README.md

Workshop 02 - Scala Collections

Goal

To get familiar with Lists, Maps, functional combinators (map, foreach, filter, zip, folds...)

Collections overview

• scala docs

Basic Collection operations

• Mutable and immutable collections - Scala support both - Scala gives preference to immutable collections (Predef contains immutable collection types) - Add import scala.collections.mutable.Map to use mutable version of Map

• Sequence - Vector: immutable version equivalent of an ArrayBuffer - Range: integer sequence, store only start end and increment

• List - In scala a collection is either Nil (empty) or an object with head (first element) and tail (rest)

    	val set = Set(1, 2, 3)
    	set.head
    	set.tail
    
    	val seq = Seq(1, 2, 3)
    	seq.head
    	seq.tail
    
    	val list = List(1, 2, 3)
    	list.head
    	list.tail
  
  - :: makes new list for given head and tail
  
    	1::2::3::Nil
  
  - Pattern matching
  
    	def sum(l: List[Int]):Int = l match {
    	  case Nil => 0
    	  case h::t => h + sum(t)
    	}
  

• Adding or removing operations - Seq: append or prepend element

    	val s = Seq(1, 2, 3)
    	s :+ 4
    	0 +: s
  
  - Set, Map: add element
  
    	val s = Set(1, 2, 3)
    	s + 4
    	s + (4, 5, 6)
  
  - Set, Map, ArrayBuffer: remove element
  
    	val s = Set(1, 2, 3)
    	s - 3
    	s - (2, 3)
  
  - Iterable:
  
    	val a = List(1, 2, 3)
    	val b = List(4, 5, 6)
  
    	a ++ b //append
    	b ++: a //prepend
  
  - Set, Map, ArrayBuffer
  
    	val a = Set(1, 2, 3)
    	val b = Set(2, 3)
  
    	a -- b //remove set
  
  - List
  
    	val a = List(1, 2, 3)
    	val b = List(4, 5, 6)
  
    	0 :: b
    	a ::: b
  
  - Set: Set union, intersection, difference,
  
    	val a = Set(1, 2, 3)
    	val b = Set(2, 3, 4)
  
    	a | b
    	a & b
    	a &~ b
  
  - Mutable collections:
  
    	import scala.collection.mutable.ListBuffer
  
    	var list = ListBuffer(1, 2, 3)
    	var list2 = ListBuffer(-3, -2, -1)
  
    	list += 4
    	list += (5, 6)
    	list ++= list2
  
    	list -= 4
    	list -= (5, 6)
    	list --= list2
  

Common Methods

• Scala kolekcje
• Cheat sheet

Exercise

1. Write a function that, given a string, produces a map of the indexes of all characters. For example indexes("Mississippi") should return a map associating 'M' with the set {0}, 'i' with the set{1, 4, 7, 10} and so on. Use a mutable map of characters to mutable sets. How can you ensure that the set is sorted?

2. Repeat the preceding exercise, using an immutable map of character to lists.

3. Implements two (word and friends) MapReduce examples from Steve Krenzel - 6. MapReduce: Finding Friends