Consider the following snippet:
val mapOfArrays = (0 to 10).map{
Array(1, 2)
}
I was expecting the output to be of type List[Array[Int]], but it is not. Any ideas?
2 Answers
Running it in the REPL shows that it actually raises an exception:
scala> val mapOfArrays = (0 to 10).map{
| Array(1, 2)
| }
java.lang.ArrayIndexOutOfBoundsException: 2
at scala.collection.mutable.WrappedArray$ofInt.apply$mcII$sp(WrappedArray.scala:155)
at scala.collection.mutable.WrappedArray$ofInt.apply(WrappedArray.scala:155)
at scala.collection.mutable.WrappedArray$ofInt.apply(WrappedArray.scala:152)
at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:245)
at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:245)
at scala.collection.immutable.Range.foreach(Range.scala:166)
at scala.collection.TraversableLike$class.map(TraversableLike.scala:245)
at scala.collection.AbstractTraversable.map(Traversable.scala:104)
... 37 elided
This is because the code is equivalent to:
val mapOfArrays = (0 to 10).map{
Array(1, 2).apply
}
or the more verbose
val mapOfArrays = (0 to 10).map{ x =>
Array(1, 2).apply(x)
}
which is indeed an IndexedSeq[Int].
The .apply method off Array returns the element at the given index, which throws an ArrayIndexOutOfBoundsException when x (from (0 to 10)) reachs 2.
Modifying the code so that it doesn't throw an exception:
scala> val mapOfArrays = (0 to 3).map{ Array(1, 2, 3, 4, 5) }
mapOfArrays: scala.collection.immutable.IndexedSeq[Int] = Vector(1, 2, 3, 4)
I'm guessing you intended to write this instead:
scala> val mapOfArrays = (0 to 10).map{ x =>
Array(1, 2)
}
mapOfArrays: scala.collection.immutable.IndexedSeq[Array[Int]] = Vector(Array(1, 2), Array(1, 2), Array(1, 2), Array(1, 2), Array(1, 2), Array(1, 2), Array(1, 2), Array(1, 2), Array(1, 2), Array(1, 2), Array(1, 2))
Comments
The map call is expecting a function as a parameter, but you are passing an Array of two values. It will then try to call apply on this array with each element of the range.
I believe this is what you intended.
val mapOfArrays = (0 to 10).map(item => Array(1,2))
1 Comment
Jörg W Mittag
An array is a
Function1[Int, T] (more precisely: a Seq is a PartialFunction[Int, T]) from its indices to its values, which is why the OP's code "works" in the first place.
List.fill(11)(Array(1,2)).