Instantiating a List of a List in Scala

Question

I have a list of a list of integers in Scala, while the interpreter/compiler doesn't throw any warnings, when I go to run the worksheet I get an error "not found: value table"

var mylist: List[List[Int]]

for (i <- 1 to 10) {
mylist = List(List(i, Random.nextInt(20000), quantity(i)))
}

As far as I know, i is an Int, nextInt will return an Int, and quantity is a predetermined list of Ints.

I'm guessing I need to instantiate the table variable, how should I go about that?

Answer 1

There's a difference between a var that holds an immutable collection, and a val that holds a mutable collection. The former ( var ) can hold a different collection at some later time. The latter ( val ) can only hold the one given collection, but the contents of that collection can change over time.

Even though mylist is a var , a List[List[Int]] is immutable. You can't modify its contents.

To create the collection you want you might try something like this.

val mylist = (1 to 10).map(x => List(x, Random.nextInt(2000), quantity(x))).toList

Answer 2

As @jwvh mentioned List s are immutable, so you should include full formula to calculate your list instead of element by element calculation

import scala.util.Random

val quantity = List(1,2,4,8,16)
val myList =  for {
  q ← quantity
} yield List.fill(q)(Random.nextInt(20000))

If you really like the imperative approach, you can use mutable builders to create your collection

import scala.collection.mutable.ListBuffer
import scala.util.Random

val quantity = List(1,2,4,8,16,32)
val myListBuffer = ListBuffer.empty[List[Int]]
for (i ← 0 until 6 )
    myListBuffer += List.fill(quantity(i))(Random.nextInt(20000))
val myList = myListBuffer.toList