C++ Example – C++ program to find prime numbers

Prime Numbers
A prime number is an integer greater than 1 that has exactly two divisors, 1 and itself.
The first ten prime numbers are

2, 3, 5, 7, 11, 13, 17, 19, 23, and 29.

Integers that are not prime are called composite numbers.
C++ program to find prime numbers in a given range

#include <iostream>
#include <iomanip>      // std::setw

using namespace std;

int main()
{
    int num,i,count,n;
    cout << "Enter max range: ";
    cin >> n;
    for(num = 1;num<=n;num++){
        count = 0;
        for(i=2;i<=num/2;i++){
            if(num%i==0){
                count++;
                break;
            }
        }

        if(count==0 && num!= 1)
            cout << num << setw(3);
    }
    return 0;
}

/*
OUTPUT:
Enter max range: 30
2  3  5  7 11 13 17 19 23 29
*/

C++ Program to Check If the number is Prime or not

#include <iostream>

using namespace std;

int main()
{
    int i,no;
    cout<<"Enter any num: ";
    cin>>no;
    if(no==1)
    {
        cout<<"Smallest prime num is 2";
    }
    for(i=2;i<no;i++)
    {
        if(no%i==0)
        {
            cout<<"Not prime number";
            break;
        }
    }
    if(no==i)
    {
        cout<<"Yes, Number is Prime";
    }
    return 0;
}

/*
OUTPUT:
Enter any num: 17
Yes, Number is Prime
*/

C++ Program to Print next Prime number

#include <iostream>

using namespace std;

int main()
{
    int i,j=2,number;
    cout<<"Enter any number: ";
    cin>>number;
    cout<<"Next prime number: ";
    for(i=number+1;i<3000;i++)
    {
        for(j=2;j <i;j++)
        {
            if(i %j==0)
            {
                break;
            } // if
        } // for
        if(i==j || i==1)
        {
            cout<<"\t"<<i;
            break;
        } // if
    }  // outer for
    return 0;
}

/*
OUTPUT:
Enter any number: 17
Next prime number:      19

*/