Instance Method

# firstIndex(where:)

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

## 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”:

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

### Finding Elements

`func contains(where: ((key: Key, value: Value)) -> Bool) -> Bool`

Returns a Boolean value indicating whether the sequence contains an element that satisfies the given predicate.

`func allSatisfy(((key: Key, value: Value)) -> Bool) -> Bool`

Returns a Boolean value indicating whether every element of a sequence satisfies a given predicate.

`func first(where: ((key: Key, value: Value)) -> Bool) -> (key: Key, value: Value)?`

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

`func min(by: ((key: Key, value: Value), (key: Key, value: Value)) -> Bool) -> (key: Key, value: Value)?`

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

`func max(by: ((key: Key, value: Value), (key: Key, value: Value)) -> Bool) -> (key: Key, value: Value)?`

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