在R中仅使用月份和年份格式化日期

我有一个很简单的问题，我在这里没有找到

我有日期格式：

times = c("Dec_2011" , "July_2011", "Dec_2010"  ,"July_2010" , "Dec_2009" , "July_2009", "Dec_2008" ,
                     "July_2008" ,"Dec_2007" , "July_2007", "Dec_2006" , "July_2006" ,"Dec_2005" , "July_2005",
                     "Dec_2004" , "July_2004" ,"Dec_2003" , "July_2003", "Dec_2002" , "July_2002", "Dec_2001" ,
                     "July_2001", "Dec_2000" , "July_2000")

31-07-2000, 31-07-2001, etc...
31-12-2000, 31-12-2001, etc...

如何将其转换为日期格式：

times = c("Dec_2011" , "July_2011", "Dec_2010"  ,"July_2010" , "Dec_2009" , "July_2009", "Dec_2008" ,
                     "July_2008" ,"Dec_2007" , "July_2007", "Dec_2006" , "July_2006" ,"Dec_2005" , "July_2005",
                     "Dec_2004" , "July_2004" ,"Dec_2003" , "July_2003", "Dec_2002" , "July_2002", "Dec_2001" ,
                     "July_2001", "Dec_2000" , "July_2000")

31-07-2000, 31-07-2001, etc...
31-12-2000, 31-12-2001, etc...

我试过：

times <- format(as.Date(time, "%B_%Y")
times
[1] NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA


times <- format(as.Date(time, "%B_%Y), "31-%m-%Y)
times
[1] NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA

times <- as.Date(paste("31", times, sep="-"), "%d-%m-%Y")
times
[1] NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA

times <- format(as.Date(time, "%b_%Y"), "31-%m-%Y")
# NA

---

times如果我们需要
31
作为所有元素的日期，请使用
paste
加入31，转换为
Date
类，并使用
format
获得所需的格式

format(as.Date(paste(times, "31", sep="_"), "%b_%Y_%d"), "%d-%m-%Y")
#[1] "31-12-2011" "31-07-2011" "31-12-2010" "31-07-2010" "31-12-2009" "31-07-2009" "31-12-2008" "31-07-2008" "31-12-2007" "31-07-2007" "31-12-2006"
#[12] "31-07-2006" "31-12-2005" "31-07-2005" "31-12-2004" "31-07-2004" "31-12-2003" "31-07-2003" "31-12-2002" "31-07-2002" "31-12-2001" "31-07-2001"
#[23] "31-12-2000" "31-07-2000"

我们可以使用
zoo
中的
as.yearmon
自动执行此操作，而不是手动粘贴
31。好处是，对于少于31天的月份，我们可以通过这样做获得最后一天

library(zoo)
format(as.Date(as.yearmon(times, "%b_%Y"), frac=1), "%d-%m-%Y")
#[1] "31-12-2011" "31-07-2011" "31-12-2010" "31-07-2010" "31-12-2009" "31-07-2009" "31-12-2008" "31-07-2008" "31-12-2007" "31-07-2007" "31-12-2006"
#[12] "31-07-2006" "31-12-2005" "31-07-2005" "31-12-2004" "31-07-2004" "31-12-2003" "31-07-2003" "31-12-2002" "31-07-2002" "31-12-2001" "31-07-2001"
#[23] "31-12-2000" "31-07-2000"