Subsetting 

There are a number of operators that can be used to extract subsets of R objects.

• [ always returns an object of the same class as the original; can be used to select more than one element (there is one exception)
• [[ is used to extract elements of a list or a data frame; it can only be used to extract a single element and the class of the returned object will not necessarily be a list or data frame
• $ is used to extract elements of a list or data frame by name; semantics(语义） are similar to that of [[. 

subsetting vector

前三个属于数字索引，后三个是逻辑索引。

Subsetting Lists 

The [[ operator can be used with computed indices（计算指令）; $ can only be used with literal names.(字面名称）

The [[ can take an integer sequence.