Lazy collections

All these combinators chaining are lovely, but there is a problem. For very large input data structures, this ends up creating intermediate copies.

Let's look at this aspect closely; you'll find that the solution is pretty revealing. Here is the Java code, just to set the stage:

public class LargeLists {
  public static void main(final String[] args) {
    final int take = 5;
    for (int i = 1000, n = 0; i < 1000000 && n < take; ++i) {
      final int j = i + 1;
      final int k = j * 2;
      if (k % 4 != 0) {
        System.out.println(k);
        ++n;
      }
    }
  }
}

Here, we iterate a range of numbers from 1000 to 1000000. And for each number, we add 1 and then multiply the result by 2. If the result is divisible by 4, we discard it. Otherwise, we print the first five numbers from the result.

For example:

scala> val list = (1000 to 1000000).toList
scala> list.map(_ + 1).map(_ * 2).filter(_ % 4 != 0).take(5)
res0: List[Int] = List(2002, 2006, 2010...