Problem processing input for online judge

Question

This is related to my previous question

I'm solving UVA's Edit Step Ladders and trying to make the online judge comply with my answer.

I have used the method ReadLn() to adapt my textfile-reading program to this:

import java.io.*;
import java.util.*;

class LevenshteinParaElJuez implements Runnable{
    static String ReadLn(int maxLength){  // utility function to read from stdin,
                                          // Provided by Programming-challenges, edit for style only
        byte line[] = new byte [maxLength];
        int length = 0;
        int input = -1;
        try{
            while (length < maxLength){//Read untill maxlength
                input = System.in.read();
                if ((input < 0) || (input == '\n')) break; //or untill end of line ninput
                line [length++] += input;
            }

            if ((input < 0) && (length == 0)) return null;  // eof
            return new String(line, 0, length);
        }catch (IOException e){
            return null;
        }
    }

    public static void main(String args[]) // entry point from OS
    {
        LevenshteinParaElJuez myWork = new LevenshteinParaElJuez();  // Construct the bootloader
        myWork.run();            // execute
    }

    public void run() {
        new myStuff().run();
    }
}
class myStuff implements Runnable{
    public void run(){

        ArrayList<String> theWords = new ArrayList<String>();
        try
        {

        /// PLACE YOUR JAVA CODE HERE

        String leido=LevenshteinParaElJuez.ReadLn(100);

        //System.out.println("lo leido fue "+leido);

        while (!leido.equals(" ")){
        theWords.add(leido);
        leido=LevenshteinParaElJuez.ReadLn(100);
        }


        }catch(Exception e){
            System.out.println("El programa genero una excepcion");
        }


        int maxEdit=0;
        int actualEdit=0;

     int wordsIndex1 =0, wordsIndex2=0;


     while (wordsIndex1<= theWords.size())
     {
      while (wordsIndex2<= theWords.size()-1){
         actualEdit=Levenshtein.computeLevenshteinDistance(theWords.get(wordsIndex1),theWords.get(wordsIndex2));
         if (actualEdit>maxEdit){maxEdit=actualEdit;}
         wordsIndex2++;
      }
     wordsIndex1++;

     }

     System.out.println(maxEdit+1);



    }


}
class Levenshtein {
    private static int minimum(int a, int b, int c) {
        if(a<=b && a<=c)
            return a;
        if(b<=a && b<=c)
            return b;
        return c;
    }

    public static int computeLevenshteinDistance(String str1, String str2) {
        return computeLevenshteinDistance(str1.toCharArray(),
                                          str2.toCharArray());
    }

    private static int computeLevenshteinDistance(char [] str1, char [] str2) {
        int [][]distance = new int[str1.length+1][str2.length+1];

        for(int i=0;i<=str1.length;i++)
                distance[i][0]=i;

        for(int j=0;j<=str2.length;j++)
            distance[0][j]=j;

        for(int i=1;i<=str1.length;i++)
            for(int j=1;j<=str2.length;j++)
                distance[i][j]= minimum(distance[i-1][j]+1,
                                        distance[i][j-1]+1,
                                        distance[i-1][j-1]+
                                        ((str1[i-1]==str2[j-1])?0:1));

        return distance[str1.length][str2.length];
    }


}

I'm supposed to read the whole of the online judge's input as it were written via keyboard, but when I run the above program I can't make it stop reading. It goes something like:

abc
cba
aba
cca

without being able to stop the console reading. How do I get around this?? I suspect the problem is in my loop's conditional:

String leido=LevenshteinParaElJuez.ReadLn(100);

            //System.out.println("lo leido fue "+leido);

            while (!leido.equals(" ")){
            theWords.add(leido);
            leido=LevenshteinParaElJuez.ReadLn(100);
            }

I have also used:

while (!leido.equals(null)){
        theWords.add(leido);
        leido=LevenshteinParaElJuez.ReadLn(100);
        }

and gotten stuck as well.

Edit: the declaration was actually:

while (leido != null)){
        theWords.add(leido);
        leido=LevenshteinParaElJuez.ReadLn(100);
        }

I don't get why it's failing. I want the reading of input via keyboard to stop when the first blank line is entered.

Edit: thanks to rodion's answer, the ReadLn method is now changed to:

if ((input < 0) || (length == 0)) return null;  // eof

instead of:

if ((input < 0) && (length == 0)) return null;  // eof

Now, it's reading two blank space before producing the integer output. How can I change it just to read one?

Answer 1

The problem is that the input will not be <0 when it reaches the line that returns null, so do:

while (leido.length() != 0) {
    ....
}

Answer 2

I am not really sure if I understand your problem correctly. If you merely want your program to stop reading from the console, you can "close" the console by hitting Ctrl+D (Linux) or Ctrl+Z (Windows). This causes System.in.read() to return -1, so that ReadLn method would return null.

Check for null as Nathaniel suggested.

Edit: (based on your comment)

The problem is with this condition in the ReadLn method

if ((input < 0) && (length == 0)) return null;

If you enter a blank line, length will be 0, input will be > 0 though (whatever your system uses to denote a newline). If you change it to

if ((input < 0) || (length == 0)) return null;

the method will return null if either the input stream is closed or a blank line is entered.strong text

Answer 3

Instead of !leido.equals(" "), simply use !leido.equals(""). The former terminates when a line containing only a space is entered, the latter when a blank line is entered.

Answer 4

I dont understand your problem much, but just a suggest from my side will be that use your condition in this way

while(!" ".equals(leido)){  
//instead of   
while (!leido.equals(" ")){

why ?, because your code will throw a Exception if your variable contains null, and the condition which i mentioned will never do that :P