R语言统计分析与应用-第二章-4

Apr 19, 2019 8 分钟阅读时长 R, 统计

2.1.2 R运算符

这个部分相对简单些。R中的运算符分为算数运算符、比较算符、逻辑算符。

算数运算符

就是指加减乘除等等啦，见下表：

算数算符	含义
+	加
-	减
*	乘
/	除
^	幂
%%	模运算
%/%	整数除法

比较算符

就是大于、小于、不等于等等啦，见下表：

比较算符	含义
==	等于
!=	不等于
>	大于
<	小于
>=	大于等于
<+	小于等于

逻辑算符

其实就是数学中的或与非啦，见下表：

逻辑算符	含义
&&	标量的“与”运算

&	向量的“与”运算

!	非

关于这个就不啰嗦了，或与非其实是高中数学内容了。大家应该注意到上面说到了标量这个词，在R中，表面上没有标量的类型，但实际上它可以看做是含有一个元素的向量。下面的例子可以看出逻辑运算符对标量和向量的区别：

x <- c(TRUE,FALSE,TRUE)
y <- c(TRUE,TRUE,FALSE)
x & y

## [1]  TRUE FALSE FALSE

x[1] && y[1]

## [1] TRUE

x && y

## [1] TRUE

if (x[1] && y[1])
  print("both true")

## [1] "both true"

if (x & y)
  print("both true")

## Warning in if (x & y) print("both true"): 条件的长度大于一，因此只能用其第
## 一元素

## [1] "both true"

可以看到最后一条语句报错了“the condition has length > 1 and only the first element will be used”。这是因为if结构判断语句的取值，只能是一个逻辑值，而不是逻辑值的向量。

运算次序

这个~~~没啥好讲的~~~

2.2 R常用函数及其应用

嗯~~前面讲了很多R的基本结构，向量啦，矩阵啦什么的，现在再进一步，开始讲一下R的常用函数。

2.2.1 数学函数

#绝对值
abs(-1)

## [1] 1

#平方根
sqrt(36)

## [1] 6

25^(0.5)

## [1] 5

#不小于x的最小整数
ceiling(3.1415926)

## [1] 4

#不大于x的最大整数
floor(3.1415926)

## [1] 3

#向0的方向截取x中的整数部分
trunc(5.99)

## [1] 5

#将x舍入为指定位数的小数
round(3.1415926, digits = 2)

## [1] 3.14

#将x舍入为指定位数的有效数字
signif(3.1415926, digits = 2)

## [1] 3.1

#一些三角函数
cos(3.1415926)

## [1] -1

sin(3.1415926)

## [1] 5.358979e-08

tan(3.1415926)

## [1] -5.358979e-08

acos(3.1415926)

## Warning in acos(3.1415926): 产生了NaNs

## [1] NaN

asin(3.1415926)

## Warning in asin(3.1415926): 产生了NaNs

## [1] NaN

atan(3.1415926)

## [1] 1.262627

#双曲余弦
cosh(3.1415926)

## [1] 11.59195

#双曲正弦
sinh(3.1415926)

## [1] 11.54874

#反双曲余弦、正弦
acosh(3.1415926)

## [1] 1.811526

asinh(3.1415926)

## [1] 1.862296

#log(x, base=n)，对x取以n为底的对数
log(8,2)

## [1] 3

#取自然对数
log(8)

## [1] 2.079442

#常用对数
log10(8)

## [1] 0.90309

log(8,10)

## [1] 0.90309

#指数函数
exp(2)

## [1] 7.389056

2.2.2 样本统计函数

#平均数
mean(c(1,2,3,4))

## [1] 2.5

#中位数
median(c(1,2,3,4))

## [1] 2.5

median(c(1,2,3,4,5))

## [1] 3

#标准差
sd(c(1,2,3,4))

## [1] 1.290994

#方差
var(c(1,2,3,4))

## [1] 1.666667

#绝对中位差
mad(c(1,2,3,4))

## [1] 1.4826

#quantile(x,probs)，求分位数，x为待求分位数的数值型向量
#probs为一个由[0,1]之间的概率组成的数值向量
#求向量x的第25和75百分位数
quantile(c(1,2,3,4,5,6,7), c(.25,.75))

## 25% 75% 
## 2.5 5.5

#求值域
range(c(1,2,3,4,5,6,7))

## [1] 1 7

#求和
sum(c(1,2,3,4,5,6,7))

## [1] 28

#最大值
max(c(1,2,3,4,5,6,7))

## [1] 7

#最小值
min(c(1,2,3,4,5,6,7))

## [1] 1

#scale(x,center=TRUE,scale=TRUE)
#将数据对象x按列进行中心化（center=TRUE）
#或标准化（center=TRUE,scale=TRUE）
#默认情况下，这个函数对矩阵或数据框的指定列进行均值为0
#标准差为1的标准化
mydata <- matrix(c(1,2,3,4,5,6), nrow = 2)
scale(mydata)

##            [,1]       [,2]       [,3]
## [1,] -0.7071068 -0.7071068 -0.7071068
## [2,]  0.7071068  0.7071068  0.7071068
## attr(,"scaled:center")
## [1] 1.5 3.5 5.5
## attr(,"scaled:scale")
## [1] 0.7071068 0.7071068 0.7071068

#要对每一列进行任意均值和标准差的标准化
#可以采用下面的语句：mydata <- scale(mydata)*SD+M
#其中M是想要的均值，SD是想要的标准差
scale(mydata)*0.25+3

##          [,1]     [,2]     [,3]
## [1,] 2.823223 2.823223 2.823223
## [2,] 3.176777 3.176777 3.176777
## attr(,"scaled:center")
## [1] 1.5 3.5 5.5
## attr(,"scaled:scale")
## [1] 0.7071068 0.7071068 0.7071068

2.2.4 字符处理函数

前面讲了很多函数，不过都是数值型的，现在接下来要讲的是有关字符处理的函数。字符处理函数可以从文本型数据中抽取信息，或者为打印输出和生成报告重设文本的格式。

#返回字符串x的字符数量
x <- c("ab","cde","dsdesd")
length(x)

## [1] 3

nchar(x)

## [1] 2 3 6

nchar(x[3])

## [1] 6

#substr(x, start, stop)返回字符串x中指定位置范围的子字符串
x <- "abcdefghij"
substr(x, 2, 4)

## [1] "bcd"

substr(x, 2, 4) <- "22222"
x

## [1] "a222efghij"

#grep(pattern, x, ignore.case=FALSE, fixed=FALSE)
#在字符串x中搜索给定的子字符串pattern。
#若fixed=FALSE，则pattern为一个正则表达式
#若fixed=TRUE，则pattern为一个文本字符串，返回值为匹配的下标
grep("A",c("b","A","c"), fixed = TRUE)

## [1] 2

grep("A",c("b","A","c"), fixed = FALSE)

## [1] 2

#sub(pattern, replacement, x, ignore.case=FALSE, fixed=FALSE)在x中搜索pattern，并以文本replacement将其替换。
#若fixed=FALSE，则pattern为一个正则表达式
#若fixed=TRUE，则pattern为一个文本字符串
#下例中“\s”是一个查找空白的正则表达式
#使用“\\s”而不是“\”的原因在于后者是R中的转义字符
sub("\\s", ".", "Hello There")

## [1] "Hello.There"

#strsplit(x,split,fixed=FALSE)将在以split处分割字符向量x中
#将元素拆分为若干个子字符串，返回这些子字符串的列表
#若fixed=FALSE，则pattern为一个正则表达式
#若fixed=TRUE，则pattern为一个文本字符串
y <- strsplit("abc","")
y

## [[1]]
## [1] "a" "b" "c"

unlist(y)[2]

## [1] "b"

sapply(y,"[",2)

## [1] "b"

strsplit("6-16-2011",split = "-")

## [[1]]
## [1] "6"    "16"   "2011"

#paste(……, sep="")把若干个字符串拼接在一起，分隔符为sep
paste("x", 1:3, sep = "")

## [1] "x1" "x2" "x3"

paste("x", 1:3, sep = "M")

## [1] "xM1" "xM2" "xM3"

paste("Today is", date())

## [1] "Today is Fri Apr 19 20:30:41 2019"

paste("North", "Pole")

## [1] "North Pole"

paste("North", "Pole", sep = " ")

## [1] "North Pole"

paste("North", "Pole", sep = "")

## [1] "NorthPole"

paste("North", "Pole", sep = ".")

## [1] "North.Pole"

paste("North", "and", "Pole", "South", sep = "")

## [1] "NorthandPoleSouth"

#大写转换
toupper("abc")

## [1] "ABC"

#小写转换
tolower("ABC")

## [1] "abc"

#regexpr(pattern,x)在字符串x中寻找pattern
#返回与pattern匹配的第一个子字符串的起始字符位置
regexpr("uat", "Equator")

## [1] 3
## attr(,"match.length")
## [1] 3
## attr(,"index.type")
## [1] "chars"
## attr(,"useBytes")
## [1] TRUE

#gregexpr(pattern,x)与前一个功能一样
#不过它会寻找与pattern匹配的全部子字符串的起始位置
gregexpr("iss", "Missppiissist")

## [[1]]
## [1] 2 8
## attr(,"match.length")
## [1] 3 3
## attr(,"index.type")
## [1] "chars"
## attr(,"useBytes")
## [1] TRUE

gregexpr("uat", "Equator")

## [[1]]
## [1] 3
## attr(,"match.length")
## [1] 3
## attr(,"index.type")
## [1] "chars"
## attr(,"useBytes")
## [1] TRUE

2.2.5 其他实用函数

对象x的长度：

x <- c(1,2,3,4,5,6)
length(x)

## [1] 6

生成一个序列：

mysequ <- seq(1,20,3)
mysequ

## [1]  1  4  7 10 13 16 19

将x重复n次：

rep("ABC",3)

## [1] "ABC" "ABC" "ABC"

rep(1:3,3)

## [1] 1 2 3 1 2 3 1 2 3

将连续型变量x分割为有着n个水平的因子：

x <- c(1:1000)
cut(x, 5)

##    [1] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##    [6] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##   [11] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##   [16] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##   [21] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##   [26] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##   [31] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##   [36] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##   [41] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##   [46] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##   [51] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##   [56] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##   [61] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##   [66] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##   [71] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##   [76] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##   [81] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##   [86] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##   [91] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##   [96] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##  [101] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##  [106] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##  [111] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##  [116] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##  [121] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##  [126] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##  [131] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##  [136] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##  [141] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##  [146] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##  [151] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##  [156] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##  [161] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##  [166] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##  [171] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##  [176] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##  [181] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##  [186] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##  [191] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##  [196] (0.001,201] (0.001,201] (0.001,201] (0.001,201] (0.001,201]
##  [201] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [206] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [211] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [216] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [221] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [226] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [231] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [236] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [241] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [246] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [251] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [256] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [261] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [266] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [271] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [276] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [281] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [286] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [291] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [296] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [301] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [306] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [311] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [316] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [321] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [326] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [331] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [336] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [341] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [346] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [351] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [356] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [361] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [366] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [371] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [376] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [381] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [386] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [391] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [396] (201,401]   (201,401]   (201,401]   (201,401]   (201,401]  
##  [401] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [406] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [411] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [416] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [421] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [426] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [431] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [436] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [441] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [446] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [451] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [456] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [461] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [466] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [471] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [476] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [481] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [486] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [491] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [496] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [501] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [506] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [511] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [516] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [521] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [526] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [531] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [536] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [541] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [546] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [551] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [556] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [561] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [566] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [571] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [576] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [581] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [586] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [591] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [596] (401,600]   (401,600]   (401,600]   (401,600]   (401,600]  
##  [601] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [606] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [611] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [616] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [621] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [626] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [631] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [636] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [641] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [646] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [651] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [656] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [661] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [666] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [671] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [676] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [681] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [686] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [691] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [696] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [701] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [706] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [711] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [716] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [721] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [726] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [731] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [736] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [741] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [746] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [751] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [756] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [761] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [766] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [771] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [776] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [781] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [786] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [791] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [796] (600,800]   (600,800]   (600,800]   (600,800]   (600,800]  
##  [801] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [806] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [811] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [816] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [821] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [826] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [831] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [836] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [841] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [846] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [851] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [856] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [861] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [866] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [871] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [876] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [881] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [886] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [891] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [896] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [901] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [906] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [911] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [916] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [921] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [926] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [931] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [936] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [941] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [946] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [951] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [956] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [961] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [966] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [971] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [976] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [981] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [986] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [991] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
##  [996] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03] (800,1e+03]
## Levels: (0.001,201] (201,401] (401,600] (600,800] (800,1e+03]

创建美观的分割点。通过选取n+1个等间距的取整值，将一个连续型变量x分割为n个区间：

x <- pretty(c(-3,3), 30)
x

##  [1] -3.0 -2.8 -2.6 -2.4 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4
## [15] -0.2  0.0  0.2  0.4  0.6  0.8  1.0  1.2  1.4  1.6  1.8  2.0  2.2  2.4
## [29]  2.6  2.8  3.0

R 统计