#include <stdio.h>
#include <stdlib.h> // atoll
#include <stdbool.h>
// Function to check primality
bool isPrime(long long n) {
if (n < 2) return false;
if (n % 2 == 0 && n != 2) return false;
for (long long i = 3; i * i <= n; i += 2) {
if (n % i == 0) return false;
}
return true;
}
// Concatenate two integers
long long concat(int a, int b) {
char buffer[64];
snprintf(buffer, sizeof(buffer), "%d%d", a, b);
return atoll(buffer);
}
int main(void) {
int limit = 12; // you can increase this
int primes[64]; // fixed-size array for simplicity
int primeCount = 0;
// Generate primes up to limit
for (int i = 2; i <= limit; i++) {
if (isPrime(i)) {
primes[primeCount++] = i;
}
}
// Check pairs
for (int i = 0; i < primeCount; i++) {
for (int j = 0; j < primeCount; j++) {
if (i == j) continue; // skip same prime if you want
long long num = concat(primes[i], primes[j]);
if (isPrime(num)) {
printf("%d + %d -> %lld is prime\n",
primes[i], primes[j], num);
}
}
}
return 0;
}
/*
run:
2 + 3 -> 23 is prime
2 + 11 -> 211 is prime
3 + 7 -> 37 is prime
3 + 11 -> 311 is prime
5 + 3 -> 53 is prime
7 + 3 -> 73 is prime
11 + 3 -> 113 is prime
*/
