Collections

Trait Iterable

Language

At the top of the collection hierarchy is trait Iterable. All methods in this trait are defined in terms of an abstract method, iterator, which yields the collection’s elements one by one.

def iterator: Iterator[A]

Collection classes that implement Iterable just need to define this method; all other methods can be inherited from Iterable.

Iterable also defines many concrete methods, which are all listed in the following table. These methods fall into the following categories:

  • Addition, concat, which appends two collections together, or appends all elements of an iterator to a collection.
  • Map operations map, flatMap, and collect, which produce a new collection by applying some function to collection elements.
  • Conversions toArray, toList, toIterable, toSeq, toIndexedSeq, toStream, toSet, toMap, which turn an Iterable collection into something more specific. All these conversions return their receiver argument unchanged if the run-time type of the collection already matches the demanded collection type. For instance, applying toList to a list will yield the list itself.
  • Copying operations copyToArray. As its name implies, this copies collection elements to an array.
  • Size info operations isEmpty, nonEmpty, size, knownSize, sizeIs. The number of elements of a collections can require a traversal in some cases (e.g. List). In other cases the collection can have an infinite number of elements (e.g. LazyList.from(1)).
  • Element retrieval operations head, last, headOption, lastOption, and find. These select the first or last element of a collection, or else the first element matching a condition. Note, however, that not all collections have a well-defined meaning of what “first” and “last” means. For instance, a hash set might store elements according to their hash keys, which might change from run to run. In that case, the “first” element of a hash set could also be different for every run of a program. A collection is ordered if it always yields its elements in the same order. Most collections are ordered, but some (e.g. hash sets) are not– dropping the ordering gives a little bit of extra efficiency. Ordering is often essential to give reproducible tests and to help in debugging. That’s why Scala collections give ordered alternatives for all collection types. For instance, the ordered alternative for HashSet is LinkedHashSet.
  • Sub-collection retrieval operations tail, init, slice, take, drop, takeWhile, dropWhile, filter, filterNot, withFilter. These all return some sub-collection identified by an index range or some predicate.
  • Subdivision operations splitAt, span, partition, partitionMap, groupBy, groupMap, groupMapReduce, which split the elements of this collection into several sub-collections.
  • Element tests exists, forall, count which test collection elements with a given predicate.
  • Folds foldLeft, foldRight, reduceLeft, reduceRight which apply a binary operation to successive elements.
  • Specific folds sum, product, min, max, which work on collections of specific types (numeric or comparable).
  • String operations mkString, addString, className, which give alternative ways of converting a collection to a string.
  • View operation: A view is a collection that’s evaluated lazily. You’ll learn more about views in later.

Two more methods exist in Iterable that return iterators: grouped and sliding. These iterators, however, do not return single elements but whole subsequences of elements of the original collection. The maximal size of these subsequences is given as an argument to these methods. The grouped method returns its elements in “chunked” increments, where sliding yields a sliding “window” over the elements. The difference between the two should become clear by looking at the following REPL interaction:

scala> val xs = List(1, 2, 3, 4, 5)
xs: List[Int] = List(1, 2, 3, 4, 5)
scala> val git = xs grouped 3
git: Iterator[List[Int]] = non-empty iterator
scala> git.next()
res3: List[Int] = List(1, 2, 3)
scala> git.next()
res4: List[Int] = List(4, 5)
scala> val sit = xs sliding 3
sit: Iterator[List[Int]] = non-empty iterator
scala> sit.next()
res5: List[Int] = List(1, 2, 3)
scala> sit.next()
res6: List[Int] = List(2, 3, 4)
scala> sit.next()
res7: List[Int] = List(3, 4, 5)

Operations in Class Iterable

WHAT IT IS WHAT IT DOES
Abstract Method:  
xs.iterator An iterator that yields every element in xs.
Other Iterators:  
xs foreach f Executes function f for every element of xs.
xs grouped size An iterator that yields fixed-sized “chunks” of this collection.
xs sliding size An iterator that yields a sliding fixed-sized window of elements in this collection.
Addition:  
xs concat ys
(or xs ++ ys)
A collection consisting of the elements of both xs and ys. ys is a IterableOnce collection, i.e., either an Iterable or an Iterator.
Maps:  
xs map f The collection obtained from applying the function f to every element in xs.
xs flatMap f The collection obtained from applying the collection-valued function f to every element in xs and concatenating the results.
xs collect f The collection obtained from applying the partial function f to every element in xs for which it is defined and collecting the results.
Conversions:  
xs.toArray Converts the collection to an array.
xs.toList Converts the collection to a list.
xs.toIterable Converts the collection to an iterable.
xs.toSeq Converts the collection to a sequence.
xs.toIndexedSeq Converts the collection to an indexed sequence.
xs.toSet Converts the collection to a set.
xs.toMap Converts the collection of key/value pairs to a map. If the collection does not have pairs as elements, calling this operation results in a static type error.
xs.to(SortedSet) Generic conversion operation that takes a collection factory as parameter.
Copying:  
xs copyToArray(arr, s, n) Copies at most n elements of the collection to array arr starting at index s. The last two arguments are optional.
Size info:  
xs.isEmpty Tests whether the collection is empty.
xs.nonEmpty Tests whether the collection contains elements.
xs.size The number of elements in the collection.
xs.knownSize The number of elements, if this one takes constant time to compute, otherwise -1.
xs.sizeCompare(ys) Returns a negative value if xs is shorter than the ys collection, a positive value if it is longer, and 0 if they have the same size. Works even if the collection is infinite, for example LazyList.from(1) sizeCompare List(1, 2) returns a positive value.
xs.sizeCompare(n) Returns a negative value if xs is shorter than n, a positive value if it is longer, and 0 if it is of size n. Works even if the collection is infinite, for example LazyList.from(1) sizeCompare 42 returns a positive value.
xs.sizeIs < 42, xs.sizeIs != 42, etc. Provides a more convenient syntax for xs.sizeCompare(42) < 0, xs.sizeCompare(42) != 0, etc., respectively.
Element Retrieval:  
xs.head The first element of the collection (or, some element, if no order is defined).
xs.headOption The first element of xs in an option value, or None if xs is empty.
xs.last The last element of the collection (or, some element, if no order is defined).
xs.lastOption The last element of xs in an option value, or None if xs is empty.
xs find p An option containing the first element in xs that satisfies p, or None if no element qualifies.
Subcollections:  
xs.tail The rest of the collection except xs.head.
xs.init The rest of the collection except xs.last.
xs.slice(from, to) A collection consisting of elements in some index range of xs (from from up to, and excluding to).
xs take n A collection consisting of the first n elements of xs (or, some arbitrary n elements, if no order is defined).
xs drop n The rest of the collection except xs take n.
xs takeWhile p The longest prefix of elements in the collection that all satisfy p.
xs dropWhile p The collection without the longest prefix of elements that all satisfy p.
xs takeRight n A collection consisting of the last n elements of xs (or, some arbitrary n elements, if no order is defined).
xs dropRight n The rest of the collection except xs takeRight n.
xs filter p The collection consisting of those elements of xs that satisfy the predicate p.
xs withFilter p A non-strict filter of this collection. Subsequent calls to map, flatMap, foreach, and withFilter will only apply to those elements of xs for which the condition p is true.
xs filterNot p The collection consisting of those elements of xs that do not satisfy the predicate p.
Subdivisions:  
xs splitAt n Split xs at a position, giving the pair of collections (xs take n, xs drop n).
xs span p Split xs according to a predicate, giving the pair of collections (xs takeWhile p, xs.dropWhile p).
xs partition p Split xs into a pair of collections; one with elements that satisfy the predicate p, the other with elements that do not, giving the pair of collections (xs filter p, xs.filterNot p)
xs groupBy f Partition xs into a map of collections according to a discriminator function f.
xs.groupMap(f)(g) Partition xs into a map of collections according to a discriminator function f, and applies the transformation function g to each element in a group.
xs.groupMapReduce(f)(g)(h) Partition xs according to a discriminator function f, and then combine the results of applying the function g to each element in a group using the h function.
Element Conditions:  
xs forall p A boolean indicating whether the predicate p holds for all elements of xs.
xs exists p A boolean indicating whether the predicate p holds for some element in xs.
xs count p The number of elements in xs that satisfy the predicate p.
Folds:  
xs.foldLeft(z)(op) Apply binary operation op between successive elements of xs, going left to right and starting with z.
xs.foldRight(z)(op) Apply binary operation op between successive elements of xs, going right to left and ending with z.
xs reduceLeft op Apply binary operation op between successive elements of non-empty collection xs, going left to right.
xs reduceRight op Apply binary operation op between successive elements of non-empty collection xs, going right to left.
Specific Folds:  
xs.sum The sum of the numeric element values of collection xs.
xs.product The product of the numeric element values of collection xs.
xs.min The minimum of the ordered element values of collection xs.
xs.max The maximum of the ordered element values of collection xs.
xs.minOption Like min but returns None if xs is empty.
xs.maxOption Like max but returns None if xs is empty.
Strings:  
xs.addString(b, start, sep, end) Adds a string to StringBuilder b that shows all elements of xs between separators sep enclosed in strings start and end. start, sep, end are all optional.
xs.mkString(start, sep, end) Converts the collection to a string that shows all elements of xs between separators sep enclosed in strings start and end. start, sep, end are all optional.
xs.stringPrefix The collection name at the beginning of the string returned from xs.toString.
Zippers:  
xs zip ys A collection of pairs of corresponding elements from xs and ys.
xs.zipAll(ys, x, y) A collection of pairs of corresponding elements from xs and ys, where the shorter sequence is extended to match the longer one by appending elements x or y.
xs.zipWithIndex An collection of pairs of elements from xs with their indices.
Views:  
xs.view Produces a view over xs.

In the inheritance hierarchy below Iterable you find three traits: Seq, Set, and Map. Seq and Map implement the PartialFunction trait with its apply and isDefinedAt methods, each implemented differently. Set gets its apply method from SetOps.

For sequences, apply is positional indexing, where elements are always numbered from 0. That is, Seq(1, 2, 3)(1) gives 2. For sets, apply is a membership test. For instance, Set('a', 'b', 'c')('b') gives true whereas Set()('a') gives false. Finally for maps, apply is a selection. For instance, Map('a' -> 1, 'b' -> 10, 'c' -> 100)('b') gives 10.

In the following, we will explain each of the three kinds of collections in more detail.

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