Print Primes Using Nested Loops
#include <stdio.h>
int main() {
int N, i, j, isPrime;
printf("Enter the upper limit: ");
scanf("%d", &N);
printf("Prime numbers between 1 and %d are: ", N);
for (i = 2; i <= N; i++) {
isPrime = 1;
for (j = 2; j < i; j++) {
if (i % j == 0) {
isPrime = 0;
break;
}
}
if (isPrime) {
printf("%d ", i);
}
}
printf("\n");
return 0;
}Enter the upper limit: 10
Prime numbers between 1 and 10 are: 2 3 5 7
This straightforward approach checks each number from 2 up to the entered limit by dividing it by every smaller integer. If none divide evenly, the number is printed as prime.
Print Primes Using Optimized Check
#include <stdio.h>
#include <math.h>
int isPrime(int num) {
int i;
if (num <= 1) return 0;
if (num == 2) return 1;
if (num % 2 == 0) return 0;
for (i = 3; i <= sqrt(num); i += 2) {
if (num % i == 0) return 0;
}
return 1;
}
int main() {
int N, i;
printf("Enter the upper limit: ");
scanf("%d", &N);
printf("Prime numbers between 1 and %d are: ", N);
for (i = 2; i <= N; i++) {
if (isPrime(i)) {
printf("%d ", i);
}
}
printf("\n");
return 0;
}Enter the upper limit: 20
Prime numbers between 1 and 20 are: 2 3 5 7 11 13 17 19
An optimized approach uses a helper function that checks divisibility only up to the square root of a number and skips even numbers after 2. Each number from 2 to N is tested with this function before being printed.
