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scalacollections

Does the grouped function from the List API return a list of lists on the same order?

I have a function that receives a list of objects. I want to partition that list in groups of fixed size, do something with the groups and return a list of objects in the same order that I received them.

I'm using the function grouped, from the List API. Something like this:

val listOfLists = inputList.grouped(FIXED_SIZE).toList

The scala-lang site is down, so what I see on the documentation of the method is:

  /** Partitions elements in fixed size ${coll}s.
   *  @see [[scala.collection.Iterator]], method `grouped`
   *
   *  @param size the number of elements per group
   *  @return An iterator producing ${coll}s of size `size`, except the
   *          last will be less than size `size` if the elements don't divide evenly.
   */
  def grouped(size: Int): Iterator[C] =
    iterator.grouped(size).map(fromSpecific)

I don't see anything related to the order there, so my question is, is this function guaranteed to always return the list of lists in order? E.g. for a batch size of 2:

List(1, 2, 3, 4, 5) returns List(List(1, 2), List(3, 4), List(5))

Any help would be greatly appreciated.

Solution

  • The simple answer is yes, this is guaranteed. The documentation may not be 100% clear (though it is arguably intrinsic to the definition of an Iterator for an ordered collection), but this ordering is assumed by a large amount of code so this behaviour is never going to change.

    Note that the grouped method is a degenerate case of the sliding method where the step and size are the same.

    def grouped[T](list: List[T], size: Int) = list.sliding(size, size)

    It makes even less sense for sliding to deliver results out of order, and in practice they are implemented using the same underlying mechanism.