###### C++ Decision Making

C++ Program to Display Prime Numbers Between Two Intervals

# C++ Program to Display Prime Numbers Between Two Intervals

In this example, we will learn a Program to check whether an integer (entered by the user) is a prime number or not using for loop and if...else statement.

To understand this example, you should have the knowledge of the following C++ programming topics:

## Example 1: Program to Display Prime Numbers Between Two Intervals

``````#include <iostream>
using namespace std;

int main() {
int low, high, i;
bool isPrime = true;

cout << "Enter two numbers (intervals): ";
cin >> low >> high;

cout << "\nPrime numbers between " << low << " and " << high << " are: " << endl;

while (low < high) {
isPrime = true;
if (low == 0 || low == 1) {
isPrime = false;
}
else {
for (i = 2; i <= low / 2; ++i) {
if (low % i == 0) {
isPrime = false;
break;
}
}
}

if (isPrime)
cout << low << " ";

++low;
}

return 0;
}``````

Output 1

```Enter two numbers (intervals): 0 20

Prime numbers between 0 and 20 are:
2 3 5 7 11 13 17 19```
##### Working

In this program, the `while` loop is iterated `(high - low - 1)` times.

In each iteration, whether low is a prime number or not is checked and the value of low is incremented by 1 until low is equal to high.

If the user enters larger number first, this program doesn't work as intended. You can solve this issue by swapping the numbers if the user enters larger number first.

## Example 2: Program to Display Prime Numbers When Larger Number is Entered first

``````#include <iostream>
using namespace std;

int main()
{
int low, high, temp, i;
bool isPrime;

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 << "\nPrime numbers between " << low << " and " << high << " are:" << endl;

while (low < high) {
isPrime = true;
if (low == 0 || low == 1) {
isPrime = false;
}
else {
for (i = 2; i <= low / 2; ++i) {
if (low % i == 0) {
isPrime = false;
break;
}
}
}

if (isPrime)
cout << low << " ";

++low;
}
return 0;
}``````

Output 1

```Enter two numbers (intervals): 20 0

Prime numbers between 0 and 20 are:
2 3 5 7 11 13 17 19```

Visit this page to learn how you can display all prime numbers between two intervals by using user-defined function.

## Next Example

We hope that this Example helped you develop better understanding of the concept of "Display Prime Numbers Between Two Intervals" in C++.

Keep Learning : )

In the next Example, we will learn about C++ `Check Whether a number is Armstrong or not`.