Functional Collections & Higher-Order Functions

Scala's default collections are immutable: List (linked list), Vector (indexed, fast random access), Set (unique elements), Map (key-value). All operations return new collections — the original is unchanged. Mutable collections (scala.collection.mutable) exist but are used sparingly. Vector is the general-purpose default: O(1) amortized for append, prepend, and random access via 32-way tree structure.

map, flatMap, and for-comprehensions

map transforms each element. filter keeps matching elements. flatMap maps then flattens — essential for working with Option, Either, and nested collections. For-comprehensions are syntactic sugar for map/flatMap/filter: 'for { x <- xs; y <- ys if x != y } yield (x, y)'. For-comprehensions work on any Monad: List, Option, Future, Either.

fold, reduce, and groupBy

foldLeft(initial)(op) reduces left-to-right with an accumulator. foldRight reduces right-to-left. reduce applies a binary operation without an initial value (requires non-empty collection). groupBy(f) partitions into a Map[K, List[V]]. sortBy, sortWith, zip, unzip, partition, span, take, drop, takeWhile, dropWhile — the full collection API replaces every loop pattern you have written in imperative languages.

// Immutable collection operations
val numbers = (1 to 20).toVector

// Chain transformations
val result = numbers .filter(_ % 2 == 0) // evens: [2,4,6,...,20] .map(n => n * n) // squares: [4,16,36,...,400] .takeWhile(_ < 200) // [4,16,36,64,100,144,196] .foldLeft(0)(_ + _) // sum: 460

println(result)  // 460

// For-comprehension: Pythagorean triples
val triples = for { c <- 1 to 100 b <- 1 to c a <- 1 to b if a*a + b*b == c*c
} yield (a, b, c)

triples.take(5).foreach(println)
// (3,4,5), (5,12,13), (6,8,10), (8,15,17), (9,12,15)

// groupBy: word frequencies
val words = 'the quick brown fox jumps over the lazy dog the'.split(' ')
val freq = words.groupBy(identity).view.mapValues(_.length).toMap
freq.toList.sortBy(-_._2).foreach { case (w, c) => println(s'$w: $c') }