Quick Sort

Concept Used What is the concept used in Merge and Quick Sort? This two sorting techniques use “DIVIDE and CONQUER “ Technique. What is Divide and Conquer Technique? The Problem is divide into similar sub problems When to stop Dividing? When the problem is small enough to be handled. Outline for divide and conquer Sorting ( NEXT )

Outline : Divide and conquer Sorting Sort ( ) {if the list has length greater than 1 then {partition the list into lowlist,highlist; sort(lowlist); sort(highlist); combine(lowlist,highlist); } Where does the Quick and merge sort differ? They differ in the Way the the List is partitioned

Quick Sort Invented by C.A.R Hoare in (Most Popular and widely used) Working: 1. Fix the First Element as the pivot element 2. SCAN L--->R for elements GREATER than PIVOT. Fix it as “i”. 3. SCAN R--->L for elements <= than PIVOT. Fix it as “j”. 4.IF ( I<J) TRUE : SWAP the array values with the index i and j. Continue the process ( GOTO 2 ) FALSE: SWAP the array value at j with the PIVOT. The List is Broken down to two sub list. GOTO 1 for each sub list.

ExampleExample ji j<= I >

Full program for QuickSort #include #include #define MAX 5 void Swap(int x, int y,int a[]) { int temp; temp=a[x];a[x]=a[y];a[y]=temp;}

int Partition (int left, int right,int a[]) { int pivot, pivotpos; pivot=a[left];pivotpos=left; for(int i=left+1;i<=right;i++) if (a[i]<pivot) Swap(++pivotpos,i,a); //move large entry to right and small to left Swap(left,pivotpos,a); return pivotpos; }

void QuickSort (int left,int right,int a[]) { int pivotpos; if (left<right) {pivotpos=Partition(left,right,a);QuickSort(left,pivotpos-1,a);QuickSort(pivotpos+1,right,a);}}

void main() { int A[MAX],i; clrscr(); cout<<"Enter "<<MAX<<" elements to be sorted:"<<endl; for (i=0;i<MAX;i++) cin>>A[i];QuickSort(0,MAX-1,A); cout<<"The elements after Quick Sort"<<endl; for (i=0;i<MAX;i++) cout<<A[i]<<endl;}