// Additive primes: primes whose sum of digits is also prime
object AdditivePrimes {
// Check if a number is prime
def isPrime(n: Int): Boolean = {
if (n < 2) return false
if (n % 2 == 0) return n == 2
if (n % 3 == 0) return n == 3
var 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
def sumDigits(n: Int): Int = {
var x = n
var sum = 0
while (x > 0) {
sum += x % 10
x /= 10
}
sum
}
// Check if a number is an additive prime
def isAdditivePrime(n: Int): Boolean =
isPrime(n) && isPrime(sumDigits(n))
def main(args: Array[String]): Unit = {
val Top = 500
var count = 0
var line = new StringBuilder
for (n <- 1 until Top) {
if (isAdditivePrime(n)) {
line.append(f"$n%3d ")
count += 1
if (count % 10 == 0) {
println(line.toString.trim)
line = new StringBuilder
}
}
}
if (line.nonEmpty) println(line.toString.trim)
println()
println(s"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
*/
