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

In this C program, we will check whether a number is prime number or not. 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.

```#include<stdio.h>

int main() {
int num, i, isPrime=0;
printf("Enter a positive number\n");
scanf("%d",&num);
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);

return 0;
}
```
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.

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

int main() {
int num, i, isPrime=0;
printf("Enter a positive number\n");
scanf("%d",&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);

return 0;
}
```
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

```#include<stdio.h>

int main(){

int num, i, isPrime, 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);
}
return 0;
}
```
Output
```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
```

Related Topics
 C program to print all prime numbers between 1 to N using for loop C program to print all prime factors of a number C program to find sum of prime numbers between 1 to N C program to check whether a number is magic number C program to check a number is palindrome or not C program to check armstrong number C program to check if two strings are anagram C program to check string is palindrome C Program for matrix multiplication List of all C programs