1. Two concepts in R: objects and functions
Object: a storage space with an associated name;
Function: a special type of R objects designed to carry out some operation.
2. 赋值：<- (e.g: x<-45);
列出现有所有对象： ls() or objects()
3. vector: 1)mode: the kind of data stored; 2)length: the number of elements
使用c()创建length()>1的向量。（e.g. v<-c(1,2,3,4)）
获取向量v的长度：length(v); 获取向量v的mode: node(v).
某向量中只能包含同一mode的元素。
NA： special value，代表缺失值。
[]获取向量某元素。（e.g. v[2]）
Vectorization on a vector leads the called function to produce a vector of the same length, with each element resulting from applying the function to the respective element of the original vector
Recycling: repeating the shorter vector until it fills in the size of the larger vector if longer object length is a multiple of shorter object length. (e.g. v1<-c(4,6,8.24), v2<-(10,2), v1+v2 = 14 8 18 26)
Factor: ......
4. List: an ordered collection of other objects (a.k.a. components) 类似于结构体。
components may be of different mode, or length, but are always numbered and may also have a name attached to them. ( the names of the components of a list are, in effect, an attribute of the list)
unlist(): can unflatten all data in a list and will create a verctor with as many elements as there are data objects in a list.
5. data frame： 一类特殊的list。类似于两维矩阵，不过每一列可存储不同类型的元素。
使用R内置的电子表格式的界面编辑或新建data frame：
1）aDataSet <- edit(aDataSet): 编辑一个存在的data frame aDataSet
2）newDataSet <- eidt(data.frame()): 新建一个data frame newDataSet
6.函数，是一类特殊的对象。一个函数，存储的“值”是一集指令：这些指令，在该函数被调用时，将被执行。新建一个函数，就是使用赋值符 (<-)将函数的内容存储于函数名。
【 function： as an object, a function can store a value. the value stored in a function is the set of instructions that will be executed at a call of this function.】
function.name <- function(<形式参数>) {<一集指令>}
7. objects, classes and methods 对象、类与方法
everything in R is an object, and every R object belongs to a class.
The class of an object determines 1)可供使用的genenral方法；2）该类对象的表示。
note: general的说法，相对于继承该类的其它子类中，可能会有更specific的方法 （polymorphism 多态）
类的表示：the representation of a class consists of a set of slots. each slot has a name and an associated class that determines the informatuon that it stores