Contents

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.

Declaration

func starts<PossiblePrefix>(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)