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.

C Program For Quick Sort:

/* Double-Click To Select Code */

#include<stdio.h>
#include<conio.h>

void quick_sort(int arr[20],int,int);

void main()
{
 int arr[20],n,i;
 clrscr();
 printf("Enter the number of elements in the Array: ");
 scanf("%d",&n);
 printf("\nEnter %d elements:\n\n",n);

 for(i=0 ; i<n ; i++)
 {
  printf(" Array[%d] = ",i);
  scanf("%d",&arr[i]);
 }

 quick_sort(arr,0,n-1);
 printf("\nThe Sorted Array is:\n\n");

 for(i=0 ; i<n ; i++)
 {
  printf(" %4d",arr[i]);
 } 
 getch();
}

void quick_sort(int arr[20],int low,int high)
{
 int pivot,j,temp,i;
 if(low<high)
 {
  pivot = low;
  i = low;
  j = high;

  while(i<j)
  {
   while((arr[i]<=arr[pivot])&&(i<high))
   {
    i++;
   }

   while(arr[j]>arr[pivot])
   {
    j--;
   }

   if(i<j)
   { 
    temp=arr[i];
    arr[i]=arr[j];
    arr[j]=temp;
   }
  }

  temp=arr[pivot];
  arr[pivot]=arr[j];
  arr[j]=temp;
  quick_sort(arr,low,j-1);
  quick_sort(arr,j+1,high);
 }
}

Output of Program:

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This article uses material from the Wikipedia article Quick Sort which is released under the Creative Commons Attribution-Share-Alike License 3.0

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