I'd like to assign only those values in the first row of a group in a data.table.
For example (simplified): my data.table is DT with the following content
x v
1 1
2 2
2 3
3 4
3 5
3 6
The key of DT is x.
I want to address every first line of a group.
This is working fine:DT[, .SD[1], by=x]
x v
1 1
2 2
3 4
Now, I want to assign only those values of v to 0.
But none of this is working:
DT[, .SD[1], by=x]$v <- 0
DT[, .SD[1], by=x]$v := 0
DT[, .SD[1], by=x, v:=0]
I searched the R-help from the package and any links provided but I just can't get it work.
I found notes there saying this would not work but no examples/solutions that helped me out.
I'd be very glad for any suggestions.
(I like this package very much and I don't wanna go back to a data.frame... where I got this working)
edit:
I'd like to have a result like this:
x v
1 0
2 0
2 3
3 0
3 5
3 6
This is not working:
DT[, .SD[1], by=x] <- DT[, .SD[1], by=x][, v:=0]
With a little help from Roland's solution, it looks like you could do the following. It simply concatenates zero with all the other grouped values of v except the first.
DT[, v := c(0L, v[-1]), by = x] ## must have the "L" after 0, as 0L
which results in
DT
# x v
# 1: 1 0
# 2: 2 0
# 3: 2 3
# 4: 3 0
# 5: 3 5
# 6: 3 6
Note: the middle section j of code could also be v := c(integer(1), v[-1])
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