force .GRP counter to start from 2 instead of 1 in data.table

Question

How to force .GRP in data.table to start the group counter from 2 instead of 1?

I have a data.table with groups which I want to sequentially order by group.

example_data.table <- data.table(Var1 = c(1,2,2,4,5,5,5), Var2 = c(1,2,3,7,1,2,3) )

When I use .GRP counter it starts with very first combination as conter 1.

Group_table   <- setDT(example_data.table)[, label := .GRP, by = c("Var1", "Var2" )]

But I want to set group with Var1 value as 4 and Var2 value as 7 to counter value 1 and then the next.

How do I use .GRP in such a way that Var1 as 4 and Var2 as 7 takes a counter as 1 and others in next order?

So, what I was thinking is manually give counter as 1 for the needed combination and for others start the counter from 2 . There are other ways too but I am just a bit confused.

You need to give a reproducible example https://stackoverflow.com/questions/5963269/how-to-make-a-great-r-reproducible-example — Kumar Manglam, Feb 21 '19 at 14:54
@KumarManglam Thanks for the comments! I have updated the qs with a reproducible example. — Deb, Feb 21 '19 at 15:15

score 3 · Answer 1 · answered Feb 21 '19 at 15:29

3

If you only have one entry with Var1 = 4 & Var2 = 7, then you can remove that entry from .GRP, and use replace to replace it with 1, i.e.

library(data.table)

dt1[-(which(dt1$Var1 == 4 & dt1$Var2 == 7)), Counter := .GRP + 1, by = c('Var1', 'Var2')][, 
                                                Counter := replace(Counter, is.na(Counter), 1)][]

which gives,

   Var1 Var2 Counter
1:    1    1       2
2:    2    2       3
3:    2    3       4
4:    4    7       1
5:    5    1       5
6:    5    2       6
7:    5    3       7

answered Feb 21 '19 at 15:29

Sotos

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Thank you so much! Just one Qs. Why have you used [ ] at last again? Can't we do without it? – Deb Feb 21 '19 at 17:44
The brackets at the end are just for displaying the result. You don't have to put them. – Sotos Feb 22 '19 at 05:43

score 3 · Answer 2 · answered Feb 22 '19 at 18:33

If you want certain groups to "start" the count, you can use order to sort during construction:

ex = copy(example_data.table)
ex[order(Var1 != 4, Var2 != 7), g := .GRP, by=.(Var1, Var2)][]

   Var1 Var2 g
1:    1    1 2
2:    2    2 3
3:    2    3 4
4:    4    7 1
5:    5    1 5
6:    5    2 6
7:    5    3 7

force .GRP counter to start from 2 instead of 1 in data.table

2 Answers2