// Additive primes: primes whose sum of digits is also prime
fn is_prime(n: u32) -> bool {
if n < 2 {
return false;
}
if n % 2 == 0 {
return n == 2;
}
if n % 3 == 0 {
return n == 3;
}
let mut i = 5;
while i * i <= n {
if n % i == 0 {
return false;
}
i += 2;
if n % i == 0 {
return false;
}
i += 4;
}
true
}
// Compute the sum of digits of a number
fn sum_digits(mut n: u32) -> u32 {
let mut sum = 0;
while n > 0 {
sum += n % 10;
n /= 10;
}
sum
}
// Check if a number is an additive prime
fn is_additive_prime(n: u32) -> bool {
is_prime(n) && is_prime(sum_digits(n))
}
fn main() {
const TOP: u32 = 500;
let mut count = 0;
for n in 1..TOP {
if is_additive_prime(n) {
print!("{:3}", n);
count += 1;
if count % 10 == 0 {
println!();
} else {
print!(" ");
}
}
}
println!("\n");
println!("Total additive primes = {}", count);
}
/*
run:
2 3 5 7 11 23 29 41 43 47
61 67 83 89 101 113 131 137 139 151
157 173 179 191 193 197 199 223 227 229
241 263 269 281 283 311 313 317 331 337
353 359 373 379 397 401 409 421 443 449
461 463 467 487
Total additive primes = 54
*/
