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 isPrimeNumber(int); 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 isPrime = isPrimeNumber(n); if(isPrime == true) cout<<n<<" "; } return 0; } // Function that checks whether n is prime or not int isPrimeNumber(int n) { 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 isPrimeNumber(int); int main() { bool isPrime; for(int n = 2; n < 100; n++) { // isPrime will be true for prime numbers isPrime = isPrimeNumber(n); if(isPrime == true) cout<<n<<" "; } return 0; } // Function that checks whether n is prime or not int isPrimeNumber(int n) { bool isPrime = true; for(int i = 2; i <= n/2; i++) { if (n%i == 0) { isPrime = false; break; } } return isPrime; }

**Output:**

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