编译原理小作业词法分析程序

词法分析程序所参照的编码如70页表3.3

（《编译技术》张莉等著.--北京：高等教育出版社,2016.9.ISBN: 978-7-04-046317-0）

用Java语言实现如下：

import java.io.IOException;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;

class Code{
    public static final int BEGINSY = 1;
    public static final int ENDSY = 2;
    public static final int IFSY = 3;
    public static final int THENSY = 4;
    public static final int ELSE = 5;
    public static final int IDSY = 20;
    public static final int INTSY = 21;
    public static final int PLUSY = 22;
    public static final int MINUSSY = 23;
    public static final int STARSY = 24;
    public static final int DIVISY = 25;
    public static final int LPARSY = 26;
    public static final int RPARSY = 27;
    public static final int COMMASY = 28;
    public static final int SEMISY = 29;
    public static final int COLONSY = 30;
    public static final int ASSIGNSY = 31;
    public static final int EQUSY = 32;
    public static Map<String, Integer> map = new HashMap<String, Integer>();
    // map to find the id of reserved.
    private static final String[] reserved = {"BEGIN", "END", "IF", "THEN", "ELSE"};
    public static void addReserv(){
        map.put("BEGIN", BEGINSY);
        map.put("END", ENDSY);
        map.put("IF", IFSY);
        map.put("THEN", THENSY);
        map.put("ELSE", ELSE);
    }
    public static boolean isReserv(String s){
        return Arrays.asList(reserved).contains(s);
    }
}


public class Word {
    private static String token = "";
    private static char lastC = 0;
    
    public static void main(String[] args) {
        char c=' ';//c is a char
        Code.addReserv();
        do {
            if(lastC==0){
                do{//erase the blank character.
                    try {
                        c = (char)System.in.read();
                    } catch (IOException e) {e.printStackTrace();}
                }while(c==' '||c=='	');
            }else{
                c = lastC;//withdraw
            }
            int tmp = getsym(c);
            if(tmp!=-1&&tmp!=0)
                System.out.println(tmp);
        } while (c!=-1);
    }
    
    private static int getsym(char c){
        lastC = 0;//give lastC a constant value to test whether it is altered.
        if(Character.isLetter(c)){// if c is a letter then link them together.
            token = "";
            do{
                token += c;
                try {
                    c = (char)System.in.read();
                } catch (IOException e) {e.printStackTrace();}
            }while(Character.isDigit(c)||Character.isLetter(c));
            lastC = c;//use lastC to withdraw;
            System.out.print(token+"	");
            return Code.isReserv(token)? Code.map.get(token).intValue() : Code.IDSY;
            //judge if token is reserved or Identifier.
        }else if(Character.isDigit(c)){// if c is digit then link them together
            token = "";
            do{
                token += c;
                try {
                    c = (char)System.in.read();
                } catch (IOException e) {e.printStackTrace();}
            }while(Character.isDigit(c));
            lastC = c;//use lastC to withdraw;
            System.out.print(token+"	");
            return Code.INTSY;
        }else if(c==':'){
            try {
                c = (char)System.in.read();
            } catch (IOException e) {e.printStackTrace();}
            if(c=='='){
                System.out.print(":=	");
                return Code.ASSIGNSY;
            }else{
                System.out.print(":	");
                lastC = c;//use lastC to withdraw;
                return Code.COLONSY;
            }
        }else if(c=='+'){
            System.out.print("+	");
            return Code.PLUSY;
        }else if(c=='-'){
            System.out.print("-	");
            return Code.MINUSSY;
        }else if(c=='*'){
            System.out.print("*	");
            return Code.STARSY;
        }else if(c=='('){
            System.out.print("(	");
            return Code.LPARSY;
        }else if(c==')'){
            System.out.print(")	");
            return Code.RPARSY;
        }else if(c==','){
            System.out.print(",	");
            return Code.COMMASY;
        }else if(c==';'){
            System.out.print(";	");
            return Code.SEMISY;
        }else if(c=='='){
            System.out.print("=	");
            return Code.EQUSY;
        }else if(c=='/'){
            try {
                c = (char)System.in.read();
            } catch (IOException e) {e.printStackTrace();}
            if(c!='*'){
                System.out.print("/	");
                lastC = c;//use lastC to withdraw;
                return Code.DIVISY;
            }else{//handle with the comments.
                do{
                    do{
                        try {
                            c = (char)System.in.read();
                        } catch (IOException e) {e.printStackTrace();}
                    }while(c!='*');
                    do{
                        try {
                            c = (char)System.in.read();
                        } catch (IOException e) {e.printStackTrace();}
                        if(c=='/')    return 0;
                    }while(c=='*');
                }while(c!='*');
            }
        }
        return -1;//-1 means wrong;
    }
    
    
}

示例：