C++ program to find given number is prime or not

C++ program to find given number is prime or not  using loops, recursion, and find prime numbers from 1 to n etc.

Method 1 : C++ program to find given number is prime or not

#include<iostream>
#include<math.h>
using namespace std;
int main()
{
    int x1,k=0,num;
    cout<<"Enter the number ";
    cin>>num;
    x1=sqrt(num);
    for(int i=2;i<=x1;i++)
    {
        if(num%i==0)
            k++;
    }
    if(k==0)
        cout<<"Number is prime";
    else
        cout<<"Number is not prime";
    return 0;
}

Method 2 : C++ program to find given number is prime or not using recursion

#include<iostream>
#include<math.h>
using namespace std;
int prime(int n,int i)
{
    if(i==1)
        return -1;
    else if(n%i==0)
        return 1;
    prime(n,i-1);
}
int main()
{
    int n,pow,result;
    cout<<"Enter the number : ";
    cin>>n;
    result=prime(n,sqrt(n));
    if(result==-1)
        cout<<"Number is Prime";
    else
        cout<<"Number is not Prime";
}

Method 1 : C++ program to find prime numbers from 1 to n

#include<iostream>
#include<math.h>
using namespace std;
int main()
{
    int prime,n,x1,k=0,c;
    cout<<"Enter the number ";
    cin>>n;
    cout<<"Prime numbers ";
    for(int i=1;i<=n;i++)
    {
        x1=sqrt(i);
        for(int y=2;y<=x1;y++)
        {
          if(i%y==0)
            k++;
        }
        if(k==0)
        cout<<i<<" ";
        k=0;
    }
    return 0;
}

Method 2 : C++ program to find prime numbers from 1 to n using recursion

#include<iostream>
#include<math.h>
#include<iomanip>
using namespace std;
int check(int n,int i)
{
    if(i==1)
        return -1;
    else if(n%i==0)
        return 1;
    check(n,i-1);
}
void prime(int n)
{
    if (n==0)
        return ;
    if(check(n,sqrt(n))==-1)
        cout<<setw(4)<<n;
    prime(n-1);
}
int main()
{
    int n;
    cout<<"Enter the number ";
    cin>>n;
    cout<<"Prime numbers :";
    prime(n);
    return 0;
}

C++ program to find sum of prime numbers

#include<iostream>
#include<math.h>
using namespace std;
int main()
{
    int prime,n,x1,k=0,c,sum=0;
    cout<<"Enter the number ";
    cin>>n;
    cout<<"Prime numbers ";
    for(int i=1;i<=n;i++)
    {
        x1=sqrt(i);
        for(int y=2;y<=x1;y++)
        {
          if(i%y==0)
            k++;
        }
        if(k==0)
        {
            cout<<i<<" ";
            sum+=i;
        }
        k=0;
    }
    cout<<endl<<"Sum = "<<sum<<endl<<endl;
    return 0;
}

C++ program to print prime numbers in an array

#include<iostream>
#include<math.h>
using namespace std;
int main()
{
    int a[20],num,sqr,k=0;
    cout<<"How many elements to be stored (max 20) : ";
    cin>>num;
    cout<<"Enter elements of array a1 : ";
    for(int i=0;i<num;i++)
        cin>>a[i];
    cout<<"Prime numbers : ";
    for(int i=0;i<num;i++)
    {
        sqr=sqrt(a[i]);
        for(int y=2;y<=sqr;y++)
            if(a[i]%y==0)
                k=1;
        if(k==1)
            k=0;
        else
            cout<<a[i]<<" ";
    }
    return 0;
}