循环语句

/// Calls the given closure on each element in the sequence in the same order
    /// as a `for`-`in` loop.
    ///
    /// The two loops in the following example produce the same output:
    ///
    ///     let numberWords = ["one", "two", "three"]
    ///     for word in numberWords {
    ///         print(word)
    ///     }
    ///     // Prints "one"
    ///     // Prints "two"
    ///     // Prints "three"
    ///
    ///     numberWords.forEach { word in
    ///         print(word)
    ///     }
    ///     // Same as above
    ///
    /// Using the `forEach` method is distinct from a `for`-`in` loop in two
    /// important ways:
    ///
    /// 1. You cannot use a `break` or `continue` statement to exit the current
    ///    call of the `body` closure or skip subsequent calls.
    /// 2. Using the `return` statement in the `body` closure will exit only from
    ///    the current call to `body`, not from any outer scope, and won't skip
    ///    subsequent calls.
    ///
    /// - Parameter body: A closure that takes an element of the sequence as a
    ///   parameter.
    public func forEach(_ body: (Element) throws -> Void) rethrows

• 数组的 for each函数和 for in 循环的区别
  forEach 不能调用continue,break
  forEach 调用Return 退出当前闭包
• for in (数组 数据区间 字典)
• while
• repeat while