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SieveOfEratosthenes

To Get the prime number in the range 2.N , you can use a simple and intersting method known as Sieve Of Eratosthenes.
For Exampl this Program :-


#include <iostream.h>
#include <conio.h>
#define Max 100
void main()
{
clrscr();
int isprime[Max+1];
int n,i;
cout<<"\tEnter N (<=100):\t";
cin>>n;
for(i=2;i<=n;i++)
isprime[i]=1;
for(i=2;i*i<=n;i++)
{
if (isprime[i]==1)
{for(int j=i;i*j<=n;j++)
isprime[i*j]=0;
}
}
int gap =0;
int bestgap=0;
int right=0;
for(i=2;i<=n;i++)
{
if (isprime[i])gap=0;
else gap++;
if (gap>bestgap)
{ bestgap=gap;
right=i;
}
}
int left=right-bestgap+1;
cout<<"\t\nThere are no prime between"<<left<<"\tand"<<right<<endl;
cout<<"\t\nThat is"<<bestgap<<"\tconsecutive integers"<<endl;
getch();
}
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