R语言笔记（二)

因子（factor）

• 分类数据/有序与无序
• 整数向量+标签（优与整数向量）

  > x<-factor(c("female","female","male","male"))
  > x
  [1] female female male   male  
  Levels: female male
  x<-factor(c("female","female","male","male"),levels=c("male","female"))#levels谁在前谁就体现基线水平
  [1] female female male   male  
  Levels: male female
  > table(x)  #对当前因子有个整体的了解
  x
    male female 
       2      2 
  > unclass(x) #去掉因子的属性
  [1] 2 2 1 1
  attr(,"levels")
  [1] "male"   "female"
  > class(unclass(x))
  [1] "integer"

  缺失值

• NA（数字，字符等）/NaN（数字的缺失值）：NaN属于NA，NA不属于NaN

• is.na()/is.nan()

• > m<-c(1,NA,2,NA,3)
  > m
  [1]  1 NA  2 NA  3
  > is.na(m)
  [1] FALSE  TRUE FALSE  TRUE FALSE
  > is.nan(m)
  [1] FALSE FALSE FALSE FALSE FALSE

  数据框

• 存储表格数据

• 设为各元素长度相同的列表

• 每个元素列表-列数据

• 每个元素的长度代表行数

元素类型可以不同
 

> df <-data.frame(id=c(1,2,3,4),name=c("a","b","c","d"))
> df
  id name
1  1    a
2  2    b
3  3    c
4  4    d

> nrow(df)
[1] 4
> ncol(df)
[1] 2
> data.matrix(df)  #转换为矩阵
     id name
[1,]  1    1
[2,]  2    2
[3,]  3    3
[4,]  4    4

日期与时间

日期：Date

> dt<-date()
> dt
[1] "Fri Feb 28 11:25:28 2020"
> class(dt)
[1] "character"
> dt <-Sys.Date()
> dt
[1] "2020-02-28"
> class(dt)
[1] "Date"
> x3 <- as.Date("2015-01-01") #存储日期
> x3
[1] "2015-01-01"
> weekdays(x3)
[1] "Thursday"
> months(x3)
[1] "January"
> quarters(x3)
[1] "Q1"
> julian(x3)
[1] 16436
attr(,"origin")
[1] "1970-01-01"
x3 <- as.Date("2015-01-01")
x4 <- as.Date("2016-01-01")
x4-x3
Time difference of 365 days
> as.numeric(x4-x3)  #强制转换
[1] 365

时间：POSIXct/POSIXlt

距离1970-01-01的秒数/Sys.time()

POSIXct：整数，常用语存入数据框

POSIXlt：列表，还包含星期，年，月，日等信息

> x5 <- Sys.time()
> x5
[1] "2020-02-28 11:34:31 CST"
> class(x5)
[1] "POSIXct" "POSIXt" 
> p<-as.POSIXlt(x5)
> p
[1] "2020-02-28 11:34:31 CST"
> class(p)
[1] "POSIXlt" "POSIXt" 
> names(unclass(p))   #获取属性名称
 [1] "sec"    "min"    "hour"   "mday"   "mon"    "year"   "wday"  
 [8] "yday"   "isdst"  "zone"   "gmtoff"
> p$sec        #查看变量的值
[1] 31.83603
> p$yday
[1] 58
> as.POSIXct(p)
[1] "2020-02-28 11:34:31 CST"

> x6 <- "Jan1,2015 01:01"
[1] "Jan1,2015 01:01"
> strptime(x6,"%B %d, %Y %H:%M")
[1] "2015-01-01 01:01:00 CST"