Instance Method

firstIndex(where:)

Returns the first index in which an element of the collection satisfies the given predicate.

Declaration

func firstIndex(where predicate: (Bound) throws -> Bool) rethrows -> Bound?
Available when Bound conforms to Strideable and Bound.Stride conforms to SignedInteger.

Parameters

predicate

A closure that takes an element as its argument and returns a Boolean value that indicates whether the passed element represents a match.

Return Value

The index of the first element for which predicate returns true. If no elements in the collection satisfy the given predicate, returns nil.

Discussion

You can use the predicate to find an element of a type that doesn’t conform to the Equatable protocol or to find an element that matches particular criteria. Here’s an example that finds a student name that begins with the letter “A”:

let students = ["Kofi", "Abena", "Peter", "Kweku", "Akosua"]
if let i = students.firstIndex(where: { $0.hasPrefix("A") }) {
    print("\(students[i]) starts with 'A'!")
}
// Prints "Abena starts with 'A'!"

Complexity: O(n), where n is the length of the collection.

See Also

Finding Elements

func first(where: (Bound) -> Bool) -> Bound?

Returns the first element of the sequence that satisfies the given predicate.

func firstIndex(of: Bound) -> Bound?

Returns the first index where the specified value appears in the collection.

func last(where: (Bound) -> Bool) -> Bound?

Returns the last element of the sequence that satisfies the given predicate.

func lastIndex(of: Bound) -> Bound?

Returns the last index where the specified value appears in the collection.

func lastIndex(where: (Bound) -> Bool) -> Bound?

Returns the index of the last element in the collection that matches the given predicate.

func min() -> Bound?

Returns the minimum element in the sequence.

func min(by: (Bound, Bound) -> Bool) -> Bound?

Returns the minimum element in the sequence, using the given predicate as the comparison between elements.

func max() -> Bound?

Returns the maximum element in the sequence.

func max(by: (Bound, Bound) -> Bool) -> Bound?

Returns the maximum element in the sequence, using the given predicate as the comparison between elements.