Check whether Given Number is Prime Number or Not

Check whether Given Number is Prime Number or Not” is again very basic problem of programming exercise. Here, we are given a number, entered by user and our task is to check whether entered number is prime number or not.

Prime Number: A number is said to be prime number if it is only divisible by 1 and by itself. 

Examples of Prime numbers are 2,3,5,7,11,…

NOTE: 1 is not a prime number.

Example:

INPUT: 
Number = 7
OUTPUT:
7 is a prime number.
INPUT: 
Number = 9
OUTPUT:
9 is not a prime number.

To check whether a number is prime number or not, we will check whether entered number is divisible from numbers 2 to (number-1). If any number from that range is divisible by entered number, then it is not a prime number. Else, it is a prime number.

C++ Program to check whether a number is prime number or not is as follows:

#include<bits/stdc++.h>  
using namespace std;  
int main()  
{  
    int num;  
      
    //Scan the number  
    cout<<"Enter the number:";  
    cin>>num;  
      
    bool res = true;  
    /*  
    We will check whether entered number is  
    divisible by numbers from 2 to (num-1). 
    If entered number is divisible by any number 
    from 2 to (num-1), then it is not prime number. 
    Else, it is prime number. 
    */   
    for(int i = 2 ; i < num ; i++)  
    {  
        if(num % i == 0)  
        {  
            res = false;  
            break;  
        }  
    }  
      
    if(res == true && num!=1) /* 1 is not a prime number */  
    cout<<num<<" is a prime number";  
    else  
    cout<<num<<" is not a prime number";  
}  

OUTPUT:
Enter the number: 11
11 is a prime number

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