BNF Grammar and reading from a txt file

[mr_zebes]

Hi everyone. I am still getting the hang of Java and I believe I have coded 90% of this task. Just a few things that I can't seem to grasp.

I am implementing a recursive descent parser that recognizes strings in the language below. The input should be from a file "input.txt" and output should be to the console.

The grammar:
A -> I = E | E
E -> T + E | T - E | T
T -> F * T | F / T | F
F -> P ^ F | P
P -> I | L | UI | UL | (A)
U -> + | - | !
I -> C | CI
C -> a | b | ... | y | z
L -> D | DL
D -> 0 | 1 | ... | 8 | 9

An example session might look like this:

String read from file: a=a+b-c*d
The string "a=a+b-c*d" is in the language.
String read from file: a=a**b++c
The string "a=a**b++c" is not in the language.

/** 
  * The Grammar
  * A -> I = E | E 
  *	E -> T + E | T - E | T
  *	T -> F * T | F / T | F
  *	F -> P ^ F | P
  *	P -> I | L | UI | UL | (A)
  *	U -> + | - | !
  *	I -> C | CI
  *	C -> a | b | ... | y | z
  *	L -> D | DL
  *	D -> 0 | 1 | ... | 8 | 9
**/

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

public class Main {
	
private static String s;
private static int i;

public static boolean A() 
{
    if (I()) 
    {
        if (i < s.length() && s.charAt(i) == '=') 
        {
            ++i;
            if (E()) 
            {
                return true;
            }
        }
            else
            {
            	return true;
            }      
    }
    return false;
}

public static boolean E() 
{
    if (T()) 
    {
        if (i < s.length() && (s.charAt(i) == '+' || s.charAt(i) == '-')) 
        {
        	++i;
            if (E()) 
            {
                return true;
            }
        }
        else 
        {
            return true;
        }
    }
    return false;
}

public static boolean T() 
{
    if (F()) 
    {
        if (i < s.length() && (s.charAt(i) == '*' || s.charAt(i) == '/')) 
        {
            ++i;
            if (T()) {
                return true;
            }
        }
        else 
        {
            return true;
        }
    }
    return false;
}

public static boolean F()
{	
	if(P())
	{
		if (i < s.length() && s.charAt(i) == '^')
		{
			i++;
			if (T())
			{
				return true;
			}
		}
		else
		{
			return true;
		}
	}
	return false;
}

public static boolean P()
{
	if(I())
	{
		return true;
	}
	else if(L())
	{
		return true;
	}
	else if(U() && I())
	{
		return true;
		
	}
	else if(U() && L())
	{
		return true;
	}
	else if (i < s.length() && s.charAt(i) == '(')
	{
		i++;
		if(A())
		{
			if(i < s.length() && s.charAt(i) ==')')
				i++;
			return true;
		}
	}
	return false;
}

public static boolean U()
{
	if (i < s.length() && (s.charAt(i) == '+' | s.charAt(i) == '-' | s.charAt(i) == '!'))
	{
		i++;
		return true;
	}
	return false;
}

public static boolean I() 
{
	if(C())
	{
		return true;
	}
	else if(C() && I())
	{
		return true;
	}
	return false;
}

public static boolean C()
{
	if(i < s.length() && 'a' <= s.charAt(i) && s.charAt(i) <='z')
	{
		i++;
		return true;
	}
	return false;
}

public static boolean L()
{
	if(D())
	{
		return true;
	}
	else if(D() && L())
	{
		return true;
	}
	return false;
}

public static boolean D() 
{
    if (i < s.length() && '0' <= s.charAt(i) && s.charAt(i) <= '9') 
    {
        ++i;
        return true;
    }
    return false;
}
    
public static void main(String[] args)
{	
    File in = new File("input.txt");

    try 
    {
    	Scanner scan = new Scanner(in);
  
        while (scan.hasNext())
        {
        	System.out.printf("The string read from file: ");
        	System.out.println(scan.next());
        } 
    }
  
    catch (IOException e) 
    {
        e.printStackTrace();
        System.exit(0);
    }
   
s = args.length == 1 ? args[0] : "";

if (A() && i == s.length()) 
{
    System.out.println("The string \"" + s + "\" is in the language.");
}
else 
{
    System.out.println("The string \"" + s + "\" is not in the language.");
}

}

}

My current output looks like this:

The string read from file: a=a+b-c*d
The string "" is not in the language.

So it seems to be reading the input file correctly. My error seems to be on this part

s = args.length == 1 ? args[0] : "";

When I put the string in that line such as: s = args.length == 1 ? args[0] : "a=a+b-c*d";

My output is:

The string read from file: a=a+b-c*d
The string "a=a+b-c*d" is in the language.

How can I go about it reading the string from the input file though?

Also, I am not too sure my boolean functions for P, I and, L are correct. Mainly when two terms are together (UI, UL, CI, DL).

Thank you.

[mr_zebes]

So further analyzing I think I am getting on the right track. Here is my revision on the main function.

public static void main(String[] args)
{	
	s = args.length == 1 ? args[0] : "" ;
    
	File in = new File("input.txt");

    try 
    {
    	Scanner scan = new Scanner(in);
  
        while (scan.hasNext())
        {
        	s = scan.next();
        	System.out.printf("The string read from file: ");
        	System.out.println(s);
        	if (A() && i == s.length())
        	{
        	    System.out.println("The string \"" + s + "\" is in the language.");
        	}
        	else 
        	{
        	    System.out.println("The string \"" + s + "\" is not in the language.");
        	}
        } 
    }
 
    catch (IOException e) 
    {
        e.printStackTrace();
        System.exit(0);
    }
}

My input text file looks like this:
a=a**b++c
a=a+b-c*d
a=b

Output in console:

The string read from file: a=a**b++c
The string "a=a**b++c" is not in the language.
The string read from file: a=a+b-c*d
The string "a=a+b-c*d" is not in the language.
The string read from file: a=b
The string "a=b" is not in the language.

So the first string is correctly analyzed as not being in the language.

The next two strings should be in the language though.

Thoughts?

[JosAH]

You should reset the current character index 'i' to zero before you start parsing.

kind regards,

Jos

ps. the few rules you where uncertain about, are correct although highly inefficient.

edit: I take that back; e.g. your method that tries to parse an integer number, return true after having parsed a single digit so it can't even recursively parse the entire number; change it into a loop ...