如何注释以“//”开头的字符串？

Question

I am learning compiler construction these days, and I am having trouble while making the code for comment in it. What is actually happening is that when ever I am writing a string in the notepad file such as Hello //World. Then it is printing "/" this div operator which I don't want. What actually I want is that Hello should be printed in the output and World should get commented. I know I have included the code for div operator but it is also necessary to include. Just wanted to know how I can achieve this comment logic while checking the logic for checking the div operator should also be there.

Here is the code!

import java.io.File;
import java.util.Scanner;
import java.io.FileNotFoundException;

public class main {
    
    public static void main(String[] args) throws FileNotFoundException{
        
        File newFile = new File("C:/temp/sourcecode.txt");
        Scanner scanFile = new Scanner(newFile);
        //Scanner scan = new Scanner(System.in);
        
        char ch;
        String str;
        
        
        
        while(scanFile.hasNextLine()){
        str = scanFile.nextLine();
        int l = str.length();
        if(!str.startsWith("//") && !str.startsWith("/*") && !str.endsWith("*/")) {
        for(int i =0; i<l ; i++) {
            ch = str.charAt(i);
            
            System.out.println(ch);
                
            if(ch == '*'){
                System.out.println("The Operator is MUL");
                System.out.println("arop\n");
            }
            if(ch == '/')
            {
                 System.out.println("The Operator is DIV");
                 System.out.println("arop\n");
            }
            

            
        }
        }
       
        
            int OP = 0;
            
            switch(OP){
                case 0: 
                    if(str.contains("<") && str.contains(">")){
                        System.out.println("The Operator is NE");
                        System.out.println("relop\n");
                        break;
                    }
                case 1: 
                    if(str.contains("<") && str.contains("=")){
                        System.out.println("The Operator is LE");
                        System.out.println("relop\n");
                        break;
                    }
                    
                case 2: 
                    if(str.contains(">") && str.contains("=")){
                        System.out.println("The Operator is GE");
                        System.out.println("relop\n");
                        break;
                    }
                    
                case 3: 
                    if(str.contains("<")){
                        System.out.println("The Operator is LT");
                        System.out.println("relop\n");
                        break;
                    }
                case 4: 
                    if(str.contains(">")){
                        System.out.println("The Operator is GT");
                        System.out.println("relop\n");
                        break;
                    }
                case 5: 
                    if(str.contains("==")){
                        System.out.println("The Operator is EQ");
                        System.out.println("relop\n");
                        break;
                    }
                case 6: 
                    if(str.contains("+")){
                        System.out.println("The Operator is ADD");
                        System.out.println("arop\n");
                        break;
                    }
                case 7: 
                    if(str.contains("-")){
                        System.out.println("The Operator is SUB");
                        System.out.println("arop\n");
                        break;
                    }
//                case 8: 
//                    if(str.contains("*")){
//                        System.out.println("The Operator is MUL");
//                        System.out.println("arop\n");
//                        break;
//                    }
//                case 9: 
//                    if(str.contains("/")){
//                        System.out.println("The Operator is DIV");
//                        System.out.println("arop\n");
//                        break;
//                    }
                case 10: 
                    if(str.contains("=")){
                        System.out.println("The Operator is ASN");
                        System.out.println("otop\n");
                        break;
                    }
                case 11: 
                    if(str.contains("'")){
                        System.out.println("The Operator is PRN");
                        System.out.println("otop\n");
                        break;
                    }
                case 12: 
                    if(str.contains(";")){
                        System.out.println("The Operator is LTRN");
                        System.out.println("otop\n");
                        break;
                    }
                case 13: 
                    if(str.contains("{")){
                        System.out.println("The Operator is LBRC");
                        System.out.println("otop\n");
                        break;
                    }
                case 14: 
                    if(str.contains("}")){
                        System.out.println("The Operator is RBRC");
                        System.out.println("otop\n");
                        break;
                    }      
            }
        }
    }
    }

Thank you in advance!

Answer 1

When programming a compiler the different input words in your code are called tokens, and the phase of recognising the role of each token is called the lexical analysis phase.

When trying to recognise tokens usually what is used is regex which is a way of implementing a finite automata.

You can read about it in much more detail here:

https://en.wikipedia.org/wiki/Lexical_analysis

You should replace the usage of contains, and use a lexer, it's the name of the tool that does lexical analysis. It uses regexes because it's not just about / and // , there can be many different situations where your compiler will need to decide which token to choose.

Here's an example of a finite automate for recognising different tokens, notice that for each prefix there can be many options for possible tokens:

In Java you can use jflex which will generate lexer code with your tokens definitions.

Answer 2

When you find a / , you need to check the next character.

if (ch == '/') {
    char nextCh = (i + 1 < l ? str.charAt(i + 1) : '\0');
    if (nextCh == '/') {
        System.out.println("The Operator is EndOfLineComment");
        System.out.println("arop\n");
        i++;
    } else if (nextCh == '*') {
        System.out.println("The Operator is TraditionalComment");
        System.out.println("arop\n");
        i++;
    } else {
        System.out.println("The Operator is DIV");
        System.out.println("arop\n");
    }
}