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)
           


        

Answer 1

As schmmd mentions, you want it to be tail recursive, and you also want it to be lazy. Fortunately there is a perfect data-structure for this: Stream.

This is a very efficient prime calculator implemented as a Stream, with a few optimisations:

object Prime {
  def is(i: Long): Boolean =
    if (i == 2) true
    else if ((i & 1) == 0) false // efficient div by 2
    else prime(i)

  def primes: Stream[Long] = 2 #:: prime3

  private val prime3: Stream[Long] = {
    @annotation.tailrec
    def nextPrime(i: Long): Long =
      if (prime(i)) i else nextPrime(i + 2) // tail

    def next(i: Long): Stream[Long] =
      i #:: next(nextPrime(i + 2))

    3 #:: next(5)

  }

    // assumes not even, check evenness before calling - perf note: must pass partially applied >= method
  def prime(i: Long): Boolean =
    prime3 takeWhile (math.sqrt(i).>= _) forall { i % _ != 0 }
}

Prime.is is the prime check predicate, and Prime.primes returns a Stream of all prime numbers. prime3 is where the Stream is computed, using the prime predicate to check for all prime divisors less than the square root of i.