Scala Appending to an empty Array

Question

I am trying to append to an array but for some reason it is just appending blanks into my Array.

  def schemaClean(x: Array[String]): Array[String] =
  {
    val array = Array[String]()
    for(i <- 0 until x.length){
      val convert = x(i).toString
      val split = convert.split('|')
      if (split.length == 5) {
        val drop = split.dropRight(3).mkString(" ")
        array :+ drop
      }
      else if (split.length == 4) {
        val drop = split.dropRight(2).mkString(" ")
        println(drop)
        array :+ drop.toString
        println(array.mkString(" "))
      }
    }
   array
  }


  val schema1 = schemaClean(schema)

prints this:

record_id string

assigned_offer_id string

accepted_offer_flag string

current_offer_flag string

If I try and print schema1 its just 1 blank line.

You are not storing the new array's reference anywhere. (array :+ drop, array :+ drop.toString) array should be a var imho (or use fold/...). — Gábor Bakos
– Gábor Bakos, Commented May 29, 2015 at 19:02

Pablo Francisco Pérez Hidalgo · Accepted Answer · 2015-05-30 13:37:34Z

7

Scala's Array size is immutable. From Scala's reference:

def
    :+(elem: A): Array[A]

[use case] A copy of this array with an element appended.

Thus :+ returns a new array whose reference you are not using.

val array = ...

Should be:

var array = ...

And you should update that reference with the new arrays obtained after each append operation.

Since there are not variable size arrays in Scala, the alternative to an Array var copied after insertion is BufferArray, use its method operator += to append new elements and obtain the resulting array from the buffer, e.g:

import scala.collection.mutable.ArrayBuffer
val ab = ArrayBuffer[String]()
ab += "hello"
ab += "world"
ab.toArray

res2: Array[String] = Array(hello, world)

Applied to your code:

def schemaClean(x: Array[String]): Array[String] =
  {
    val arrayBuf = ArrayBuffer[String]()
    for(i <- 0 until x.length){
      val convert = x(i).toString
      val split = convert.split('|')
      if (split.length == 5) {
        val drop = split.dropRight(3).mkString(" ")
        arrayBuf += drop
      }
      else if (split.length == 4) {
        val drop = split.dropRight(2).mkString(" ")
        println(drop)
        arrayBuf += drop.toString
        println(arrayBuf.toArray.mkString(" "))
      }
    }
   arrayBuf.toArray
  }

edited May 30, 2015 at 13:37

answered May 29, 2015 at 19:10

Pablo Francisco Pérez Hidalgo

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3 Comments

theMadKing Over a year ago

Wonderful thank you for not only giving the answer but also explaining why! New to this language and its subtle nuances!

Steve Waldman Over a year ago

A bit of a correction: Scala arrays are NOT immutable. As in Java, the length of a Scala array cannot be modified, so :+ necessarily creates a copy. But arrays can be updated in-place with statements like myArray(0) = newVal;. You'll get yourself into all kinds of trouble if you treat arrays as immutable and therefore thread-safe.

Pablo Francisco Pérez Hidalgo Over a year ago

@SteveWaldman Thanks for the observation, I've edited my answer to include the correction.

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