How does one flatten an array of arrays in Scala?

Question

I have the following array:

val input = Array(Array(), Array(22,33), Array(), Array(77), Array(88,99))

It contains empty arrays. I want to get a flattened array without any empty arrays, so the output should be:

Array(22,33,77,88,99)

I tried the flatten function, but it seems to not work with the type of Array[_ <: Int].

Jordan Cutler · Accepted Answer · 2018-02-16 23:08:45Z

13

Another way of writing:

input.flatMap(_.toList)

The empty arrays get converted to Nils and since it's a flatMap the Nils get flattened out

answered Feb 16, 2018 at 23:08

Jordan Cutler

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Comments

Euge · Accepted Answer · 2018-02-16 20:04:55Z

7

Monads are your friends:

for { a <- input; b <- a.toList } yield b

Edit: If you specify the type, flatten works fine

val input: Array[Array[Int]] = Array(Array(), Array(22,33), Array(), Array(77), Array(88,99))
input.flatten

answered Feb 16, 2018 at 20:04

Euge

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And even more concise input.flatMap(_.toList)

Alvaro Carrasco · Accepted Answer · 2018-02-16 19:43:10Z

4

It's inferring Array[_ <: Int] because some of the arrays are empty. Try this:

val input = Array(Array[Int](), Array(22,33), Array[Int](), Array(77), Array(88,99)).flatten

That ensures that the resulting type is Array[Array[Int]] which should be flattenable.

answered Feb 16, 2018 at 19:43

Alvaro Carrasco

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Actually all you need is type ascription: val input:Array[Array[Int]] = ...