Explain the merge sort technique in C language

Sorting is the process of arranging the elements either in ascending (or) descending order.

Types of sorting

C language provides five sorting techniques, which are as follows −

• Bubble sort (or) Exchange Sort
• Selection sort
• Insertion sort(or) Linear sort
• Quick sort (or) Partition exchange sort
• Merge Sort (or) External sort

Merge sort

Merge sort is a divided and conquer method. It divides an array in two half’s, conquer recursively and merge (combine).

Let’s consider an example which is given below −

Take an unsorted array and apply merge sort technique to sort the array.

38, 27, 43, 3, 9, 82, 10

Now, combine an array by sorting as shown below −

Example

Following is the C program to sort the elements by using the merge sort technique −

#include <stdio.h>
#define max 10
int a[11] = { 10, 14, 19, 26, 27, 31, 33, 35, 42, 44, 0 };
int b[10];
void merging(int low, int mid, int high) {
   int l1, l2, i;
   for(l1 = low, l2 = mid + 1, i = low; l1 <= mid && l2 <= high; i++) {
      if(a[l1] <= a[l2])
         b[i] = a[l1++];
      else
         b[i] = a[l2++];
   }
   while(l1 <= mid)
      b[i++] = a[l1++];
   while(l2 <= high)
      b[i++] = a[l2++];
   for(i = low; i <= high; i++)
      a[i] = b[i];
   }
   void sort(int low, int high) {
      int mid;
      if(low < high) {
         mid = (low + high) / 2;
         sort(low, mid);
         sort(mid+1, high);
         merging(low, mid, high);
      } else {
      return;
   }
}
int main() {
   int i;
   printf("List before sorting\n");
   for(i = 0; i <= max; i++)
   printf("%d ", a[i]);
   sort(0, max);
   printf("\nList after sorting\n");
   for(i = 0; i <= max; i++)
   printf("%d ", a[i]);
}

Output

When the above program is executed, it produces the following output −

List before sorting
10 14 19 26 27 31 33 35 42 44 0
List after sorting
0 10 14 19 26 27 31 33 35 42 44