Quick Sort

1. Quick Sort
Dr. G. Jasmine Beulah,
Assistant Professor, Kristu Jayanti College,
Bengaluru

2. What is Quick Sort
• Most widely used sorting algorithm
• It follows Dive and Conquer paradigm
• Recursion is used in quicksort implementation
• In each recursive call, a pivot is chosen then the array is partitioned in
such a way that all the elements less than pivot lie to the left and all
the elements greater than pivot lie to the right.
• After every call, the chosen pivot occupies its correct position in the
array which it is supposed to as in sorted array
• So with each step, our problem gets reduced by 2 which leads to quick
sorting
• Pivot can be last element of current array, first element of current array
or any random element.

3. //program to sort a list of N integers using quicksort
#include<stdio.h>
void main()
{
int i,n,a[50];
void quicksort(int a[],int,int);
clrscr();
printf("nEnter the size of the array : ");
scanf("%d",&n);
printf("Enter the elements of the arrynt");
for(i=0;i<n;i++)
{
scanf("%d",&a[i]);
}
printf("nThe entered unsorted arrayn");

4. for(i=0;i<n;i++)
{
printf("t%d",a[i]);
}
quicksort(a,0,n-1);
printf("nSorted Arrayn");
for(i=0;i<n;i++)
{
printf("t%d",a[i]);
}
getch();
}

5. void quicksort(int a[],int low,int high)
{
int pivot,down,up,key,temp;
if(low<high)
{
down=low;
pivot=low;
up=high;
key=a[pivot];
while(down<up)
{
while((a[down]<=key)&&(down<=high))
down++;
while((a[up]>key)&&(up>=low))
up--;

6. if(down<up)
{
temp=a[up];
a[up]=a[down];
a[down]=temp;
}
}
temp=a[up];
a[up]=a[pivot];
a[pivot]=temp;
quicksort(a,low,up-1);
quicksort(a,up+1,high);
}
return;
}

7. Algorithm for mainegg
A is an array of n elements.
Step 1: Start
Step 2: Read the size of the array into n
Step 3: Read the array elements
Step 4: Print unsorted list
Step 5: Call the function quicksort(a,0,n-1)
Step 6: Print the sorted list
Step 7 : Stop

8. Algorithm for Quicksort
Quicksort(a[],lb, ub)
Step1: if( lb<ub)
up=lb; down=ub; key=a[lb];
Step 2: while(lb<=ub)
Step 3: while(a[up] < key) up++;
[End of while]
Step 4: while(a[down] > key) down--;
[End of while]
Step 5 : if(up<down)
[Interchange a[up] with a[down]]
temp=a[up]; a[up]=a[down]; a[down]=temp;
[End of if]
[End of while loop in step 2]

9. Step 6: if(up>down)
[Interchange the key element a[lb] with a[down]]
temp=a[lb];a[lb]=a[down];a[down]=temp;
[End of if]
Step 7: Call quicksort(a, lb, down-1);
Step 8: Call quicksort(a, down+1,ub);
[End of if in step 1]