C++ Program:

Example:

#include <iostream>
using namespace std;

int prime(int n){
  if(n<2)return 0;
  for(int i=2; i<=n/2; i++) {
    if(n%i==0) return 0;
  }
  return 1;
}

int main() {
  int num;
  cout<<"Enter any +ve number : ";
  cin>>num;
  cout<<"Your prime numbers between 1 & "<<n<<" are : \n";
  for(int i=2; i<num; i++)
    if(prime(i))cout<<i<<"  ";
  return 0;
}

Output:

  Enter any +ve number : 20
  Your prime numbers between 1 & 20 are :
  2 3 5 7 11 13 17 19

You should know first :

Prime numbers: Numbers greater than 1 & have only 2 factors ( 1 and itself )

Main Logic to check prime :

int prime(int n){
  for(int i=2;i<=n/2;i++) {
    if(n%i==0) return 0;
  }
  return 1;
}

We are running the loop n/2 Since, any number can have only factors under its half, except itself

To print all primes :

for(int i=2; i<num; i++) {
  //check every number for prime [ if(prime(i)) ], if prime then print the number
}