AdjacentPairs

[Source | Tests]

Lazily iterates over tuples of adjacent elements.

let numbers = (1...5)
let pairs = numbers.adjacentPairs()
// Array(pairs) == [(1, 2), (2, 3), (3, 4), (4, 5)]

Detailed Design

The adjacentPairs() method is declared as a Sequence extension returning AdjacentPairsSequence and as a Collection extension returning AdjacentPairsCollection.

extension Sequence {
    public func adjacentPairs() -> AdjacentPairsSequence<Self>
}

extension Collection {
    public func adjacentPairs() -> AdjacentPairsCollection<Self>
}

The AdjacentPairsSequence type is a sequence, and the AdjacentPairsCollection type is a collection with conditional conformance to BidirectionalCollection and RandomAccessCollection when the underlying collection conforms.

Complexity

Calling adjacentPairs is an O(1) operation.

Naming

This method is named for clarity while remaining agnostic to any particular domain of programming. In natural language processing, this operation is akin to computing a list of bigrams; however, this algorithm is not specific to this use case.

Comparison with other languages

This function is often written as a zip of a sequence together with itself, minus its first element.

Haskell: This operation is spelled s `zip` tail s.

Python: Python users may write zip(s, s[1:]) for a list with at least one element. For natural language processing, the nltk package offers a bigrams function akin to this method.

Note that in Swift, the spelling zip(s, s.dropFirst()) is undefined behavior for a single-pass sequence s.