[R] date

[Duncan Murdoch]

Another way to do this is to use the xtfrm() function.  That function 
creates numerical values from many different starting types, so you can 
just change the sign to change the sort order:

df[order(df$ID, -xtfrm(df$date2)),]

I never did figure out where the name came from.

Duncan Murdoch

On 21/09/2020 3:41 p.m., Sarah Goslee wrote:
> Hi,
> 
> Nice reproducible example.
> 
> rev(df$date2) isn't doing what you think it's doing - try looking at
> it by itself.
> 
> Some digging into ?order will get you what you are after:
> 
> df[order(df$ID, df$date2, decreasing=c(FALSE, TRUE), method="radix"),]
> 
>> df[order(df$ID, df$date2, decreasing=c(FALSE, TRUE), method="radix"),]
>    ID      date2
> 2 A1 2005-01-27
> 1 A1 2004-09-17
> 3 A1 2003-05-07
> 4 A2 2017-05-21
> 5 A2 2016-09-12
> 6 A3 2013-01-25
> 7 A4 2019-09-27
> 
> Sarah
> 
> On Mon, Sep 21, 2020 at 2:41 PM Val <valkremk using gmail.com> wrote:
>>
>> Hi All,
>>
>> I am trying to sort dates within a group. My sample data is
>>
>> df <-read.table(text="ID date
>> A1   09/17/04
>> A1   01/27/05
>> A1   05/07/03
>> A2   05/21/17
>> A2   09/12/16
>> A3   01/25/13
>> A4   09/27/19",header=TRUE,stringsAsFactors=F)
>> df$date2 = as.Date(strptime(df$date,format="%m/%d/%y"))
>> df$date =NULL
>>
>> I want to sort  date2  from recent to oldest.  within the ID group and
>> I used this,
>> df <- df[order(df$ID, rev((df$date2))),]. It did not work and teh
>> output is  shown below.
>>
>> ID      date2
>> 2 A1 2005-01-27
>> 3 A1 2003-05-07
>> 1 A1 2004-09-17
>> 5 A2 2016-09-12
>> 4 A2 2017-05-21
>> 6 A3 2013-01-25
>> 7 A4 2019-09-27
>> What am I missing?
>> Thank you.
>>
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> 
> 
>