# starts(with:by:) Returns a Boolean value indicating whether the initial elements of the sequence are equivalent to the elements in another sequence, using the given predicate as the equivalence test. ``` func starts(with possiblePrefix: PossiblePrefix, by areEquivalent: (Self.Element, PossiblePrefix.Element) throws -> Bool) rethrows -> Bool where PossiblePrefix : Sequence ``` ## Parameters `possiblePrefix` A sequence to compare to this sequence. `areEquivalent` A predicate that returns `true` if its two arguments are equivalent; otherwise, `false`. ## Return Value `true` if the initial elements of the sequence are equivalent to the elements of `possiblePrefix`; otherwise, `false`. If `possiblePrefix` has no elements, the return value is `true`. ## Discussion The predicate must be an *equivalence relation* over the elements. That is, for any elements `a`, `b`, and `c`, the following conditions must hold: - `areEquivalent(a, a)` is always `true`. (Reflexivity) - `areEquivalent(a, b)` implies `areEquivalent(b, a)`. (Symmetry) - If `areEquivalent(a, b)` and `areEquivalent(b, c)` are both `true`, then `areEquivalent(a, c)` is also `true`. (Transitivity) > Complexity: O(*m*), where *m* is the lesser of the length of the > sequence and the length of `possiblePrefix`. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)