Prime Numbers in C Programming – Complete Guide
What is a Prime Number?
A prime number is a natural number greater than 1 that has only two factors: 1 and itself.
Examples → 2, 3, 5, 7, 11, 13 …
Not Prime (Composite) → 4 (factors: 1,2,4), 6 (factors: 1,2,3,6)
Logic to Check Prime Number in C
Take a number
nas input.If
n <= 1, it’s not prime.Check divisibility from
2ton/2(or √n for efficiency).If divisible, then it’s not prime.
Otherwise, it’s prime.
#include <stdio.h>
int main() {
int n, i, flag = 0;
printf("Enter a number: ");
scanf("%d", &n);
if (n <= 1) {
printf("%d is not a prime number.\n", n);
return 0;
}
for (i = 2; i <= n/2; i++) {
if (n % i == 0) {
flag = 1; // Not prime
break;
}
}
if (flag == 0)
printf("%d is a prime number.\n", n);
else
printf("%d is not a prime number.\n", n);
return 0;
}
Example Run
Optimized Code (Check till √n only)
#include <stdio.h>
#include <math.h>
int main() {
int n, i, flag = 0;
printf("Enter a number: ");
scanf("%d", &n);
if (n <= 1) {
printf("%d is not a prime number.\n", n);
return 0;
}
for (i = 2; i <= sqrt(n); i++) {
if (n % i == 0) {
flag = 1;
break;
}
}
if (flag == 0)
printf("%d is a prime number.\n", n);
else
printf("%d is not a prime number.\n", n);
return 0;
}
Checking up to √n reduces execution time for large numbers.
Program to Print Prime Numbers in a Range
#include <stdio.h>
int main() {
int start, end, i, j, flag;
printf("Enter start and end values: ");
scanf("%d %d", &start, &end);
printf("Prime numbers between %d and %d are: ", start, end);
for (i = start; i <= end; i++) {
if (i <= 1)
continue;
flag = 0;
for (j = 2; j <= i/2; j++) {
if (i % j == 0) {
flag = 1;
break;
}
}
if (flag == 0)
printf("%d ", i);
}
return 0;
}
Example Run
Key Points
First prime number is 2 (smallest and only even prime).
Prime checking is important in cryptography, hashing, and algorithms.
Optimization is important when working with large numbers.
