Scala Array Copy looks to be working differently - arrays

Question

object Solution extends App {
val arr1 = Array(
Array(1,2,3),
Array(4,5,6)
)
var arr2 = Array.ofDim[Int](2,3)
Array.copy(arr1,0,arr2,0,arr1.length)
arr1(0)(1) = 23
println(arr1.map(_.mkString(",")).mkString("\n"))
println()
println(arr2.map(_.mkString(",")).mkString("\n"))
}
1,23,3
4,5,6
1,23,3
4,5,6
what is wrong, why is the 23 appearing in both arrays

Answer 1

Because Array in Scala, or if to be more precise in JVM, because of Scala interop with Java - is a mutable structure, and you performing shallow copy and not a deep copy. Meaning - you copying the upper structure (or top array in your case) and not entire structure recursively, like all downstream array.
Solution might look like:
val source = Array(
Array(1, 2, 3),
Array(4, 5, 6)
)
val target = Array.ofDim[Int](2, 3)
source.zipWithIndex.foreach { case (row, index) =>
Array.copy(source(index), 0, target(index), 0, source.length)
}
target(0)(1) = 23
println(source.map(_.mkString(",")).mkString("\n"))
println()
println(target.map(_.mkString(",")).mkString("\n"))
which will print out result:
1,2,3
4,5,6
1,23,0
4,5,0
Scatie example: https://scastie.scala-lang.org/lrrHyGqZRxKk7mZ6CbLoiA
UPDATE
As correctly stated #Luis Miguel Mejía Suárez in the comment section - zipWithIndex expensive operation. More optimal solution would be
(0 until source.length).foreach { index =>
Array.copy(source(index), 0, target(index), 0, source.length)
}

Answer 2

Array.copy uses System.arrayCopy which modifies both the arrays. In the doc:
Copy one array to another. Equivalent to Java's System.arraycopy(src, srcPos, dest, destPos, length), except that this also works for polymorphic and boxed arrays.
Note that the passed-in dest array will be modified by this call.
You can try a simple map with identity:
scala> val arr1 = Array(Array(1,2,3),Array(4,5,6))
arr1: Array[Array[Int]] = Array(Array(1, 2, 3), Array(4, 5, 6))
scala> val arr3 = arr1.map(_.map(identity))
arr3: Array[Array[Int]] = Array(Array(1, 2, 3), Array(4, 5, 6))
scala> arr1(0)(1) = 23
scala> arr1
res16: Array[Array[Int]] = Array(Array(1, 23, 3), Array(4, 5, 6))
scala> arr3
res17: Array[Array[Int]] = Array(Array(1, 2, 3), Array(4, 5, 6))