collections

The sequence traits Seq, IndexedSeq, and LinearSeq

The Seq trait represents sequences. A sequence is a kind of iterable that has a length and whose elements have fixed index positions, starting from 0.

The operations on sequences, summarized in the table below, fall into the following categories:

  • Indexing and length operations apply, isDefinedAt, length, indices, and lengthCompare. For a Seq, the apply operation means indexing; hence a sequence of type Seq[T] is a partial function that takes an Int argument (an index) and which yields a sequence element of type T. In other words Seq[T] extends PartialFunction[Int, T]. The elements of a sequence are indexed from zero up to the length of the sequence minus one. The length method on sequences is an alias of the size method of general collections. The lengthCompare method allows you to compare the lengths of a sequences with an Int even if the sequences has infinite length.
  • Index search operations indexOf, lastIndexOf, indexOfSlice, lastIndexOfSlice, indexWhere, lastIndexWhere, segmentLength, prefixLength, which return the index of an element equal to a given value or matching some predicate.
  • Addition operations +:, :+, padTo, which return new sequences obtained by adding elements at the front or the end of a sequence.
  • Update operations updated, patch, which return a new sequence obtained by replacing some elements of the original sequence.
  • Sorting operations sorted, sortWith, sortBy, which sort sequence elements according to various criteria.
  • Reversal operations reverse, reverseIterator, reverseMap, which yield or process sequence elements in reverse order.
  • Comparisons startsWith, endsWith, contains, containsSlice, corresponds, which relate two sequences or search an element in a sequence.
  • Multiset operations intersect, diff, union, distinct, which perform set-like operations on the elements of two sequences or remove duplicates.

If a sequence is mutable, it offers in addition a side-effecting update method, which lets sequence elements be updated. As always in Scala, syntax like seq(idx) = elem is just a shorthand for seq.update(idx, elem), so update gives convenient assignment syntax for free. Note the difference between update and updated. update changes a sequence element in place, and is only available for mutable sequences. updated is available for all sequences and always returns a new sequence instead of modifying the original.

Operations in Class Seq

WHAT IT IS WHAT IT DOES
Indexing and Length:  
xs(i) (or, written out, xs apply i). The element of xs at index i.
xs isDefinedAt i Tests whether i is contained in xs.indices.
xs.length The length of the sequence (same as size).
xs lengthCompare n Returns -1 if xs is shorter than n, +1 if it is longer, and 0 if it is of length n. Works even if the sequence is infinite, for example Stream.from(1) lengthCompare 42 equals +1.
xs.indices The index range of xs, extending from 0 to xs.length - 1.
Index Search:  
xs indexOf x The index of the first element in xs equal to x (several variants exist).
xs lastIndexOf x The index of the last element in xs equal to x (several variants exist).
xs indexOfSlice ys The first index of xs such that successive elements starting from that index form the sequence ys.
xs lastIndexOfSlice ys The last index of xs such that successive elements starting from that index form the sequence ys.
xs indexWhere p The index of the first element in xs that satisfies p (several variants exist).
xs segmentLength (p, i) The length of the longest uninterrupted segment of elements in xs, starting with xs(i), that all satisfy the predicate p.
xs prefixLength p The length of the longest prefix of elements in xs that all satisfy the predicate p.
Additions:  
x +: xs A new sequence that consists of x prepended to xs.
xs :+ x A new sequence that consists of x appended to xs.
xs padTo (len, x) The sequence resulting from appending the value x to xs until length len is reached.
Updates:  
xs patch (i, ys, r) The sequence resulting from replacing r elements of xs starting with i by the patch ys.
xs updated (i, x) A copy of xs with the element at index i replaced by x.
xs(i) = x (or, written out, xs.update(i, x), only available for mutable.Seqs). Changes the element of xs at index i to x.
Sorting:  
xs.sorted A new sequence obtained by sorting the elements of xs using the standard ordering of the element type of xs.
xs sortWith lt A new sequence obtained by sorting the elements of xs using lt as comparison operation.
xs sortBy f A new sequence obtained by sorting the elements of xs. Comparison between two elements proceeds by mapping the function f over both and comparing the results.
Reversals:  
xs.reverse A sequence with the elements of xs in reverse order.
xs.reverseIterator An iterator yielding all the elements of xs in reverse order.
xs reverseMap f A sequence obtained by mapping f over the elements of xs in reverse order.
Comparisons:  
xs startsWith ys Tests whether xs starts with sequence ys (several variants exist).
xs endsWith ys Tests whether xs ends with sequence ys (several variants exist).
xs contains x Tests whether xs has an element equal to x.
xs containsSlice ys Tests whether xs has a contiguous subsequence equal to ys.
(xs corresponds ys)(p) Tests whether corresponding elements of xs and ys satisfy the binary predicate p.
Multiset Operations:  
xs intersect ys The multi-set intersection of sequences xs and ys that preserves the order of elements in xs.
xs diff ys The multi-set difference of sequences xs and ys that preserves the order of elements in xs.
xs union ys Multiset union; same as xs ++ ys.
xs.distinct A subsequence of xs that contains no duplicated element.

Trait Seq has two subtraits LinearSeq, and IndexedSeq. These do not add any new operations, but each offers different performance characteristics: A linear sequence has efficient head and tail operations, whereas an indexed sequence has efficient apply, length, and (if mutable) update operations. Frequently used linear sequences are scala.collection.immutable.List and scala.collection.immutable.Stream. Frequently used indexed sequences are scala.Array and scala.collection.mutable.ArrayBuffer. The Vector class provides an interesting compromise between indexed and linear access. It has both effectively constant time indexing overhead and constant time linear access overhead. Because of this, vectors are a good foundation for mixed access patterns where both indexed and linear accesses are used. You’ll learn more on vectors later.

Buffers

An important sub-category of mutable sequences is Buffers. They allow not only updates of existing elements but also element insertions, element removals, and efficient additions of new elements at the end of the buffer. The principal new methods supported by a buffer are += and ++= for element addition at the end, +=: and ++=: for addition at the front, insert and insertAll for element insertions, as well as remove and -= for element removal. These operations are summarized in the following table.

Two often used implementations of buffers are ListBuffer and ArrayBuffer. As the name implies, a ListBuffer is backed by a List, and supports efficient conversion of its elements to a List, whereas an ArrayBuffer is backed by an array, and can be quickly converted into one.

Operations in Class Buffer

WHAT IT IS WHAT IT DOES
Additions:  
buf += x Appends element x to buffer, and returns buf itself as result.
buf += (x, y, z) Appends given elements to buffer.
buf ++= xs Appends all elements in xs to buffer.
x +=: buf Prepends element x to buffer.
xs ++=: buf Prepends all elements in xs to buffer.
buf insert (i, x) Inserts element x at index i in buffer.
buf insertAll (i, xs) Inserts all elements in xs at index i in buffer.
Removals:  
buf -= x Removes element x from buffer.
buf remove i Removes element at index i from buffer.
buf remove (i, n) Removes n elements starting at index i from buffer.
buf trimStart n Removes first n elements from buffer.
buf trimEnd n Removes last n elements from buffer.
buf.clear() Removes all elements from buffer.
Cloning:  
buf.clone A new buffer with the same elements as buf.
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