In Swift Array, is there a function that returns the last index based in where clause?

Question

The function index of an Swift array returns the first element based on the condition inside the where clause. Is there a way to get the last element with this condition?

For example, I want something like this (I know there is no function called lastIndex. This function or similar is what I'm searching for):

let array = [1, 2, 3, 4, 5, 3, 6]

let indexOfLastElementEquals3 = array.lastIndex(where: { $0 == 3 })

print(indexOfLastElementEquals3) //5 (Optional)

Answer 1

lastIndex(where:) and related methods were added in Swift 4.2, see

• SE-0204 Add last(where:) and lastIndex(where:) Methods.

In earlier Swift versions you can use index(where:) on the reversed view of the collection:

let array = [1, 2, 3, 4, 5, 3, 6]

if let revIndex = array.reversed().index(where: { $0 % 2 != 0 } ) {
    let indexOfLastOddElement = array.index(before: revIndex.base)
    print(indexOfLastOddElement) // 5
}

Or as a single expression:

let indexOfLastOddElement =  array.reversed().index(where: { $0 % 2 != 0 } )
    .map { array.index(before: $0.base) }

print(indexOfLastOddElement) // Optional(5)

revIndex.base returns the position after the position of revIndex in the underlying collection, that's why we have to “subtract” one from the index.

For arrays this can be simplified to

    let indexOfLastOddElement = revIndex.base - 1

but for collections with non-integer indices (like String) the above index(before:) methods is needed.