[datatable-help] New column with conditions for first and last observation by id group

[Matthew DeAngelis]

Hi Frank,

Not sure about a one-liner, but this seems to do what you want:

> DT <- data.table(obs=1:7, id=c(1,1,1,4,4,4,4), time=c(3,4,7,5,8,10,15))
> DT[,value:=0]
> DT[!duplicated(id),value:=2]
> DT[!duplicated(id,fromLast=T),value:=1]
> DT

   obs id time value
1:   1  1    3     2
2:   2  1    4     0
3:   3  1    7     1
4:   4  4    5     2
5:   5  4    8     0
6:   6  4   10    0
7:   7  4   15    1

Seems too straightforward, though, so maybe I am missing something about
your problem. Please elaborate if so.


Regards,
Matt


On Fri, Jul 25, 2014 at 8:29 AM, Frank S. <f_j_rod at hotmail.com> wrote:

> Hi all, I'm very novice in data.table management, and I have the
> following doubt about this data:
>
>
>
> > DT <- data.table(obs=1:7, id=c(1,1,1,4,4,4,4), time=c(3,4,7,5,8,10,15))
> > DT
>    obs id time
> 1:   1  1    3
> 2:   2  1    4
> 3:   3  1    7
> 4:   4  4    5
> 5:   5  4    8
> 6:   6  4   10
> 7:   7  4   15
>
>
>
> In general, I know that I can select respectively the first and the last
> observation within "id" group with:
>
>
>
> First observation: *DT[!duplicated(id)] *
>
> Last observation:* DT[!duplicated(id, fromLast=T)] *
>
>
>
> But, how can I add a new column, called "value", which contains all zeros
> except:
>
> 1) The first observation within each "id" group, which is equal to 2
>
> 2) The last observation within each "id" group, which is equal to 1 ?
>
>
>
>    obs id time value
> 1:   1  1    3     2
> 2:   2  1    4     0
> 3:   3  1    7     1
> 4:   4  4    5     2
> 5:   5  4    8     0
> 6:   6  4   10     0
> 7:   7  4   15     1
>
>
>
> I've tried with conditionslas, ifelse, etc, but I get an error message.
> Please, can you help me?
>
>
>
> Thanks in advance!!
>
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