R提供了两种不同的方法来访问列表或data.frame的元素-[]和[[]]运算符。

两者有什么区别？ 在什么情况下我应该使用另一个？

R语言定义可轻松回答以下类型的问题：

R has three basic indexing operators, with syntax displayed by the following examples

For vectors and matrices the [[ forms are rarely used, although they have some slight semantic differences from the [ form (e.g. it drops any names or dimnames attribute, and that partial matching is used for character indices). When indexing multi-dimensional structures with a single index, x[[i]] or x[i] will return the ith sequential element of x.

For lists, one generally uses [[ to select any single element, whereas [ returns a list of the selected elements.

The [[ form allows only a single element to be selected using integer or character indices, whereas [ allows indexing by vectors. Note though that for a list, the index can be a vector and each element of the vector is applied in turn to the list, the selected component, the selected component of that component, and so on. The result is still a single element.

两种方法之间的显着区别在于，它们在用于提取时返回的对象的类别，以及它们是否可以接受一定范围的值，或者在赋值期间仅接受单个值。

考虑以下列表中数据提取的情况：

为了帮助新手克服手动迷雾，将[[ ... ]]表示法视为折叠功能可能会有所帮助-换句话说，这就是当您只想从命名矢量，列表或数据框"获取数据"时。如果要使用这些对象中的数据进行计算，则最好这样做。这些简单的例子将说明。