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
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