CODEWITHSUNDEEP
scala
flavian
flavian

Reputation: 28511

Scala View + Stream combo causing OutOfMemory Error. How do I replace it with a View?

I was looking at solving a very simple problem, Eratosthenes sieve, using idiomatic Scala, for learning purposes.

I've learned a Stream caches, so it is not so performant when determining the nth element because it's an O(n) complexity access with memoisation of data, therefore not suitable for this situation.

    def primes(nums: Stream[Int]): Stream[Int] = {
        Stream.cons(nums.head,
            primes((nums tail) filter (x => x % nums.head != 0)))
    }

    def ints(n: Int): Stream[Int] = {
        Stream.cons(n, ints(n + 1))

    };
    def nthPrime(n: Int): Int = {
        val prim = primes(ints(2)).view take n toList;
        return prim(n - 1);
    };

The Integer stream is the problematic one. While the prime number filtering is done, JVM runs OutOfMemory. What is the correct way to achieve the same functionality without using Streams?

Basically take a view of primes from a view of ints and display the last element, without memoisation?

Upvotes: 0

Views: 104

Answers (1)

EECOLOR
EECOLOR

Reputation: 11244

I have had similar cases where a stream was a good idea, but I did not need to store it's values. In order to consume the stream without storing it's values I created (what I called) ThrowAwayIterator:

class ThrowAwayIterator[T](var stream: Stream[T]) extends Iterator[T] {
  def hasNext: Boolean = stream.nonEmpty
  def next(): T = {
    val next = stream.head
    stream = stream.tail
    next
  }
}

Make sure that you do not store a reference to the instance of stream that is passed in.

Upvotes: 1

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