Given the following data.table:
library(data.table)
dt <- data.table(
a=c(1,3,1,3,5:8)
, b=c(2,4,2,4,9:12)
, signal = c("up", "up", "down", "down")
)
I need to create a column that lists all the signal instances for a given pair of a and b. The following code works:
dt_out <- dt[
, .(temp = signal |> list(), signal)
, .(a,b)
]
Which correctly outputs:
> dt_out
a b temp signal
<num> <num> <list> <char>
1: 1 2 up,down up
2: 1 2 up,down down
3: 3 4 up,down up
4: 3 4 up,down down
5: 5 9 up up
6: 6 10 up up
7: 7 11 down down
8: 8 12 down down
However, when trying to create temp directly within dt by reference it does not work as expected:
dt[
, temp := signal |> list()
, .(a,b)]
Which output does not show temp as a list:
> dt
a b signal temp
<num> <num> <char> <char>
1: 1 2 up up
2: 3 4 up up
3: 1 2 down down
4: 3 4 down down
5: 5 9 up up
6: 6 10 up up
7: 7 11 down down
8: 8 12 down down
How do I fix to create temp by reference in dt?
You've got to double up the list:
dt[, temp := .(.(signal)), by=.(a,b)]
## or
dt[, temp := list(list(signal)), by=.(a,b)]
dt
# a b signal temp
# <num> <num> <char> <list>
#1: 1 2 up up,down
#2: 3 4 up up,down
#3: 1 2 down up,down
#4: 3 4 down up,down
#5: 5 9 up up
#6: 6 10 up up
#7: 7 11 down down
#8: 8 12 down down