What's the meaning of _*

Question

val numbers = scala.collection.parallel.mutable.ParTrieMap(
    (1 until 100) zip (1 until 100): _*
  ) map { case (k, v) => (k.toDouble, v.toDouble) }

For the above code I know that (1 until 100) zip (1 until 100) will return something like a Map, but what's the meaning of _* following it?

here check this out: http://stackoverflow.com/questions/7938585/what-does-param-mean-in-scala — Gaurav, Apr 27 '13 at 13:33
you can search SO for character n-grams: http://stackoverflow.com/search?q=scala+%22_%2A%22 — Gene T, Apr 27 '13 at 22:21

Vikas Pandya · Answer 1 · 2013-04-27T14:26:42.930

Programming in Scala Section 8.8 Repeated parameters has a quick explanation with an example: _* notation tells compiler to pass each element as its own argument to ParTrieMap

from section 8.8 : val arr = Array("What's", "up", "doc?") when you call

echo(arr: _*)

it will print

What's
up
doc?

arr: _* is equivalent to type declaration that tells the compiler to treat "arr" as repeated parameter e.g. vararg

It's explained well on this post as well.

score 3 · Accepted Answer · answered Apr 27 '13 at 14:11

By calling zip method on Range, you are making IndexedSeq structure with of type: IndexedSeq[(Int, Int)]. To pass your (Int, Int) tuples into ParTrieMap you have to make a type ascription( your : _* annotation), cause you can't pass Seq to ParTrieMap (there is no method with apply(seq: Seq[(A, B)]) signature defined in ParTrieMap companion object, but there is a apply(a: (A, B)*) method). Basically you extract all tuples from Seq and passing them into Map.

What's the meaning of _*

2 Answers2