Subsetting （提取子集）

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.

Subsetting a Matrix 

Matrices can be subsetted in the usual way with (i,j) type indices.

By default, when a single element of a matrix is retrieved, it is returned as a vector of length 1 rather than a 1 × 1 matrix. This behavior can be turned off by setting drop = FALSE.

Partial Matching （模糊匹配）

Partial matching of names is allowed with [[ and $.

Removing NA Values 

A common task is to remove missing values (NAs).

（1）移除单一对象中的缺失值

（2)移除多个对象中的缺失值，使用complete.cases函数。