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Thread: Reversing lines using LinkedList

  1. #1
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    Default 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;
       }
    }


  2. #2
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    Default Re: Reversing lines using LinkedList

    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);

  3. #3
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    Default Re: Reversing lines using LinkedList

    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?

  4. #4
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    Default Re: Reversing lines using LinkedList

    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?

  5. #5
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    Default Re: Reversing lines using LinkedList

    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]

  6. #6
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    Default Re: Reversing lines using LinkedList

    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?

  7. #7
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    Default Re: Reversing lines using LinkedList

    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);

  8. #8
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    Default Re: Reversing lines using LinkedList

    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?

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