What is the correct way to get a subarray in Scala?

Question

I am trying to get a subarray in scala, and I am a little confused on what the proper way of doing it is. What I would like the most would be something like how you can do it in python:

x = [3, 2, 1]
x[0:2]

but I am fairly certain you cannot do this.

The most obvious way to do it would be to use the Java Arrays util library.

import java.util.Arrays
val start = Array(1, 2, 3)
Arrays.copyOfRange(start, 0, 2)

But it always makes me feel a little dirty to use Java libraries in Scala. The most "scalaic" way I found to do it would be

def main(args: List[String]) {
    val start = Array(1, 2, 3)
    arrayCopy(start, 0, 2)
}
def arrayCopy[A](arr: Array[A], start: Int, end: Int)(implicit manifest: Manifest[A]): Array[A] = {
    val ret = new Array(end - start)
    Array.copy(arr, start, ret, 0, end - start)
    ret
}

but is there a better way?

score 146 · Accepted Answer · answered May 31 '12 at 09:36

146

You can call the slice method:

scala> Array("foo", "hoo", "goo", "ioo", "joo").slice(1, 4)
res6: Array[java.lang.String] = Array(hoo, goo, ioo)

It works like in python.

answered May 31 '12 at 09:36

paradigmatic

40,153
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147

Yes you're right. I was referring to the bound conventions. The lower bound is included and the upper is excluded. There is no trailing `:` neither. – paradigmatic Jun 01 '12 at 04:53
@nnythm The link is head. – Amir Shabani Sep 08 '20 at 16:00
Looks like I can't edit a comment that stale, but an alive link is: https://www.scala-lang.org/api/2.12.8/scala/collection/mutable/ArrayOps.html – nnythm Oct 23 '20 at 21:46
@nnythm You can however _delete_ the older comment ;) – WestCoastProjects Mar 08 '21 at 19:53

KeyMaker00 · Answer 2 · 2018-08-11T08:17:59.960

Imagine you have an array with elements from a to f

scala> val array = ('a' to 'f').toArray // Array('a','b','c','d','e','f')

Then you can extract a sub-array from it in different ways:

Dropping the first n first elements with drop(n: Int)

array.drop(2) // Array('c','d','e','f')
Take the first n elements with take(n: Int)

array.take(4) // Array('a','b','c','d')
Select any interval of elements with slice(from: Int, until: Int). Note that until is excluded.

array.slice(2,4) // Array('c','d')

The slice method is stricly equivalent to:
array.take(4).drop(2) // Array('c','d')
Exclude the last n elements with dropRight(n: Int):

array.dropRight(4) // Array('a','b')
Select the last n elements with takeRight(n: Int):

array.takeRight(4) // Array('c','d','e','f')

Reference: Official documentation

score 1 · Answer 3 · answered Jun 16 '20 at 14:10

An example of extracting specific columns from a 2D Scala Array (original_array):

import scala.collection.mutable.ArrayBuffer

val sub_array = ArrayBuffer[Array[String]]()
val columns_subset: Seq[String] = Seq("ColumnA", "ColumnB", "ColumnC")
val columns_original = original_array(0)

for (column_now <- columns_subset) {
      sub_array += original_array.map{_(columns_original.indexOf(column_now))}
    }
sub_array

What is the correct way to get a subarray in Scala?

3 Answers3

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