iBAT Session > June 17' 2011 > Meetu Maltiar

Scala Collections

An Example

package com.inphina.collections.person class Person(val name: String, val age: Int) { }

Somewhere in the code:val (minors, adults) = people partition (_.age < 18)

Three concepts: - Simple pattern matching - An Infix method call - A function value

The Scala Way Of Collections

scala> val ys = List(1, 2, 3)ys: List[Int] = List(1, 2, 3)

scala> val xs: Seq[Int] = ysxs: Seq[Int] = List(1, 2, 3)

scala> xs map (_ + 1)res0: Seq[Int] = List(2, 3, 4)

scala> ys map (_ + 1)res1: List[Int] = List(2, 3, 4)

Collection Properties

Object-oriented

Generic: List[T], Map[K, V]

optionally persistent: scala.collections.immutable

Higher-order with methods like foreach, map, filter

uniform return type principle: operations return collections of same type as their left operand

The Uniform Return Type Principle

Bulk operations return collections of the same type (constructor) as their left operand.

scala> val ys = List(1,2,3)ys: List[Int] = List(1, 2, 3)

scala> val xs: Seq[Int] = ysxs: Seq[Int] = List(1, 2, 3)

scala> xs map(_ + 1)res0: Seq[Int] = List(2, 3, 4)

scala> ys map(_ + 1)res1: List[Int] = List(2, 3, 4)

Using Collections: Map and Filterscala> val xs = List(1, 2, 3)xs: List[Int] = List(1, 2, 3)

scala> val ys = xs map (x => x + 1)ys: List[Int] = List(2, 3, 4)

scala> val ys = xs map (_ + 1)ys: List[Int] = List(2, 3, 4)

scala> val zs = ys filter (_ % 2 == 0)zs: List[Int] = List(2 , 4)

scala> val as = ys map (0 to _)as: List[scala.collection.immutable.Range.Inclusive] = List(Range(0, 1), Range(0, 1, 2), Range(0, 1, 2, 3))

Using Collections: flatMap and groupByscala> val bs = as.flattenbs: List[Int] = List(0, 1, 0, 1, 2, 0, 1, 2, 3)

scala> val bs = ys flatMap (0 to _)bs: List[Int] = List(0, 1, 0, 1, 2, 0, 1, 2, 3)

scala> val fruit = Vector("apples", "oranges", "ananas")fruit: scala.collection.immutable.Vector[java.lang.String] = Vector(apples, oranges, ananas)

scala> fruit groupBy (_.head)res3: scala.collection.immutable.Map[Char,scala.collection.immutable.Vector[java.lang.String]] = Map(a -> Vector(apples, ananas), o -> Vector(oranges))

Using Collections: For Notation

scala> for (x <- xs) yield x + 1 // mapres7: Seq[Int] = List(2, 3, 4)

scala> for (x <- res7 if x % 2 == 0 ) yield x // filterres8: Seq[Int] = List(2, 4)

scala> for (x <- xs; y <- 0 to x) yield y // flatMap res9: Seq[Int] = List(0, 1, 0, 1, 2, 0, 1, 2, 3)

String also a Collection

Even String is also a collection that means we can apply higher order functions on it.

scala> val aString = "hello world"aString: java.lang.String = hello world

scala> aString map (_.toUpper)res12: String = HELLO WORLD

Using Maps

scala> val m = Map(1 -> "ABC", 2 -> "DEF", 3 -> "GHI")m: scala.collection.immutable.Map[Int,java.lang.String] = Map(1 -> ABC, 2 -> DEF, 3 -> GHI)

scala> m(2)res10: java.lang.String = DEF

scala> m + (4 -> "JKL")res11: scala.collection.immutable.Map[Int,java.lang.String] = Map(1 -> ABC, 2 -> DEF, 3 -> GHI, 4 -> JKL)

scala> m map {case (k, v) => (v, k)}res12: scala.collection.immutable.Map[java.lang.String,Int] = Map(ABC -> 1, DEF -> 2, GHI -> 3)

Scala Collection Hierarchy

All Collection classes are in scala.collection or one of its sub-packages mutable, immutable and generic

Root collections in the package scala.collection define thesame interface as immutable collections and mutablecollections add some modification operations to make itmutable

The generic package contains building block for implementingCollections.

Scala.Collection Hierarchy

Scala.Collection.Immutable

Scala.Collection.Mutable

Overview Of CollectionsTraversable Iterable Seq IndexedSeq LinearSeq mutable.Buffer Range Set SortedSet immutable.HashSet mutable.HashSet mutable.LinkedHashSet BitSet Map SortedMap immutable.HashMap mutable.HashMap mutable.LinkedHashMap

Commonality In collections

All classes are quite common. For instance, everykind of collection can be created by same uniform Syntax Traversable(1, 2, 3) Iterable("x", "y", "z") Map("x" -> 24, "y" -> 25, "z" -> 26) Set(Color.red, Color.green, Color.blue) SortedSet("hello", "world") Buffer(x, y, z) IndexedSeq(1.0, 2.0) LinearSeq(a, b, c)

The same principle applies with specific collection implementations List(1, 2, 3) HashMap("x" -> 24, "y" -> 25, "z" -> 26)

Commonality In collections...

All these Collections get displayed with toString in the same waythey are written above

All collections support the API provided by Traversable butspecializes types wherever it makes sense

map method in class Traversable returns another Traversable asits result. But this result type is overridden in subclasses

scala> List(1, 2, 3) map (_ + 1) res0: List[Int] = List(2, 3, 4) scala> Set(1, 2, 3) map (_ * 2) res0: Set[Int] = Set(2, 4, 6)

This behavior in the collections libraries is called the uniform return type principle.

Trait TraversableTop of collection hierarchy. Its abstract method is foreach:def foreach[U](f: Elem => U)

Traversable also provides lot of concrete methods they fall infollowing categories Addition: ++, appends two traversables together Map operations: map, flatMap, and collect Conversions: toArray, toList, toIterable, toSeq, toIndexedSeq, toStream, toSet, toMap, Copying operations: copyToBuffer and copyToArray Size info operations: isEmpty, nonEmpty, size, and hasDefiniteSize Element retrieval operations: head, last, headOption, lastOption, and find Sub-Collection retrieval operations: tail, init, slice, take, drop, takeWhile, dropWhile, filter, filterNot, withFilter Subdivision operations: splitAt, span, partition, groupBy Element tests: exists, forall, count Folds: foldLeft, foldRight, /:, :\, reduceLeft, reduceRight Specific Folds: sum, product, min, max String Operations: mkString, addString, stringPrefix

Trait Iterable

This is the next Trait in Collection hierarchy. All methods in this traitare defined in terms of an abstract method iterator, which returnscollection results one by one.

The foreach method from trait Traversable is implemented inIterable in terms of iterator

def foreach[U](f: Elem => U): Unit = { val it = iterator while (it.hasNext) f(it.next())}

An Example ..Task: Phone keys has mnemonics attached to them

val mnemonics = Map( '2' -> "ABC", '3' -> "DEF", '4' -> "GHI", '5' -> "JKL", '6' -> "MNO", '7' -> "PQRS", '8' -> "TUV", '9' -> "WXYZ")

Assume that you are given a dictionary dict as a list of words. Design a class Coder with a method translate such that new Coder(dict).translate(phoneNumber)

Produces all phrases of words which can serve as mnemonics for the phone number

Example The phone number “7225276257” should have a mnemonic Scala rocksas one element of the list of solution phrases

An Example

Creating a mnemonics Mapval mnemonics = Map('2' -> "ABC", '3' -> "DEF", '4' -> "GHI", '5' -> "JKL", '6' -> "MNO", '7' -> "PQRS", '8' -> "TUV", '9' -> "WXYZ")

Creating reverse map with upper codeval upperCode: Map[Char, Char] = for ((digit, str) <- mnemonics; ltr <- str) yield (ltr -> digit)

Creating a word for which digit string will be generatedval word = "java"

Gettng a digit string from a word eg; "java" -> "5282"word.toUpperCase map (upperCode(_))

Everything is a library

Collections feel they are language constructsLanguage does not contain any collection related constructs - no collection types - no collection literals - no collection operatorsEverything is in libraryThey are extensible

Conclusion

De-emphasize destructive updates

Focus on Transformers that map collections to collections

Have complete range of persistent collections

Next Steps

Making and extending Scala Collection

Parallel Collections:Move from Mutable → Persistent → Parallel

References

Martin Odersky's talk at Parleys

Scala Collection documentation