length();p++){
ch=str.charAt(p);
token+=ch;
if (ch=='"'){
break;
}
}
if (token.charAt(token.length()-1)!='"'){
System.out.println(lines+"line"+": "+token+" is wrong");
}else {
System.out.println("("+6+","+token+")");
}
}
}
实验结果
1、对于正确的输入:
public static void test(){
int a=10;
double b=20.2;
double c=a+b;
}
2、有不正确的输入:
public static double test(){
double a=20.3.2.1;
int 1111bbbb=10;
System.out.println(a+b);
String str="helloWorld";
double c=124.、
a++;
return a+b+c;
}