nth.swift

extension Collection {
  func nth(_ n: Int) -> Element? {
    assert(n >= 0, "Can't get a negative-th element")
    guard let i = index(startIndex, offsetBy: numericCast(n), limitedBy: endIndex),
      i != endIndex
      else { return nil }
    return self[i]
  }
}

let str = "Hello!"
str.nth(4)      // "o"
str.nth(6)      // nil

let nums = [10, 20, 30, 40]
nums.nth(3)     // 40
nums.nth(100)   // nil