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# C Program to Find Prime Number #### In this Program, you’ll learn how to find Prime number entered by User.

What is the prime number?
To understand this example, you should have knowledge of C programming :
A number is considered as the prime number when it satisfies the below conditions.

• It should be the whole number
• It should be greater than 1
• It should have only 2 factors. They are 1 and the number itself.

Example for prime numbers: 2, 3, 5, 7, 11, 13, 17, 19, 23 etc.

Why 4, 6, 8, 9, 10, 12, 14, 15, 16 etc are not prime numbers?Advertisement

Because the number 4 can be factored as 2*2 and 1*4. As per the rule of a prime number, there should be 2 factors only. They are 1 and the number itself. But, number 4 has 2*2 also. Like this, all remaining numbers 6, 8, 9, 10, 12, 14, 15, 16 have factors other than 1 and the number itself. So, these are not called as prime numbers.

#### Program to find Prime Numbers Entered by User

``````#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 nonprime number
if(n%i==0)
{
flag=1;
break;
}
}

if (flag==0)
printf("%d is a prime number.",n);
else
printf("%d is not a prime number.",n);

return 0;
}``````

Output:

``````Please enter a number: 13
Entered number is 13 and it is a prime number.``````