#清除所有变量
rm(list=ls())

#清除单个变量
rm(data)

#清除多个变量
rm(list=c(‘data1‘,‘data2‘))

> str(a)
‘data.frame‘:	13 obs. of  3 variables:
 $ class: Factor w/ 7 levels "浜岀彮","涓夌彮",..: 7 6 5 4 3 1 1 1 1 2 ...
 $ name : Factor w/ 10 levels "","122","闄堜附鐏\xb5,115",..: 1 2 1 1 2 8 7 6 5 4 ...
 $ score: int  NA NA NA NA NA 119 96 NA NA NA ...

> attributes(a)
$names
[1] "class" "name"  "score"

$class
[1] "data.frame"

$row.names
 [1]  1  2  3  4  5  6  7  8  9 10 11 12 13

> vector(‘numeric‘,5)#numeric(5)
[1] 0 0 0 0 0
> vector(‘logical‘,5)#logical(5)
[1] FALSE FALSE FALSE FALSE FALSE
> vector(‘character‘,5)#character(5)
[1] "" "" "" "" ""
> vector(‘list‘,2)
[[1]]
NULL

[[2]]
NULL

> letters[4]
[1] "d"
> letters[1:4]
[1] "a" "b" "c" "d"
> letters[]
 [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l"
[13] "m" "n" "o" "p" "q" "r" "s" "t" "u" "v" "w" "x"
[25] "y" "z"

#runif(n,min,max),默认产生0-1之间的随机数
> runif(3)
[1] 0.3308360 0.5573855 0.9720616
> runif(3,min=1,max=9)
[1] 8.946071 4.915268 5.763918
#rnom可指定参数mean:平均数，sd:标准差，默认mean=0,sd=1
> rnorm(5)
[1] 0.55080064 -0.24792330 -0.09480838 0.65215600
[5] -0.96422061
> rnorm(5,mean=2,sd=0.3)
[1] 2.373522 2.176204 1.948216 2.371155 2.070872

#seq.int创建一个序列，与冒号有所不同，指定步长时也不一样
> 2:8
[1] 2 3 4 5 6 7 8
> seq.int(2:8)
[1] 1 2 3 4 5 6 7
> seq.int(4,10,3)#指定步长3
[1]  4  7 10
#seq_len函数创建一个从1到它的输入值的序列，输入为0时创建一个空值
> seq_len(0)
integer(0)
> attributes(seq_len(0))
NULL
#seq_along创建一个从1开始、长度为其输入值的序列
> seq_along(letters[1:10])
 [1]  1  2  3  4  5  6  7  8  9 10

#整体重复三遍
> rep(1:5,3)
 [1] 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5
#每个元素重复三遍
> rep(1:5,each=3)
 [1] 1 1 1 2 2 2 3 3 3 4 4 4 5 5 5
#每个元素重复个数：1:5
> rep(1:5,times=1:5)
 [1] 1 2 2 3 3 3 4 4 4 4 5 5 5 5 5
#重复矢量直到达到15的长度
> rep(1:5,length.out=15)
 [1] 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5

#array创建数组，另个参数：值，维度dim
> array(1:12,dim=c(3,4))
     [,1] [,2] [,3] [,4]
[1,]    1    4    7   10
[2,]    2    5    8   11
[3,]    3    6    9   12
#matrix创建矩阵，需要指定行数或者列数，默认输入的值按照列填充，或者指定参数byrow=TRUE按行填充
> matrix(1:12,nrow=3)
     [,1] [,2] [,3] [,4]
[1,]    1    4    7   10
[2,]    2    5    8   11
[3,]    3    6    9   12
> matrix(1:12,nrow=3,byrow=TRUE)
     [,1] [,2] [,3] [,4]
[1,]    1    2    3    4
[2,]    5    6    7    8
[3,]    9   10   11   12

> length(1:5)
[1] 5
#缺失值NA也被计入长度
> length(c(TRUE,FALSE,NA))
[1] 3
#返回字符串的数目
> length(c(‘A‘,‘B‘,‘C‘))
[1] 3
#若想返回每个字符串中字符的长度用nchar
> nchar(c(‘Axc‘,‘Bcdsfs‘,‘Cdsss‘))
[1] 3 6 5

#kiwi fruit用引号括起来了，因为其名称无效：有空格
#可以忽略其他元素命名
> c(apple=1,"kiwi fruit"=2,3)
     apple kiwi fruit            
         1          2          3 
> x <- 1:3
> names(x) <- c("apple","kiwi fruit",‘‘)
> x
     apple kiwi fruit            
         1          2          3 
#names函数返回名称，若没有一个元素有名字则返回NULL
> names(x)
[1] "apple"      "kiwi fruit" ""   
> names（1:4）
NULL
#创建数组时命名，矩阵同样
> x <- array(1:6,dim=c(2,3),dimnames=list(c("one","tow"),c("ein","zwei","drei")))
> x
ein zwei drei
one 1 3 5
tow 2 4 6

#rownames、colnames和dimnames函数
> rownames(x)
[1] "one" "tow"
> colnames(x)
[1] "ein" "zwei" "drei"
> dimnames(x)
[[1]]
[1] "one" "tow"

[[2]]
[1] "ein" "zwei" "drei"

> x <- matrix(1:12,nrow=3);x
     [,1] [,2] [,3] [,4]
[1,]    1    4    7   10
[2,]    2    5    8   11
[3,]    3    6    9   12
> y <- matrix(13:24,nrow=3);y
     [,1] [,2] [,3] [,4]
[1,]   13   16   19   22
[2,]   14   17   20   23
[3,]   15   18   21   24
> c(x,y)
 [1]  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16
[17] 17 18 19 20 21 22 23 24
> cbind(x,y)
     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
[1,]    1    4    7   10   13   16   19   22
[2,]    2    5    8   11   14   17   20   23
[3,]    3    6    9   12   15   18   21   24
> rbind(x,y)
     [,1] [,2] [,3] [,4]
[1,]    1    4    7   10
[2,]    2    5    8   11
[3,]    3    6    9   12
[4,]   13   16   19   22
[5,]   14   17   20   23
[6,]   15   18   21   24

向量内乘
> 1:3%*%2:4
     [,1]
[1,]   20
#向量外乘
> 1:3%o%2:4
     [,1] [,2] [,3]
[1,]    2    3    4
[2,]    4    6    8
[3,]    6    9   12
> x<-matrix(1:6,nrow=2);x
     [,1] [,2] [,3]
[1,]    1    3    5
[2,]    2    4    6
> y<-matrix(1:6,nrow=3);y
     [,1] [,2]
[1,]    1    4
[2,]    2    5
[3,]    3    6
#矩阵相乘
> x%*%y
     [,1] [,2]
[1,]   22   49
[2,]   28   64
> x<-matrix(runif(9),nrow=3);x
          [,1]       [,2]      [,3]
[1,] 0.6136448 0.05304645 0.7378249
[2,] 0.1922476 0.18177634 0.9545918
[3,] 0.9050422 0.65400112 0.2954112
#矩阵求逆
> solve(x)
           [,1]      [,2]       [,3]
[1,]  1.6984882 -1.389698  0.2484943
[2,] -2.4026083  1.448092  1.3214365
[3,]  0.1154494  1.051693 -0.3016769
#矩阵转置
> t(x)
           [,1]      [,2]       [,3]
[1,] 0.98387863 0.6292973 0.03753178
[2,] 0.09061386 0.5971163 0.10537436
[3,] 0.80727162 0.5403768 0.55767250