CODEWITHSUNDEEP
scala
S0rin
S0rin

Reputation: 1293

Scala lazy elements in iterator

Does anyone know how to create a lazy iterator in scala?

For example, I want to iterate through instantiating each element. After passing, I want the instance to die / be removed from memory.

If I declare an iterator like so:

val xs = Iterator(
 (0 to 10000).toArray,
 (0 to 10).toArray,
 (0 to 10000000000).toArray)

It creates the arrays when xs is declared. This can be proven like so:

def f(name: String) = {
  val x =  (0 to 10000).toArray
  println("f: " + name) 
  x
}

val xs = Iterator(f("1"),f("2"),f("3"))

which prints:

scala> val xs = Iterator(f("1"),f("2"),f("3"))
f: 1
f: 2
f: 3
xs: Iterator[Array[Int]] = non-empty iterator

Anyone have any ideas?

Streams are not suitable because elements remain in memory.

Note: I am using an Array as an example, but I want it to work with any type.

Upvotes: 1

Views: 2290

Answers (2)

incrop
incrop

Reputation: 2738

Use one of Iterator factory methods which accepts call-by-name parameter.

For your first example you can do one of this:

val xs1 = Iterator.fill(3)((0 to 10000).toArray)
val xs2 = Iterator.tabulate(3)(_ => (0 to 10000).toArray)
val xs3 = Iterator.continually((0 to 10000).toArray).take(3)

Arrays won't be allocated until you need them.

In case you need different expressions for each element, you can create separate iterators and concatenate them:

val iter = Iterator.fill(1)(f("1")) ++ 
           Iterator.fill(1)(f("2")) ++ 
           Iterator.fill(1)(f("3"))

Upvotes: 1

thSoft
thSoft

Reputation: 22670

Scala collections have a view method which produces a lazy equivalent of the collection. So instead of (0 to 10000).toArray, use (0 to 10000).view. This way, there will be no array created in the memory. See also https://stackoverflow.com/a/6996166/90874, https://stackoverflow.com/a/4799832/90874, https://stackoverflow.com/a/4511365/90874 etc.

Upvotes: 2

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