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In this example we show how to tell if a number is a prime number or not in C++
Lets first talk about what a prime number is
A prime number (or a prime) is a natural number greater than 1 that is not a product of two smaller natural numbers.
A natural number greater than 1 that is not prime is called a composite number. For example, 5 is prime because the only ways of writing it as a product, 1 × 5 or 5 × 1, involve 5 itself.
However, 4 is composite because it is a product (2 × 2) in which both numbers are smaller than 4.
Example
#include <iostream>
#include <math.h>
using namespace std;
//Returns true if the given number is a Prime number
bool isPrime(int n)
{
if (n == 1)
return false; // as 1 is not a prime number
for (int i = 2; i <= sqrt(n); i++)
{
if (n % i == 0)
{
return false;
}
}
return true;
}
int main()
{
cout << "Is a Prime or not.\n";
//variable declarations
int number;
bool prime = false;
//taking input from the user
cout << "Enter a positive integer other than 1 : ";
cin >> number;
//Calling a method that returns true if the number is Prime
prime = isPrime(number);
if (prime)
{
cout << "\n\nThe number " << number << " is a Prime number.";
}
else
{
cout << "\n\nThe number " << number << " is not a Prime number.";
}
cout << "\n";
return 0;
}
When you run this you will see something like this
Is a Prime or not. Enter a positive integer other than 1 : 9 The number 9 is not a Prime number. -------------------------------- Process exited after 4.384 seconds with return value 0 Press any key to continue . . .