Question

I found this example of merge_sort here at cprogramming. I’m tyring to break down the algorithm to understand the alogrithm with in the context of quick sort another SO post.

EDIT:

I’ve updated/fixed the code and posted it as an answer below. This is a working merge_sort.

Answer 1

The code posted here could use some improvements in readability…Here they are. Verified this is a working merge_sort:

#include "c_arclib.cpp"
void merge_helper(int *input, int left, int right, int *scratch)
  {
  if(right == left + 1)
    {
    return;
    }
  else
    {
    int i = 0;
    int length = right - left;
    int midpoint_distance = length/2;
    int l = left, r = left + midpoint_distance;
    merge_helper(input, left, left + midpoint_distance, scratch);
    merge_helper(input, left + midpoint_distance, right, scratch);
    for(i = 0; i < length; i++)
      {
      if((l < (left + midpoint_distance)) && (r == right || input[l] > input[r]))
        {
        scratch[i] = input[l];
        l++;
        }
      else
        {
        scratch[i] = input[r];
        r++;
        }
      }
    for(i = left; i < right; i++)
      {
      input[i] = scratch[i - left];
      }
    }
  }
int merge_sort(int *input, int size)
  {
    int *scratch = (int *)malloc(size * sizeof(int));
    if(scratch != NULL)
    {
        merge_helper(input, 0, size, scratch);
        free(scratch);
        return 1;
    }
    else
    {
        return 0;
    }
  }

int main()
  {
  int size=1000;
  int* array = new int[size]();
  util::rand_to_array(array,size);
  util::print_array(array, size);
  merge_sort(array, size);
  cout << endl; util::print_array(array, size);
  return 0;
  }