Question

Write a c program for implementation of quick sort

Answer 1

Make a list of items that need to be sorted, lets apply in an array.

2.) Choose any element as pivot element from the array list.(Complexity largely depends on choosing the pivot element)

3.) Rearrange the array list so that all the elements with value less than the pivot will come before the pivot and the element with value greater will come after the pivot with in the same array, which make pivot element in the sorted position.(If the reverse the order we are reversing the sorting order that is descending).

4.) Apply recursively the 3rd step to the sub array of the element with smaller values and separate the sub array of the elements
with the greater values.

Note: In the below implemented quick sort program, always first element is set as Pivot element. But if array is already sorted, then the time complexity becomes worse, as quick sort will take quadratic time. So pivot element should be chosen in such a way, that partitioning of array becomes half.

C program for Quick Sort

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

//quick Sort function to Sort Integer array list
void quicksort(int array[], int firstIndex, int lastIndex)
{
//declaaring index variables
int pivotIndex, temp, index1, index2;

if(firstIndex < lastIndex)
{
//assigninh first element index as pivot element
pivotIndex = firstIndex;
index1 = firstIndex;
index2 = lastIndex;

//Sorting in Ascending order with quick sort
while(index1 < index2)
{
while(array[index1] <= array[pivotIndex] && index1 < lastIndex)
{
index1++;
}
while(array[index2]>array[pivotIndex])
{
index2--;
}

if(index1<index2)
{
//Swapping opertation
temp = array[index1];
array[index1] = array[index2];
array[index2] = temp;
}
}

//At the end of first iteration, swap pivot element with index2 element
temp = array[pivotIndex];
array[pivotIndex] = array[index2];
array[index2] = temp;

//Recursive call for quick sort, with partiontioning
quicksort(array, firstIndex, index2-1);
quicksort(array, index2+1, lastIndex);
}
}