Updating existing String elements of an ArrayBuffer in Scala

Question

I have an ArrayBuffer of Strings as below:

var myList = ArrayBuffer[String]()
myList += "abc"
myList += "def"

Now, I'm trying to update the String in the ArrayBuffer based on some condition:

for(item <- myList){
  if(some condition){
    item = "updatedstring"
  }
}

When I try to do this, I get an error saying val cannot be reassigned. Why do I get this error even though I have declared myList as a var? If I cannot update it this way, how else can I update the elements when iterating through the ArrayBuffer? I'm new to Scala, so I apologize if I got this all wrong.

childofsoong · Accepted Answer · 2015-07-27 19:53:16Z

1

The first thing I would point out is that item is not the same as myList - it's an element within myList, and the way that Scala iteration works, it's a val. There are various reasons for this related to immutability which I won't get into here.

I would recommend this instead:

val myNewList = myList.map(originalString =>
    if (someCondition) "xyz"
    else originalString
)

Then, if you feel so inclined, you could do myList = myNewList (or just forgo having a myNewList entirely and do myList = myList.map(...)).

answered Jul 27, 2015 at 19:53

childofsoong

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Comments

Sascha Kolberg · Accepted Answer · 2015-07-27 19:56:36Z

1

In the for loop item is a local val. Therefore, it cannot be changed. You could:

either iterate through the array and update each item

myList.zipWithIndex foreach { case (item, index) if (condition) => myList.update(index, "updated") }

or create a new ArrayBuffer

myList = (0 until myList.length).map { index => 
  val item = myList(index)
  if (condition) "updated" else item
}

answered Jul 27, 2015 at 19:56

Sascha Kolberg

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1 Comment

Peter Neyens Over a year ago

You could use myList.indices in your last solution, which returns the same Range as your (0 until myList.length).

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