Swift_Array.firstIndex(of: eleme

【使用】：返回指定值在集合中出现的索引。
值得注意的是：element在数组中有可能不存在，此时会返回nil。

/// Returns the first index where the specified value appears in the collection.
/// : 返回指定值在集合中出现的索引。
//
/// After using `firstIndex(of:)` to find the position of a particular element
/// in a collection, you can use it to access the element by subscripting.
/// This example shows how you can modify one of the names in an array of
/// students.
// : 在使用' firstIndex(of:) '查找集合中特定元素的位置之后，可以使用它通过下标访问或修改元素。这个示例展示了如何修改学生数组中的一个名称。
///
///     var students = ["Ben", "Ivy", "Jordell", "Maxime"]
///     if let i = students.firstIndex(of: "Maxime") {
///         students[i] = "Max"
///     }
///     print(students)
///     // Prints "["Ben", "Ivy", "Jordell", "Max"]"
///
/// - Parameter element: An element to search for in the collection.
/// - Returns: The first index where `element` is found. If `element` is not
///   found in the collection, returns `nil`.
// ：在集合中找到element时，返回下标，否则返回nil.
///
    @inlinable public func firstIndex(of element: Element) -> Int?