let tools = ["a", "b", "c", "d"]
let index = 1
let target = "a"

let leftDirectionArray = Array(tools.reversed())
let leftDirection = leftDirectionArray[(tools.count - index) - 1 ..< tools.count]
print("leftDirectionArray: \(leftDirectionArray)")
print("leftDirection: \(leftDirection)")
print("leftDirection count: \(leftDirection.count)")

let firstIndexA = leftDirection.firstIndex { $0 == target } ?? 0
let firstIndexB = leftDirection.firstIndex(of: target) ?? 0

print("firstIndexA: \(firstIndexA)")
print("firstIndexB: \(firstIndexB)")

Outputs

leftDirectionArray: ["d", "c", "b", "a"]
leftDirection: ["b", "a"]
leftDirection count: 2
firstIndexA: 3
firstIndexB: 3

So my question is why is the return for firstIndex is 3 when there should be only 2 in the array? does it mean the firstIndex still does the operation on the original array?

CodePudding user response：

leftDirection is not an Array but an ArraySlice and this is described in the documentation as

an ArraySlice instance presents a view onto the storage of a larger array

So the elements you see are the elements of leftDirectionArray and their position in the real array

print(type(of: leftDirection))
leftDirection.indices.forEach { print($0) }

ArraySlice
2
3