A prime number is a number which have only two factors as 1 and the number iteself. For example, 13 is prime number, because it is only divisible by 1 and 13. A non-prime number have more than two factors such as 25 and it is divissible by 1, 5, and 25. 2 is the only even number which is a prime number and 0, 1 is neither prime nor composite. To check a given number we can follow various approaches, as follows:

1. Iterative Approach (1st Method)

Start dividing the number from 1 to iteself and use a counter to count once the reminder is zero. Later on we can use the value of count, if it is 2 then number is prime else non-prime.

#include<stdio.h>

void checkPrime(int num){

int count = 0;

if(num==0 || num==1)

printf("%d is neither prime nor composite",num);

else{

for(int i=1;i<=num;i++){

if(num%i==0){

count++;

}

}

}

if(count==2) printf("%d has %d factors and a prime number",num,count);

else printf("%d has %d factors and a non-prime number",num);

}

int main(){

int num;

printf("Enter Any number::");

scanf("%d",&num);

checkPrime(num);

return 0;

}

2. Iterative Approach (2nd Method)

Another and better approach we can follow, instead of start dividing that number from 1, this process will start from 2 and will continue till the half of that number. In this approach, it is number of iterations will less than the previous approach. Instead of checking a number prime by counting number of factors, we can look for its factors (excluding 1 and itself).

#include<stdio.h>

int checkPrime(int num){

int check = 0;

if(num==0 || num==1){

printf("%d is neither prime nor composite",num);

return 0;

}

else{

for(int i=2;i<=num/2;i++){

if(num%i==0){

check = 1;

break;

}

}

}

if(check==1) printf("%d is a non-prime number",num);

else printf("%d is a prime number",num);

}

int main(){

int num;

printf("Enter Any number::");

scanf("%d",&num);

checkPrime(num);

return 0;

}