Oct 13, 2011

C program to implement MERGE sort.

Merge sort is based on the divide conquer strategy. Array is divided in to two halves.if the array length is n, then it is divided into n/2,n/4,n/8.... and each part is sorted independently, then conquered into the sorted array. The efficiency of merge sort is O(n log n). Read more about C Programming Language .
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#include <stdio.h>
#include <stdlib.h>

#define MAX_ARY 10

void merge_sort(int x[], int end, int start);

int main(void) {
 int ary[MAX_ARY];
 int j = 0;

 printf("\n\nEnter the elements to be sorted: \n");
 for(j=0;j<MAX_ARY;j++)
  scanf("%d",&ary[j]);

 /* array before mergesort */
 printf("Before    :");
 for(j = 0; j < MAX_ARY; j++)
  printf(" %d", ary[j]);

 printf("\n");

 merge_sort(ary, 0, MAX_ARY - 1);

 /* array after mergesort */
 printf("After Merge Sort :");
 for(j = 0; j < MAX_ARY; j++)
  printf(" %d", ary[j]);

 printf("\n");
 getch();
}

/* Method to implement Merge Sort*/
void merge_sort(int x[], int end, int start) {
 int j = 0;
 const int size = start - end + 1;
 int mid  = 0;
 int mrg1 = 0;
 int mrg2 = 0;
 int executing[MAX_ARY];

 if(end == start)
  return;

 mid  = (end + start) / 2;

 merge_sort(x, end, mid);
 merge_sort(x, mid + 1, start);

 for(j = 0; j < size; j++)
  executing[j] = x[end + j];

 mrg1 = 0;
 mrg2 = mid - end + 1;

 for(j = 0; j < size; j++) {
  if(mrg2 <= start - end)
   if(mrg1 <= mid - end)
    if(executing[mrg1] > executing[mrg2])
     x[j + end] = executing[mrg2++];
    else
     x[j + end] = executing[mrg1++];
   else
    x[j + end] = executing[mrg2++];
  else
   x[j + end] = executing[mrg1++];
 }
}
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