write the prime no program in c++?

Answer Posted / prateek

I found some good and new ways to write this programe.
Thankx to all. Well even I have also tried to make this
programe. By making use of "while loop";

#include<iostream>
#include<conio.h>

using namespace std;

void main(){

int num,n,i=0,flag=0;;
cout<<"Enter the number";
cin>>num;

n=num/2;
while(i<n)
{
++i;
if(num%i==0 && i!=1)
{
flag=1;
break;
}
else
{
flag;
}
}
if(flag)
{
cout<<"The number is not a prime number";
}
else
{
cout<<"The number is a prime number";
}

getch();


}

Plz notice, that I have divided the number by 2. Suppose
user input 42. The highest divisible value of 42 will be
its half, i.e., 21(21*2=42). So there is no need to check
the loop condition until the value of 'i' reach num. Becoz
it is for sure, that the values more than its half are not
divisible. This will increase the efficiency of the
programe.

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