C++ Program For Prime Number Between 1 to N

C++ Program For Prime Number Between 1 to N | A natural number which has only two factors ( 1 and itself ) is called a prime number. For example, 5 is a prime number because it has only two factors 1 and 5. Similarly, 9 is not a prime number because it has more than 2 factors that are 1,3, and 9.

To develop a C++ program to check prime number; first, you should know how to find out all factors of a number. If any number has more than 2 factors then only, it is a prime number. All negative numbers, 0 and 1 are not the prime numbers.

#include<iostream>
using namespace std;
int main()
{
  // declare variables
  int m, n, count;

  // take input
  cout << "Enter min range: ";
  cin >> m;
  cout << "Enter max range: ";
  cin >> n;
  
  // don't check for -ve numbers
  if(m <= 1) m = 1;
  
  // display result
  cout << "Prime numbers between " 
       << m << " to " << n 
       << " are: " << endl;
  
  for(int i=m; i <= n; i++) 
  {
     count = 0;
     // check for prime
     for (int j=2; j <= i/2; j++)
     {
        if (i % j == 0)
        {
           count++;
           break;
        }
    }
    
    // display result
    if (count == 0)
    cout << i << " ";
  }

  return 0;
}

Output:-

Enter min range: 1
Enter max range: 50
Prime numbers between 1 to 50 are:
1 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47

Optimized Prime Number Program in C++

In the previous post, C++ program to check prime number we had written a optimized program. Using that program we can develop a C++ program for prime number between 1 to N.

#include<iostream>
#include<math.h>
using namespace std;
int isPrime(int number) ;
int main()
{
    // declare variable
    int m, n;
    int prime;

    // take input
    cout << "Enter min range: ";
    cin >> m;
    cout << "Enter max range: ";
    cin >> n;
    
    // display result
    cout << "Prime numbers between " 
         << m << " to " << n 
         << " are: " << endl;
    
    // loop to repeat the process
    for(int i=m; i<=n; i++) 
    {
        // check prime 
        if(isPrime(i) != 0)
        cout << i << " ";
    }
    
    return 0;
}

int isPrime(int number) 
{

  // negative numbers, 0 and 1 are 
  // not a prime number
  if( number <= 1 ) return 0;

  // 2 and 3 are prime numbers
  if( number <= 3 ) return 1;

  // numbers divisible by 2 and 3
  // are not prime number
  if(number%2==0 || number%3==0)
      return 0;

  // logic for remaining numbers
  for(int i=5; i <= sqrt(number); i=i+6) {

      // 6k+1 => number%i
      // 6k-1 => number % (i+2)
      if(number%i == 0 || number%(i+2) == 0) 
          return 0;
  }

  // if all above conditions are not satisfied
  return 1;
}

Output:-

Enter min range: 1
Enter max range: 100
Prime numbers between 1 to 100 are:
2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97

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