Merge Sort - Invalid Output ??

This is a discussion on Merge Sort - Invalid Output ?? within the C++ Programming forums, part of the General Programming Boards category; Code: #include <iostream> using namespace std; void mergeSort(int a[], int low, int high); void merge (int a[], int low1, int ...

  1. #1
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    Merge Sort - Invalid Output ??

    Code:
    #include <iostream>
    
    using namespace std;
    
    
    void mergeSort(int a[], int low, int high);
    void merge (int a[], int low1, int high1, int low2, int high2);
    
    int main()
       {
       int sort[10]={6, 5, 8, 2, 12, 21, 11, 15, 25, 32};
    
       mergeSort(sort,0,9);
    
       for(int i =0; i<10; i++)
          {
          cout << sort[i] << endl;
          }
    
       return 0;
       }
    
    void mergeSort(int a[], int low, int high)
       {
       if (low >=high) return;
       int mid = (low+high)/2;
    
       mergeSort(a, low, mid);
       mergeSort(a, mid+1, high);
       merge(a, low, mid, mid+1, high);
       }
    
    void merge(int a[], int low1, int high1, int low2, int high2)
       {
       int t, i1, i2;
       int *temp;
       i1 = low1;
       i2 = low2;
       t = 0;
    
      while (i1 <= high1 && i2 <= high2)
         {
         if (a[i1] < a[i2])
           temp[t++] = a[i1++] ;
    
         else
           temp[t++] = a[i2++] ;
         }
    
       while (i1 <= high1)
         temp[t++] = a[i1++] ;
    
       while (i2 <= high2)
         temp[t++] = a[i2++] ;
    
       for (t = low1; t <= high2; t++)
          {
          a[t] = temp[t - low1] ;
          }
    
       }
    Output I get is

    11
    11
    11
    11
    11
    11
    15
    21
    25
    32


  2. #2
    and the hat of wrongness Salem's Avatar
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    > int *temp;
    Well this isn't pointing anywhere at the moment.

    Even if it did, it would still need to be copied back to the original array.

    I thought the merge was supposed to take place in the original array?
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  3. #3
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    Quote Originally Posted by Salem
    > int *temp;
    Well this isn't pointing anywhere at the moment.

    Even if it did, it would still need to be copied back to the original array.

    I thought the merge was supposed to take place in the original array?

    thanks I fixed it now it works

    int temp[10];

  4. #4
    Code Goddess Prelude's Avatar
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    >I thought the merge was supposed to take place in the original array?
    Not if you want it to be reasonably efficient. An in-place merge is a pain to get right, and it's much slower than using O(N) extra space. That's why merge sort isn't such a good option for arrays, but kicks butt for linked lists. So for arrays you merge to the auxiliary array, then copy back to the original array and wonder why you didn't use quicksort.
    My best code is written with the delete key.

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