Assign the value of the first row of a group to the whole group

Question

I have a data.table like the following:

DT<- data.table(id=c(1,1,1,1,2,2,2,2), date = c(10,11,12,13,12,12,14,18), seq = c(1,2,3,4,1,2,3,4,1,2,3,4)) 
setkey(DT,id)
setorder(DT,id,seq)

> DT
   id date seq
1:  1   10   1
2:  1   11   2
3:  1   12   3
4:  1   13   4
5:  2   12   1
6:  2   12   2
7:  2   14   3
8:  2   18   4

I want to create a new column, "id.date" which will indicate the value of "date" of the first row per id. So, the new data.table would be:

   id date seq id.date
1:  1   10   1      10
2:  1   11   2      10
3:  1   12   3      10
4:  1   13   4      10
5:  2   12   1      12
6:  2   12   2      12
7:  2   14   3      12
8:  2   18   4      12

I know I can get the value of the first row by

DT[,.SD[1,date],by=id]$V1

but how can I assign this value to all the rows of the group?

Is it possible to do the same with dplyr?

Thanx Sotos, it was much simpler than I thought! Any `dplyr` solution? — User800701, Dec 24 '18 at 08:54
Same concept, `df %>% group_by(id) %>% mutate(id.date = first(date))` — Sotos, Dec 24 '18 at 08:55

score 1 · Accepted Answer · answered Dec 24 '18 at 08:56

1

DT %>%
  group_by(id) %>%
  mutate(id.date = date[1])

answered Dec 24 '18 at 08:56

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Assign the value of the first row of a group to the whole group

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