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Scala Collections

The Scala collections are predefined data-structures that are available to you to structure and organize the data in your programs. A full description of the Scala collections is available on the Scala website. Also, this post on Java Collections provides useful background information.

Here, we are going to look at just the collections that we will use to begin with:

ArrayBuffer
HashSet
HashMap

ArrayBuffer

An ArrayBuffer is a mutable (modifiable), indexed sequence of elements of a given type. An element’s location in the ArrayBuffer (its index) is based on the order in which it was added to the buffer. More info.

Example usage:

clearOutput()
// create an empty arraybuffer
val ab = ArrayBuffer.empty[Int]
// add an element to it
ab.append(10)
// add two more elements to it
ab.append(11, 12)
// do something with all the elements
ab.foreach { x =>
    println(x)    
}
// do something with all the elements using our old friend repeatFor
// this does the same thing as above
println("---")
repeatFor(ab) { x =>
    println(x)
}

// use indexing to access an element
println("---")
val e = ab(0)
val e2 = ab(1)
println(e, e2)

// convert ab to a new arraybuffer via a rule for how each element 
// is to be changed
val ab2 = ab.map { x =>
    x * 2    
}
println("---")
println(ab2)

// filter ab to get a new arraybuffer via a rule for whether an element 
// is to be selected or not
val ab3 = ab.filter { x =>
    x > 10
}
println("---")
println(ab3)

HashSet

A HashSet is a mutable set of elements of a given type. A set is an unordered sequence. Use a set when you don’t care about the order of the elements inside, but care only about whether an element is present inside or not. More info.

Example Usage:

clearOutput()
// create an empty hashset
val hs = HashSet.empty[Int]
// add an element to it
hs.add(10)
// add another elements to it
hs.add(12)

// do something with all the elements
hs.foreach { x =>
    println(x)    
}
// do something with all the elements using our old friend repeatFor
// this does the same thing as above
println("---")
repeatFor(hs) { x =>
    println(x)
}

// foreach and repeatFor (above) work the same for all sequences.

// test for membership; this is mostly why you want to use a set
println("---")
val c = hs.contains(11)
val c2 = hs.contains(12) 
println(c, c2)

// filter and map (below) work the same for all sequences.

// convert hs to a new hashset via a rule for how each element 
// is to be changed
val hs2 = hs.map { x =>
    x * 2    
}
println("---")
println(hs2)

// filter hs to get a new hashset via a rule for whether an element 
// is to be selected or not
val hs3 = hs.filter { x =>
    x > 10
}
println("---")
println(hs3)

HashMap

A HashMap is a mutable map/dictionary of associations between keys and values. You add (key, value) pairs to the map, and can then look up values based on keys. More info.

Example:

clearOutput()
// create an empty hashmap
val hm = HashMap.empty[Int, Int]
// add a key, value pair to it
hm(10) = 2

// add another key, value pair to it
hm(11) = 5

// get the value for a key
println("---")
val k = hm(10)
val k2 = hm(11)
println(k, k2)

// To guard against a key not being present, use getOrElse
val k3 = hm.getOrElse(12, 0)
println(k3)

Kojo Wiki

docs for Kojo

Scala Collections

ArrayBuffer

HashSet

HashMap