Thread: Reversing lines using LinkedList

Hi, I'm writing a program that reverses the lines of a String using the LinkedList structure, which includes an iterator. I have my code written out, but every time my program runs, it takes a very long time before it outputs the lines. I'm having difficulty proceeding to the next step. If anyone could point me to the right direction on what to do next, I would greatly appreciate it.

Also note: I'm unable to call the reverse() method from the Collections class.

import java.util.*;
import java.util.ListIterator;
import java.io.*;
 
public class ReverseList
{
   public static void main (String[] args)
   {
 
        LinkedList<String> phrase = new LinkedList<String>();
        phrase.add("Four");
        phrase.add("score");
        phrase.add("and");
        phrase.add("seven");
        phrase.add("years");
        phrase.add("ago");        
        System.out.println("Phrase in normal order: " + phrase);
 
        reverse(phrase);
 
        System.out.println("Phrase in reverse order: " + phrase);
    }
 
    public static LinkedList<String> reverse(LinkedList<String> a)
    {
      ListIterator<String> order = a.listIterator(a.size());
      LinkedList<String> reversed = new LinkedList<String>();
      while(order.hasPrevious())
      {              
         reversed.add(order.previous());
      }
      return reversed;
   }
}

I don't understand your problem. On my computer your program runs normally and fast. However it doesn't reverse the list because the correct is phrase = reverse(phrase);

Yes, I too was able to receive the output I expected by modifying it as:

System.out.println("Phrase in reversed order: " + reverse(phrase));

Would I be able to use the reverse method to reverse the words of a file? For instance, if I called a text file and used the reverse method, would it be able to reverse the words of said file?

You can get the text file using the BufferedReader and store it in a variable. So, you can to split the variable using a function split("[ ]+").

For example,

 
    // load text using BufferedReader
 
    String words[] = text.split("[ ]+");
 
    for(String word: words)
    {
         phrase.add(word);
    }
 
  phrase = reverser(phrase)
 
   // print phrase

understand?

Not quite, this is what I have so far.

	   Scanner scan = new Scanner (System.in);
	   File f = new File("prezodent.txt");	
	  System.out.print("Enter file name: ");
	  Scanner input = new Scanner (new File(scan.nextLine()));
	  scan.useDelimiter(" ");
	  System.out.println();
	  System.out.println(input);

But it gives me this long stretch of a runtime error every time I enter the file name.

java.util.Scanner[delimiters=\p{javaWhitespace}+][position=0][match valid=false][need input=false][source closed=false][skipped=false][group separator=\,][decimal separator=\.][positive prefix=][negative prefix=\Q-\E][positive suffix=][negative suffix=][NaN string=\Q?\E][infinity string=\Q?\E]

You are printing the object "input" and not its content. You can print the contents of the file this way:

 
 Scanner scan = new Scanner (System.in);
	   File f = new File("prezodent.txt");	
	  System.out.print("Enter file name: ");
	  Scanner input = new Scanner (new File(scan.nextLine()));
	  scan.useDelimiter(" ");
	  System.out.println();
	  while(input.hasNextLine())
	  {
		  System.out.println(input.nextLine());
	  }

helped?

Yes. I have one last question. How can I take the text file/input and convert it to a linkedlist, so it can use the reverse method?

LinkedList<String> list = new LinkedList<String>();
while(input.hasNextLine())
{  
   System.out.println(input.nextLine());
}
System.out.println("\nWords in normal order: " + list);
System.out.println("Words in reverse order: " + list);

You can divide the lines of file in words so you add the words into linkedlist.

For example:

 
           while(input.hasNextLine())
 	   {
    	         String[] words = input.nextLine().split(" ");
 
    	          for(String word: words)
    	         {
    		       phrase.add(word);
    	         }
 	   }
 
        System.out.println("Phrase in normal order: " + phrase);
 
        System.out.println("Phrase in reverse order: " +  reverse(phrase));

This line is the important

 String[] words = input.nextLine().split(" ");

It get one line of file and split the words using space how separator.

understand?