Oct 11, 2011

C Program to implement quick sort

C Program to implement quick sort. Quick sort algorithm is based on divide and conquer strategy. In a quick sort we take the one element called as pivot,then we list all the smaller elements than pivot, and greater than pivot. after partitioning we have pivot in the final position. After recursively sorting the partition array, we get the sorted elements. Read more about C Programming Language .

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#include<stdio.h>

void swap (int a[], int left, int right)
{
 int temp;
 temp=a[left];
 a[left]=a[right];
 a[right]=temp; 
}//end swap

void quicksort( int a[], int low, int high )
{
 int pivot;
 // Termination condition! 
 if ( high > low )
 {
  pivot = partition( a, low, high );
  quicksort( a, low, pivot-1 );
  quicksort( a, pivot+1, high );
 }
} //end quicksort

int partition( int a[], int low, int high )
{
 int left, right;
 int pivot_item;
 int pivot = left = low; 
 pivot_item = a[low]; 
 right = high;
 while ( left < right ) 
 {
  // Move left while item < pivot 
  while( a[left] <= pivot_item ) 
   left++;
  // Move right while item > pivot 
  while( a[right] > pivot_item ) 
   right--;
  if ( left < right ) 
   swap(a,left,right);
 }
 // right is final position for the pivot 
 a[low] = a[right];
 a[right] = pivot_item;
 return right;
}//end partition

// void quicksort(int a[], int, int);
void printarray(int a[], int);

int main()
{
 int a[50], i, n;
 printf("\nEnter no. of elements: "); 
 scanf("%d", &n);
 printf("\nEnter the elements: \n");
 for (i=0; i<n; i++)
  scanf ("%d", &a[i]);
 printf("\nUnsorted elements: \n");
 printarray(a,n);
 quicksort(a,0,n-1);
 printf("\nSorted elements: \n");
 printarray(a,n);

}//end main


void printarray(int a[], int n)
{
 int i;
 for (i=0; i<n; i++)
  printf(" %d ", a[i]);
 printf("\n");
}//end printarray
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