collections

Trait Traversable

At the top of the collection hierarchy is trait Traversable. Its only abstract operation is foreach:

def foreach[U](f: Elem => U) 

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

The foreach method is meant to traverse all elements of the collection, and apply the given operation, f, to each element. The type of the operation is Elem => U, where Elem is the type of the collection’s elements and U is an arbitrary result type. The invocation of f is done for its side effect only; in fact any function result of f is discarded by foreach.

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

  • Addition, ++, which appends two traversables together, or appends all elements of an iterator to a traversable.
  • 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 a Traversable 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 copyToBuffer and copyToArray. As their names imply, these copy collection elements to a buffer or array, respectively.
  • Size info operations isEmpty, nonEmpty, size, and hasDefiniteSize: Traversable collections can be finite or infinite. An example of an infinite traversable collection is the stream of natural numbers Stream.from(0). The method hasDefiniteSize indicates whether a collection is possibly infinite. If hasDefiniteSize returns true, the collection is certainly finite. If it returns false, the collection has not been not fully elaborated yet, so it might be infinite or finite.
  • 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, groupBy, 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, stringPrefix, which give alternative ways of converting a collection to a string.
  • View operations, consisting of two overloaded variants of the view method. A view is a collection that’s evaluated lazily. You’ll learn more about views in later.

Operations in Class Traversable

WHAT IT IS WHAT IT DOES
—— ——
Abstract Method:  
xs foreach f Executes function f for every element of xs.
Addition:  
xs ++ ys A collection consisting of the elements of both xs and ys. ys is a TraversableOnce collection, i.e., either a Traversable 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.toStream Converts the collection to a lazily computed stream.
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.
Copying:  
xs copyToBuffer buf Copies all elements of the collection to buffer buf.
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.hasDefiniteSize True if xs is known to have finite size.
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 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 two 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.
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:  
(z /: xs)(op) Apply binary operation op between successive elements of xs, going left to right and starting with z.
(xs :\ z)(op) Apply binary operation op between successive elements of xs, going right to left and starting with z.
xs.foldLeft(z)(op) Same as (z /: xs)(op).
xs.foldRight(z)(op) Same as (xs :\ z)(op).
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.
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.
Views:  
xs.view Produces a view over xs.
xs view (from, to) Produces a view that represents the elements in some index range of xs.
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