A prime number is a number which can be divided by 1 and itself i.e. a number which can not be divided by any other number other than 1 or itself is a prime number.

Example: 2,3,5,7,11….so on are the prime number as we can see they only have two divisors, 1 and the number itself.

### Algorithm

- Start
- Read a “num” value to check prime or not.
- set i=1,div=0.
- if i<=num if true go to step 5, else go to step 8.
- Check the condition num%i==0 if true then evaluate step 6, else go to step 7.
- set div=div+1
- i=i+1, go to step 4
- check div, if div==2 display prime, else display not prime
- Stop

### Working

- This program takes a number from user and checks whether the number entered by the user is perfectly divisible by i or not.
- If true then value of div is incremented else no change the loop continues with the increment in i until i is equal to the entered number.
- After the loop the value of div is checked and based on that we get whether the number is prime or not.

### C++ Code

```
//C++ Program
//Check Prime or Not
#include<iostream>
using namespace std;
int main()
{
int i,num,div=0; //initializing variables
cout<<"Enter number:";
cin>>num; //user input
for(i=1;i<=num;i++) //checking for number of divisor
{
if(num%i==0)
{
div++;
}
}
if(div==2) //no divisors other than 1 and itself
{
cout<<num<<" is a prime number";
}
else
{
cout<<num<<" is not a prime number";
}
return 0;
```

### Output

Enter number:31

31 is a prime number

## Program to Check Prime Number or not in c.

```
#include <stdio.h>
int main() {
int n, i, flag = 0;
printf("Enter a positive integer: ");
scanf("%d", &n);
for (i = 2; i <= n / 2; ++i) {
// condition for non-prime
if (n % i == 0) {
flag = 1;
break;
}
}
if (n == 1) {
printf("1 is neither prime nor composite.");
}
else {
if (flag == 0)
printf("%d is a prime number.", n);
else
printf("%d is not a prime number.", n);
}
return 0;
}
```

### Output

Enter a positive integer: 29 29 is a prime number.