Quick Sort Algorithm And C++ Code

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Quicksort, or partition-exchange sort, is a sorting algorithm that, on average, makes O(n log n) comparisons to sort n items.  It was developed by Tony Hoare. Quicksort is faster in practice than other O(n log n) algorithms such as Bubble sort or Insertion Sort. Quicksort can be implemented with an in-place partitioning algorithm, so the entire sort can be done with only O(log n) additional space. Quicksort is a comparison sort and is not a stable sort.
Its complexity is as follows: Best Case - O(n log n) Worst Case - O(n^2)  Average Case - O(n log n)
Quicksort is a divide and conquer algorithm. Quicksort first divides a large list into two smaller sub-lists: the low elements and the high elements. Quicksort can then recursively sort the sub-lists. The steps are: 1. Pick an element, called a pivot, from the list. 2. Reorder the list so that all elements with values less than the pivot come before the pivot, while all elements with values greater than the pivot come after it (equal values can go either way). After this partitioning, the pivot is in its final position. This is called the partition operation. 3. Recursively sort the sub-list of lesser elements and the sub-list of greater elements. The base case of the recursion are lists of size zero or one, which never need to be sorted.

Algorithm/Pseudo-code For Quicksort:

/* Double-Click To Select Code */

  function quicksort('array')
      if length('array') ≤ 1
         return 'array'  // an array of zero or one elements is already sorted
      select and remove a pivot value 'pivot' from 'array'
      create empty lists 'less' and 'greater'
      for each 'x' in 'array'
         if 'x' ≤ 'pivot' then append 'x' to 'less'
         else append 'x' to 'greater'
      return concatenate(quicksort('less'), 'pivot', quicksort('greater')) 
// two recursive calls

Here is an Image depicting Quick Sort:

In-place partition in action on a small list. The boxed element is the pivot element, blue elements are less or equal, and red elements are larger.

If you want to see the C Code for Quick Sort, you can refer to my post here.

C++ Program For Quick Sort:

/* Double-Click To Select Code */
#include<iostream.h>
#include<conio.h>

using namespace std;


int part(int low,int high,int *a)
{
//Function for partitioning array
     int i,h=high,l=low,p,t;  //p==pivot
     p=a[low];
     while(low<high)
     {
                    while(a[l]<p)
                    {
                                   l++;
                    }
                    while(a[h]>p)
                    {
                                   h--;
                    }
                    if(l<h)
                    {
                                t=a[l];
                                a[l]=a[h];
                                a[h]=t;
                    }
                    else
                    {
                        t=p;
                        p=a[l];
                        a[l]=t;
                        break;
                    }
     }
     return h;    
}

void quick(int l,int h,int *a)
{
  int index,i;
  if(l<h)
  {
          index=part(l,h,a);
          quick(l,index-1,a);
          quick(index+1,h,a);
  }
}

void main()
{
      int a[100],n,l,h,i;
      cout<<"Enter number of elements:";
      cin>>n;
      cout<<"Enter the elements (Use Space As A Separator):";
      for(i=0;i<n;i++)
      cin>>a[i];
      cout<<"\nInitial Array:\n";
      for(i=0;i<n;i++)
      {
                      cout<<a[i]<<"\t";
      }   
      h=n-1;
      l=0;
      quick(l,h,a);
      cout<<"\nAfter Sorting:\n";
      for(i=0;i<n;i++)
      {
                cout<<a[i]<<"\t";
      }
      getch();
}

Output of Program:

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