parallel collections

Parallel Collection Conversions

Converting between sequential and parallel collections

Every sequential collection can be converted to its parallel variant using the par method. Certain sequential collections have a direct parallel counterpart. For these collections the conversion is efficient– it occurs in constant time, since both the sequential and the parallel collection have the same data-structural representation (one exception is mutable hash maps and hash sets which are slightly more expensive to convert the first time par is called, but subsequent invocations of par take constant time). It should be noted that for mutable collections, changes in the sequential collection are visible in its parallel counterpart if they share the underlying data-structure.

Sequential Parallel
Array ParArray
HashMap ParHashMap
HashSet ParHashSet
TrieMap ParTrieMap
Vector ParVector
Range ParRange
HashMap ParHashMap
HashSet ParHashSet

Other collections, such as lists, queues or streams, are inherently sequential in the sense that the elements must be accessed one after the other. These collections are converted to their parallel variants by copying the elements into a similar parallel collection. For example, a functional list is converted into a standard immutable parallel sequence, which is a parallel vector.

Every parallel collection can be converted to its sequential variant using the seq method. Converting a parallel collection to a sequential collection is always efficient– it takes constant time. Calling seq on a mutable parallel collection yields a sequential collection which is backed by the same store– updates to one collection will be visible in the other one.

Converting between different collection types

Orthogonal to converting between sequential and parallel collections, collections can be converted between different collection types. For example, while calling toSeq converts a sequential set to a sequential sequence, calling toSeq on a parallel set converts it to a parallel sequence. The general rule is that if there is a parallel version of X, then the toX method converts the collection into a ParX collection.

Here is a summary of all conversion methods:

Method Return Type
toArray Array
toList List
toIndexedSeq IndexedSeq
toStream Stream
toIterator Iterator
toBuffer Buffer
toTraversable GenTraverable
toIterable ParIterable
toSeq ParSeq
toSet ParSet
toMap ParMap
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