C++ Program To Check Whether a Number is Prime or not
Given a positive integer N. The task is to write a C++ program to check if the number is prime or not.Â
Definition:Â
A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. The first few prime numbers are {2, 3, 5, 7, 11, ….}
The idea to solve this problem is to iterate through all the numbers starting from 2 to sqrt(N) using a for loop and for every number check if it divides N. If we find any number that divides, we return false. If we did not find any number between 2 and sqrt(N) which divides N then it means that N is prime and we will return True.Â
Why did we choose sqrt(N)?Â
The reason is that the smallest and greater than one factor of a number cannot be more than the sqrt of N. And we stop as soon as we find a factor. For example, if N is 49, the smallest factor is 7. For 15, smallest factor is 3.
Below is the C++ program to check if a number is prime:Â
C++
#include <iostream>
#include <math.h>
using namespace std;
int main()
{
int n, i, flag = 1;
cout << "Enter a number: " ;
cin >> n;
for (i = 2; i <= sqrt (n); i++) {
if (n % i == 0) {
flag = 0;
break ;
}
}
if (n <= 1)
flag = 0;
if (flag == 1) {
cout << n << " is a prime number" ;
}
else {
cout << n << " is not a prime number" ;
}
return 0;
}
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Output:Â
Enter a number: 11
11 is a prime number
Time Complexity: O(n1/2)
Auxiliary Space: O(1)
Method 2 : Optimized Approach using Wilsons theorem with O(N) complexity
If  ((n-1)! + 1) % n == 0  then  n is prime and else it is not prime
Example :Â
Input : 11
Output : 11 is a prime number
Explanation : (11-1)! + 1 = 3628801
            3628801 % 11 = 0Â
       Â
Input : 8
Output : 8 is not prime number
Explanation : (8-1)! + 1 = 5041
            5041 % 8 = 1Â
Implementation of the above approach :Â
C++
#include <iostream>
using namespace std;
int main()
{
int n = 11;
int m = n - 1;
int factm = 1;
for ( int i = 1; i <= m; i++) {
factm *= i;
}
int factn = factm + 1;
if (factn % n == 0) {
cout << n << " is prime number" ;
}
else {
cout << n << " is not prime" ;
}
return 0;
}
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Output
11 is prime number
Time Complexity: O(N) complexity only for calculating factorial  of (n-1) checking it is 0 or 1 using % takes constant time
Auxiliary Space: O(1) Â Â Â Â Â Â Â Â Â Â
Last Updated :
17 Oct, 2022
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