Find prime numbers using Scala. Help me to improve

Question

I wrote this code to find the prime numbers less than the given number i in scala.

def findPrime(i : Int) : List[Int] = i match {
    case 2 => List(2)
    case _ => {
    val primeList = findPrime(i-1)
    if(isPrime(i, primeList)) i :: primeList else primeList
    }
}

def isPrime(num : Int, prePrimes : List[Int]) : Boolean = prePrimes.forall(num % _ != 0)    

But, I got a feeling the findPrime function, especially this part:

case _ => {
    val primeList = findPrime(i-1)
    if(isPrime(i, primeList)) i :: primeList else primeList
}

is not quite in the functional style.

I am still learning functional programming. Can anyone please help me improve this code to make it more functional.

Many thanks.

Answer 1

Here's a functional implementation of the Sieve of Eratosthenes, as presented in Odersky's "Functional Programming Principles in Scala" Coursera course :

  // Sieving integral numbers
  def sieve(s: Stream[Int]): Stream[Int] = {
    s.head #:: sieve(s.tail.filter(_ % s.head != 0))
  }

  // All primes as a lazy sequence
  val primes = sieve(Stream.from(2))

  // Dumping the first five primes
  print(primes.take(5).toList) // List(2, 3, 5, 7, 11)