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Thread: MergeSort

  1. #1
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    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
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    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
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    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
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    Default Re: MergeSort

    Hey m8, you may find the Mergesort I have here: Quick Question about Mergesort 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 07:52 PM.