基础知识 | R语言数据处理之日期值的转换

R语言数据处理之日期值

可能，刚开始学习R的人都会觉得日期值的处理非常简单，却常常在数据的深度分析，特别是利用时间序列绘制循环静态图、日历图、旭日图、螺旋图或者动态GIF/VIDEO等时出现Bug，罪魁祸首往往是因为日期值与字符型变量的相互转换、日期值的算术运算以及函数使用错误导致的。

as.Date()函数

1、把字符串转换为日期,系统默认格式yyyy-mm-dd

> Date1<-as.Date(c("2020-07-19","2020-07-20"))
> Date1
[1] "2020-07-19" "2020-07-20"

2、把系统非默认格式的日期转换为默认格式的日期yyyy-mm-dd

> Date<-c("06/20/20","06/21/20","06/22/20","06/23/20","06/24/20","06/25/20")
> Date2<-as.Date(Date,"%m/%d/%Y")
> Date2
[1] "2020-06-20" "2020-06-21" "2020-06-22" "2020-06-23" "2020-06-24" "2020-06-25"

3、修改数据框中的日期格式

#构建数据框
> PatientID<-c(1,2,3,4,5,6)
> Date<-c("06/20/20","06/21/20","06/22/20","06/23/20","06/24/20","06/25/20")
> Age<-c(18,25,26,35,48,50)
> Gender<-c("M","F","F","M","F","M")
> City<-c("福州","厦门","泉州","龙岩","漳州","三明")
> Pr1<-c("69","57","67","65","72","55")
> Pr2<-c("180","155","159","184","167","175")
> Pr3<-c("11","14","34","23","35","12")
> Pr4<-c("6","7","5","8","6","9")
> Pr5<-c("20","34","35","56","47","19")
> S1<-c(88,78,NA,56,45,60)
> S2<-c(67,87,45,25,NA,96)
> mydata<-data.frame(PatientID,Date,Age,Gender,City,Pr1,Pr2,Pr3,Pr4,Pr5,S1,S2)
> mydata
  PatientID     Date Age Gender City Pr1 Pr2 Pr3 Pr4 Pr5 S1 S2
1         1 06/20/20  18      M 福州  69 180  11   6  20 88 67
2         2 06/21/20  25      F 厦门  57 155  14   7  34 78 87
3         3 06/22/20  26      F 泉州  67 159  34   5  35 NA 45
4         4 06/23/20  35      M 龙岩  65 184  23   8  56 56 25
5         5 06/24/20  48      F 漳州  72 167  35   6  47 45 NA
6         6 06/25/20  50      M 三明  55 175  12   9  19 60 96

#修改数据框日期格式
> mydata$Date<-as.Date(mydata$Date,"%m/%d/%Y")
> mydata
  PatientID       Date Age Gender City Pr1 Pr2 Pr3 Pr4 Pr5 S1 S2
1         1 2020-06-20  18      M 福州  69 180  11   6  20 88 67
2         2 2020-06-21  25      F 厦门  57 155  14   7  34 78 87
3         3 2020-06-22  26      F 泉州  67 159  34   5  35 NA 45
4         4 2020-06-23  35      M 龙岩  65 184  23   8  56 56 25
5         5 2020-06-24  48      F 漳州  72 167  35   6  47 45 NA
6         6 2020-06-25  50      M 三明  55 175  12   9  19 60 96

Sys.Date()、date()、format()

1、Sys.Date()#系统当天日期

> Today<-Sys.Date()
> Today
[1] "2020-07-19"

2、date()返回当前的日期和时间

> date()
[1] "Sun Jul 19 14:59:10 2020"

3、format()输出指定格式的日期值

> Today<-Sys.Date()#系统当天日期
> Today
[1] "2020-07-19"
> date()
[1] "Sun Jul 19 14:59:10 2020"
> Today<-Sys.Date()#系统当天日期
> format(Today,format="%B %d %Y")#%B表示非缩写月份，%d表0-31的数字日期，%Y表四位数的年份
[1] "七月 19 2020"
> format(Today,format="%b %d %y")#%b表示缩写月份，%y表示两位数的年份
[1] " 7 19 20"
> format(Today,format="%m")#%m表示00-12月份
[1] "07"
> format(Today,format="%A")#%A非缩写星期名
[1] "星期日"

4、将日期值转换为字符型

> str<-as.character(Sys.Date(),"%m/%d/%y")
> str
[1] "07/19/20"

日期值的算术运算

1、计算两个日期之间间隔天数

> Start<-as.Date("2019-12-01")
> End<-Sys.Date()
> days<-Start-End
> days
Time difference of -231 days

2、日期的加减

> z <- Sys.Date()
> z + 10
[1] "2020-07-29"
> z-10
[1] "2020-07-09"

3、seq.Date()生成有规律的日期序列

> seq(as.Date("1992/1/1"), as.Date("2020/1/1"), "years")
 [1] "1992-01-01" "1993-01-01" "1994-01-01" "1995-01-01" "1996-01-01" "1997-01-01" "1998-01-01"
 [8] "1999-01-01" "2000-01-01" "2001-01-01" "2002-01-01" "2003-01-01" "2004-01-01" "2005-01-01"
[15] "2006-01-01" "2007-01-01" "2008-01-01" "2009-01-01" "2010-01-01" "2011-01-01" "2012-01-01"
[22] "2013-01-01" "2014-01-01" "2015-01-01" "2016-01-01" "2017-01-01" "2018-01-01" "2019-01-01"
[29] "2020-01-01"
> seq(as.Date("2020/1/1"), by = "month", length.out = 12)
 [1] "2020-01-01" "2020-02-01" "2020-03-01" "2020-04-01" "2020-05-01" "2020-06-01" "2020-07-01"
 [8] "2020-08-01" "2020-09-01" "2020-10-01" "2020-11-01" "2020-12-01"
> seq(as.Date("2019/1/1"), as.Date("2020/7/1"), by = "quarter")
[1] "2019-01-01" "2019-04-01" "2019-07-01" "2019-10-01" "2020-01-01" "2020-04-01" "2020-07-01"
> #找出两个日期之间的每个月的19号
> st <- as.Date("2019-12-01")
> en <- Sys.Date()
> ll <- seq(en, st, by = "-1 month")
> rev(ll[ll > st & ll < en])
[1] "2019-12-19" "2020-01-19" "2020-02-19" "2020-03-19" "2020-04-19" "2020-05-19" "2020-06-19"

4、difftime函数计算时间间隔

> Today<-Sys.Date()
> Sb<-as.Date("1990-01-12")
> difftime(Today,Sb,units="weeks")
Time difference of 1592.286 weeks

5、as.difftime计算时间

> as.difftime(c("0:5:26", "11:22:45"))
Time differences in mins
[1]   5.433333 682.750000
> as.difftime(c("5:26", "22:45", "2:"), format = "%H:%M")
Time differences in hours
[1]  5.433333 22.750000        NA
> z <- as.difftime(c(0,30,60), units = "mins")
> as.numeric(z, units = "secs")
[1]    0 1800 3600
> as.numeric(z, units = "hours")
[1] 0.0 0.5 1.0
> format(z)
[1] " 0 mins" "30 mins" "60 mins"

小结

1、

seq(from, to, by, length.out = NULL, along.with = NULL, ...)#length.out是指序列所需长度。

2、

difftime(time1, time2, tz,units = c("auto", "secs", "mins", "hours","days", "weeks")),tz用于转换的可选时区规范，主要用于“POSIXlt”对象。

3、

as.difftime(tim, format = "%X", units = "auto")tim:指定时间间隔的字符串或数值。format:指定tim格式的字符:参见strptime。默认是特定于地区的时间格式.