Welcome to the Java Programming Forums


The professional, friendly Java community. 21,500 members and growing!


The Java Programming Forums are a community of Java programmers from all around the World. Our members have a wide range of skills and they all have one thing in common: A passion to learn and code Java. We invite beginner Java programmers right through to Java professionals to post here and share your knowledge. Become a part of the community, help others, expand your knowledge of Java and enjoy talking with like minded people. Registration is quick and best of all free. We look forward to meeting you.


>> REGISTER NOW TO START POSTING


Members have full access to the forums. Advertisements are removed for registered users.

Results 1 to 4 of 4

Thread: MergeSort

  1. #1
    Junior Member
    Join Date
    Oct 2009
    Posts
    26
    Thanks
    3
    Thanked 0 Times in 0 Posts

    Default MergeSort

    I am trying to implement recursive mergesort algorithm and it is almost done the only thing is that if i want to mergesort ( 7, 1 ,2, 3, 5, 6, 8, 4) when i run the program my output is (0, 0, 1, 2, 3, 4, 5, 6, 7, 8)

    What is wrong with my code???

    import java.util.*;
    import java.io.*;
     
    public class MergeSort
    {
     
    public static void mergeSort(double[] a, int length)
       {  
         Sort(a, 0, a.length - 1);
       }
    public static void Sort(double[] a, int left, int right)
       {  
         if (left < right)
         {
          int mid = (left + right) / 2;
           // sort the first and the second half
          Sort(a, left, mid);
          Sort(a, mid + 1, right);
          merge(a, left, mid, right);
         }
       }
       public static void merge(double[] a, int left, int mid, int right)
       {  
     
          double[] b = new double[right - left + 1]; // merge both halves into a temporary array b
     
          int index1 = left;         // next element to consider in the first range
          int index2 = mid + 1;
          int last = right;
          int index = 0;             // next open position in b
     
          // as long as neither i1 nor i2 past the end, move the smaller into b
          while(index1 <= mid && index2 <= last)
          {  
            if(a[index1] < a[index2])
             {  
              b[index] = a[index1];
                index1++;
             }
             else
             { 
               b[index] = a[index2];
                index2++;
             }
             index++;
          }
     
          // note that only one of the two while loops
          // below is executed
     
          // copy any remaining entries of the first half
          while (index1 <= mid)
          { 
            b[index] = a[index1];
             index1++;
             index++;
          }
     
          // copy any remaining entries of the second half
          while (index2 <= last)
          {  
            b[index] = a[index2];
             index2++;
             index++;
          }
          int s = 0;
          // copy back from the temporary array
          for(int i = left; i <= right; i++)
          {
             a[i] = b[s];
             s++;
          }
       }
     
    public static void main(String[] args)throws FileNotFoundException
    {
      Scanner input = new Scanner(new FileReader("input.txt"));
     
      int count = 0;
      int numOfElements = 0;  
      double[] array = new double[10];
     
     
      while(input.hasNextDouble())
      {
        array[count] = input.nextDouble();
        count++;
      }
            System.out.println("Unsorted Array");
        for(int i = 0; i < count; i++)
        {
           System.out.print(array[i] + " ");
      }
        System.out.println();
        mergeSort(array, array.length);
        System.out.println("Sorted Array");
        for(int i = 0; i < count; i++)
        {
          System.out.print(array[i] + " ");
        }
     
    input.close();
     }
    }


  2. #2
    Junior Member
    Join Date
    Oct 2009
    Posts
    26
    Thanks
    3
    Thanked 0 Times in 0 Posts

    Default Re: MergeSort

    correction my output when i run the program is (0,0,1,2,3,4,5,6) so it leaves out that last two integers of my array

  3. #3
    Member
    Join Date
    Feb 2010
    Location
    Auburn, AL
    Posts
    31
    Thanks
    0
    Thanked 8 Times in 8 Posts

    Default Re: MergeSort

    I don't see any problem with your code right off. When I run your code and manually set the array to be random-ish values, it works. Is it possible you aren't reading what you think you're reading from the file?
    Let me know what you think of my website:
    http://www.auburn.edu/~carpept
    Comments or suggestions are appreciated!

  4. #4
    Junior Member
    Join Date
    Feb 2010
    Posts
    10
    Thanks
    3
    Thanked 0 Times in 0 Posts

    Default Re: MergeSort

    Hey m8, you may find the Mergesort I have here: http://www.javaprogrammingforums.com...mergesort.html useful as a reference. It works for the numbers I have tried.

    You may also find this psudocode useful:
    function mergesort(m)
       var list left, right, result
       if length(m) ≤ 1
           return m
       else
           var middle = length(m) / 2
           for each x in m up to middle - 1
               add x to left
           for each x in m at and after middle
               add x to right
           left = mergesort(left)
           right = mergesort(right)
           if last(left) ≤ first(right) 
              append right to left
              return left
           result = merge(left, right)
           return result
     
    function merge(left,right)
       var list result
       while length(left) > 0 and length(right) > 0
           if first(left) ≤ first(right)
               append first(left) to result
               left = rest(left)
           else
               append first(right) to result
               right = rest(right)
       if length(left) > 0 
           append rest(left) to result
       if length(right) > 0 
           append rest(right) to result
       return result
    Source: http://rosettacode.org/wiki/Sorting_...hms/Merge_sort
    Last edited by Shadow703793; March 3rd, 2010 at 08:52 PM.