# C Program to find Prime Numbers

Last updated on September 23, 2020

## What are Prime numbers? #

Numbers which are either divisible by 1 or itself are known as Prime Numbers. For example, 11 is a prime number because it is only divisible by 1 and 11. However, number 6 is not prime since it is divisible by 2 and 3. The following is a C program to print the prime numbers between 2 and 50.

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 /************************************************************** * Program to find the Prime numbers between 2 and 50 ***************************************************************/ #include int main() { int is_prime = 1, n; for(int n = 2; n < 50; n++) { is_prime = 1; for(int i = 2; i < n; i++) { if(n % i == 0) { is_prime = 0; break; } } if(is_prime) { printf("%d is prime\n", n); } } return 0; } 

Expected Output:

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 2 is prime 3 is prime 5 is prime 7 is prime 11 is prime 13 is prime 17 is prime 19 is prime 23 is prime 29 is prime 31 is prime 37 is prime 41 is prime 43 is prime 47 is prime 

## How it works: #

To check for a prime number we successively divide a number n from 2 to n-1. If n is not divisible by any number between this range then it is a prime number.

Example 1: Is 5 a prime number?

Question Statement Result
Is 5 is divisible by 2 5 % 2 == 0 0 (false)
Is 5 is divisible by 3 5 % 3 == 0 0 (false)
Is 5 is divisible by 4 5 % 4 == 0 0 (false)

Hence, 5 is a prime number.

Example 2: Is 6 a prime number

Question Statement Result
Is 6 is divisible by 2 6 % 2 == 0 1 (true)

Number 6 is divisible by 2. Hence, it is not a prime number.

## Check Prime Numbers using a user-defined function #

The following is a C program to check prime numbers using a user-defined function.

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 /************************************************************** * Program to check whether the number entered by the user * is prime or not ***************************************************************/ #include int check_prime_number(int); int main() { int n; printf("Enter a number: "); scanf("%d", &n); if(check_prime_number(n)) { printf("%d is prime", n); } else { printf("%d is not prime", n); } return 0; } int check_prime_number(int n) { int is_prime = 1; if(n == 1) { return 0; } for(int i = 2; i < n; i++) { if(n % i == 0) { is_prime = 0; break; } } return is_prime; } 

Expected Output: 1st run:

 1 2 Enter a number: 11 11 is prime 

2nd run:

 1 2 Enter a number: 45 45 is not prime