Generic Instance Method

starts(with:)

Returns a Boolean value indicating whether the initial elements of the sequence are the same as the elements in another sequence.

Declaration

func starts<PossiblePrefix>(with possiblePrefix: PossiblePrefix) -> Bool where PossiblePrefix : Sequence, Element == PossiblePrefix.Element
Available when Element conforms to Equatable.

Parameters

possiblePrefix

A sequence to compare to this sequence.

Return Value

true if the initial elements of the sequence are the same as the elements of possiblePrefix; otherwise, false. If possiblePrefix has no elements, the return value is true.

Discussion

This example tests whether one countable range begins with the elements of another countable range.

let a = 1...3
let b = 1...10

print(b.starts(with: a))
// Prints "true"

Passing a sequence with no elements or an empty collection as possiblePrefix always results in true.

print(b.starts(with: []))
// Prints "true"

Complexity: O(m), where m is the lesser of the length of the sequence and the length of possiblePrefix.