# C Program to Check Whether a Number is Prime or Not

• Write a C program to check whether a number is prime or not
• Algorithm check prime number.

A Prime number is a natural number greater than 1 that is only divisible by either 1 or itself. All numbers other than prime numbers are known as composite numbers. Any non-prime number can be expressed as a factor of prime numbers. There are infinitely many prime numbers, here is the list of first few prime numbers
2 3 5 7 11 13 17 19 23 29 31 37….

### C program to check a number is prime or not

In this program, we will use brute force approach by testing whether n is a multiple of any integer between 2 and N/2. This is the most basic method of checking the primality of a given integer n is called trial division. /*
* C program to check whether a number is prime number or not
*/

#include<stdio.h>
#include<conio.h>

int main() {
int num, i, isPrime=0;
printf("Enter a positive number\n");
scanf("%d",&num);
/* Check whether num is divisible by any number between 2 to (num/2)
*/
for(i = 2; i <=(num/2); ++i) {
if(num%i==0) {
isPrime=1;
break;
}
}

if(isPrime==0)
printf("%d is a Prime Number",num);
else
printf("%d is NOT a Prime Number",num);

getch();
return 0;
}

Program Output

Enter a positive number
23
23 is a Prime Number
Enter a positive number
30
30 is NOT a Prime Number

Below program is the optimized version of above program in which we only test with numbers between 2 to sqrt(N). Testing with numbers till N/2 is not required. /*
* C program to check whether a number is prime number or not
*/

#include<stdio.h>
#include<conio.h>
#include<math.h>

int main() {
int num, i, isPrime=0;
printf("Enter a positive number\n");
scanf("%d",&num);
/* Check whether num is divisible by any number between 2 to sqrt(num)
*/
for(i = 2; i <= (int)sqrt(num); ++i) {
if(num%i==0) {
isPrime=1;
break;
}
}

if(isPrime==0)
printf("%d is a Prime Number",num);
else
printf("%d is NOT a Prime Number",num);

getch();
return 0;
}

Program Output

Enter a positive number
71
71 is a Prime Number

Enter a positive number
45
45 is NOT a Prime Number


### C program to find all prime numbers between 1 to N /*
* C Program to print all Prime numbers between 1 to N
*/
#include<stdio.h>
#include<conio.h>

int main(){

int num, i, isPrime, n;
printf("To print all prime numbers between 1 to N\n");
printf("Enter value of N\n");
scanf("%d",&num);

for(n = 1; n <= num; n++){
isPrime = 0;
for(i=2;i<=n/2;i++){
if(n%i==0){
isPrime = 1;
break;
}
}

if(isPrime==0 && n!= 1)
printf("%d ",n);
}
getch();
return 0;
}

Program Output

To print all prime numbers between 1 to N
Enter value of N
100
2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97