Today we will learn C++ program for prime number between 1 to n. So before starting you should have a little bit of knowledge about prime numbers.

### What are Prime Numbers?

Any natural number which can be divisible by itself or the number which has only two factors i.e 1 and the number itself are called prime numbers.

For example 2,3,5,7,11 and so on…

Note: The number 2 is only even prime number because most of the numbers are divisible by 2.

There are different methods to print prime numbers from 1 to n. We will see it one by one.

## 1. C++ Program for Prime Number Between 1 to n

In this program, we will ask the user to enter the starting and the ending number so that the compiler will fetch and prints all the prime numbers between those intervals.

```//Learnprogramo - programming made simple
#include <iostream>
using namespace std;
int main()
{
int low, high, i, flag;
cout << "Enter two numbers(intervals): ";
cin >> low >> high;
cout << "Prime numbers between " << low << " and " << high << " are: ";
while (low < high)
{
flag = 0;
for(i = 2; i <= low/2; ++i)
{
if(low % i == 0)
{
flag = 1;
break;
}
}
if (flag == 0)
cout << low << " ";
++low;
}
return 0;
}```

Output:

In the above program, the lower interval is assigned to variable low and the highest interval is assigned to the high. So in the output, we have given low as 1 and the high as 10 and the compiler have printed all the prime numbers between 1 and 10.

## 2. C++ Program for Prime Number Between 1 to n Using Functions

In this program, we will declare a user-defined function to print all the prime numbers between 1 and n.

```//Learnprogramo - programming made simple
#include <iostream>
using namespace std;
int main() {
bool isPrime;
int count;
cout<<"Enter the value of n:";
cin>>count;
for(int n = 2; n < count; n++)
{
// isPrime will be true for prime numbers
if(isPrime == true)
cout<<n<<" ";
}
return 0;
}
// Function that checks whether n is prime or not
bool isPrime = true;
for(int i = 2; i <= n/2; i++) {
if (n%i == 0)
{
isPrime = false;
break;
}
}
return isPrime;
}```

Output:

After compiling the above program we have given the input n as 15 so the compiler has printed all the prime numbers up to 15.

## 3. Print Prime Numbers When Greatest Number is Entered first

In this program, we will first give inputs lowest and greatest number between which we want to print all prime numbers.

```//Learnprogramo - programming made simple
#include <iostream>
using namespace std;
int main()
{
int low, high, flag, temp;
cout << "Enter two numbers(intevals): ";
cin >> low >> high;
//swapping numbers if low is greater than high
if (low > high) {
temp = low;
low = high;
high = temp;
}
cout << "Prime numbers between " << low << " and " << high << " are: ";
while (low < high)
{
flag = 0;
for(int i = 2; i <= low/2; ++i)
{
if(low % i == 0)
{
flag = 1;
break;
}
}
if (flag == 0)
cout << low << " ";
++low;
}
return 0;
}```

Output:

## 4. Printing all Prime Numbers Between 1 and 100

In this program we will print all the prime numbers between 1 and 100.

```//Learnprogramo - programming made simple
#include <iostream>
using namespace std;
int main()
{
bool isPrime;
for(int n = 2; n < 100; n++) {
// isPrime will be true for prime numbers
if(isPrime == true)
cout<<n<<" ";
}
return 0;
}
// Function that checks whether n is prime or not