Scala: adding element to ArrayBuffer on index

Question

I just started learning Scala, trying to code a simple game to learn it's syntax and some basic rules. I'd be grateful if anyone could help me with a simple problem. I created simple board filled with ".". Now I want to change one "." on the certain index and put there "X". In java it would look like this:

board[col][row] = symbol;

This is the board part:

val board = {
 collection.mutable.ArrayBuffer[String]()
 collection.mutable.ArrayBuffer.fill(cols, rows)(".")
}

def show(): Unit = {
 board.foreach(x => println(x.mkString))
}

def setField(col:Int, row:Int, fieldSymbol:String): Unit = {
 //fieldSymbol on index, which is (col*row) as it's 1d array
 //thought board.apply(col*row): fieldSymbol might work but it's not 
 //correct I guess
}

Any help would be much appreciated. Thanks in advance for advice.

Use board(i) to access the i-th value of a collection (same as Java's [i]). — Marth
– Marth, Commented Feb 3, 2016 at 13:52
collection.mutable.ArrayBuffer[String]() This is creating an empty array then (since you don't do anything with it), discarding it. You only need the fill line — The Archetypal Paul
– The Archetypal Paul, Commented Feb 3, 2016 at 14:01
Let me ask some more questions so I won't be creating many trash topics. When I have (cols, rows) then it means I have something like 2d array? Like, can I do board(col, row) = fieldSymbol ? — Cauchy
– Cauchy, Commented Feb 3, 2016 at 14:05
You have a two-d array, but it's represented as an array of arrays. It's just Java (in fact a Scala Array is a Java Array). So you need board(col)(row) (I'm working so I'm not going to be a real-time further-question-answering service, post "proper" questions if required (or consult a Scala tutorial!) — The Archetypal Paul
– The Archetypal Paul, Commented Feb 3, 2016 at 14:09

dth · Accepted Answer · 2016-02-03 21:29:53Z

2

You could implement setField for a flat array like this

def setField(c: Int, r: Int, v: String) {
  board(c * rows + row) = v
}

This is however only a good idea if you want to store your multi dimensional data in a single array (which can be interesting if you care about allocation structure and stuff like that). In this case I would recommend to write a generic method setField though.

If you instead want arrays of arrays fill already gives you that

val board = ArrayBuffer.fill(rows, cols)(".")

and then just update like this

board(x)(y) = "something"

You should however ask yourself whether you really need a mutable data structure and your program cannot be expressed more elegantly using immutable data structures. And also, if you really want a two dimensional vector if you want to represent something as a board of objects, especially if most of the board is "empty". It could be much more elegant to use a map:

val board = Map[(Int, Int), String]() // empty board
val nextBoard = bord + ((2,3) -> "something")

edited Feb 3, 2016 at 21:29

answered Feb 3, 2016 at 14:11

dth

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6 Comments

Cauchy Over a year ago

What is the advantage of tabulate over fill? Or maybe fill just will not work here?

dth Over a year ago

tabulate returns a nested structure, i.e. in this case ArrayBuffer[ArrayBuffer[String]] which is what you would typically have in Java as well.

dth Over a year ago

By the way, there is also Array.tabulate(...) which would be preferable if you do not need arrays of variable size.

Cauchy Over a year ago

Yeah, I don't. I just overcomplicated everything. Thanks for help.

Seth Tisue Over a year ago

There is no advantage of using tabulate here, as you are not doing anything depending on the values of x and y. You should just use fill, as follows: ArrayBuffer.fill(rows, cols)(".")

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