# base The position after this position in the underlying collection. ``` let base: Base.Index ``` ## Discussion To find the position that corresponds with this index in the original, underlying collection, use that collection’s `index(before:)` method with the `base` property. The following example declares a function that returns the index of the last even number in the passed array, if one is found. First, the function finds the position of the last even number as a `ReversedIndex` in a reversed view of the array of numbers. Next, the function calls the array’s `index(before:)` method to return the correct position in the passed array. ``` func indexOfLastEven(_ numbers: [Int]) -> Int? { let reversedNumbers = numbers.reversed() guard let i = reversedNumbers.firstIndex(where: { $0 % 2 == 0 }) else { return nil } return numbers.index(before: i.base) } let numbers = [10, 20, 13, 19, 30, 52, 17, 40, 51] if let lastEven = indexOfLastEven(numbers) { print("Last even number: \(numbers[lastEven])") } // Prints "Last even number: 40" ``` --- 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)