在列表的指定位置插入新元素 [英] Insert a new element in a specified position of a list

问题描述

List 没有内置函数或方法可以允许用户在 List 的某个位置添加新元素.我已经编写了一个执行此操作的函数，但我不确定以这种方式执行此操作是否是一个好主意，即使它运行得非常好:

There is no built-in function or a method of a List that would allow user to add a new element in a certain position of a List. I've wrote a function that does this but I'm not sure that its a good idea to do it this way, even though it works perfectly well:

def insert(list: List[Any], i: Int, value: Any) = {
  list.take(i) ++ List(value) ++ list.drop(i)
}

用法:

scala> insert(List(1,2,3,5), 3, 4)
res62: List[Any] = List(1, 2, 3, 4, 5)

推荐答案

类型安全

我看到的最明显的事情是缺乏类型安全/丢失类型信息.我将使该方法在列表的元素类型中通用:

Type Safety

The most glaring thing I see is the lack of type safety / loss of type information. I would make the method generic in the list's element type:

def insert[T](list: List[T], i: Int, value: T) = {
  list.take(i) ++ List(value) ++ list.drop(i)
}

风格

如果正文只包含一个表达式，则不需要花括号:

Style

If the body only consists of a single expression, there is no need for curly braces:

def insert[T](list: List[T], i: Int, value: T) = 
  list.take(i) ++ List(value) ++ list.drop(i)

效率

@Marth 关于使用 List.splitAt 避免两次遍历列表也是一个好方法:

Efficiency

@Marth's comment about using List.splitAt to avoid traversing the list twice is also a good one:

def insert[T](list: List[T], i: Int, value: T) = {
  val (front, back) = list.splitAt(i)
  front ++ List(value) ++ back
}

界面

一次插入多个值可能会很方便:

Interface

It would probably be convenient to be able to insert more than one value at a time:

def insert[T](list: List[T], i: Int, values: T*) = {
  val (front, back) = list.splitAt(i)
  front ++ values ++ back
}

接口，取2

你可以把它作为List的扩展方法:

implicit class ListWithInsert[T](val list: List[T]) extends AnyVal {
  def insert(i: Int, values: T*) = {
    val (front, back) = list.splitAt(i)
    front ++ values ++ back
  }
}

List(1, 2, 3, 6).insert(3, 4, 5)
// => List(1, 2, 3, 4, 5, 6)

结束语

但是请注意，插入列表中间并不适合 cons 列表.使用(可变)链表或动态数组会更好.

Closing remarks

Note, however, that inserting into the middle of the list is just not a good fit for a cons list. You'd be much better off with a (mutable) linked list or a dynamic array instead.

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