如何将字符数据帧转换为相应的日期?

我已经注意到有几次使用dates不允许使用R中常用的技巧,比如我有一个带有日期的dataframe数据(见下文),我想将完整的dataframe转换成一个date类。到目前为止,我能想到的唯一解决办法是:

for (i in 1:ncol(Data)){
    Data[,i] <- as.Date(Data[,i],format="%d %B %Y")
}

这将提供具有正确结构的数据帧:

> str(Data)
'data.frame':   6 obs. of  4 variables:
 $ Rep1:Class 'Date'  num [1:6] 12898 12898 13907 13907 13907 ...
 $ Rep2:Class 'Date'  num [1:6] 13278 13278 14217 14217 14217 ...
 $ Rep3:Class 'Date'  num [1:6] 13600 13600 14340 14340 14340 ...
 $ Rep4:Class 'Date'  num [1:6] 13831 13831 14669 14669 14669 ...

使用经典的apply方法会产生完全不同的效果。尽管所有变量都属于同一类,并且都属于同一类,但我无法获得正确类的数据帧或矩阵作为输出:

> str(sapply(Data,as.Date,format="%d %B %Y"))
 num [1:6, 1:4] 12898 12898 13907 13907 13907 ...
 - attr(*, "dimnames")=List of 2
  ..$ : NULL
  ..$ : chr [1:4] "Rep1" "Rep2" "Rep3" "Rep4"
> str(apply(Data,2,as.Date,format="%d %B %Y"))
 num [1:6, 1:4] 12898 12898 13907 13907 13907 ...
 - attr(*, "dimnames")=List of 2
  ..$ : NULL
  ..$ : chr [1:4] "Rep1" "Rep2" "Rep3" "Rep4"

如果要在日期对象中再次变换这些矩阵,则需要一个原点。来源可能因系统而异,因此使用截止日期或者apply()之后的另一个函数也没有多大帮助。如果你应用原点,你会得到一个向量。

Data <- structure(list(Rep1 = c(" 25 April 2005 ", " 25 April 2005 ", 
" 29 January 2008 ", " 29 January 2008 ", " 29 January 2008 ", 
" 29 January 2008 "), Rep2 = c(" 10 May 2006 ", " 10 May 2006 ", 
" 4 December 2008 ", " 4 December 2008 ", " 4 December 2008 ", 
" 4 December 2008 "), Rep3 = c(" 28 March 2007 ", " 28 March 2007 ", 
" 6 April 2009 ", " 6 April 2009 ", " 6 April 2009 ", " 6 April 2009 "
), Rep4 = c(" 14 November 2007 ", " 14 November 2007 ", " 1 March 2010 ", 
" 1 March 2010 ", " 1 March 2010 ", " 1 March 2010 ")), .Names = c("Rep1", 
"Rep2", "Rep3", "Rep4"), row.names = c("1", "2", "3", "4", "5", 
"6"), class = "data.frame")