data.table aggregation to list column
data.table, r
Solution
Update: The behaviour `DT[, list(list(.)), by=.]` sometimes resulted in wrong results in R version >= 3.1.0. This is now fixed in commit #1280 in the current development version of data.table v1.9.3. From NEWS:
- `DT[, list(list(.)), by=.]` returns correct results in R >=3.1.0 as well. The bug was due to recent (welcoming) changes in R v3.1.0 where `list(.)` does not result in a copy. Closes #481.
With this update, it's not necessary for `I()` anymore. You can just do: `DT[, list(list(.)), by=.]` as before.
This seems to be a similar issue as the known bug #5585. In your case, I think you could just use
dt[, paste(letter, collapse=","), by = id]
to fix your problem.
As @ilir pointed out, if it is actually desirable to get a list (rather than the displayed character), you could use the workaround suggested in the bug report:
dt[, list(list(I(letter))), by = id]
Problem
I'm trying to aggregate a data from a data.table to create a new column which is a list of previous rows. It's easier to see by example: ``` dt <- data.table(id = c(1,1,1,1,2,2,3,3,3), letter = c('a','a','b','c','a','c','b','b','a')) ``` I would like to aggregate this in such a ways that the result should be ``` id letter 1: 1 a,a,b,c 2: 2 a,c 3: 3 b,b,a ``` Intuitively I tried ``` dt[,j = list(list(letter)), by = id] ``` but that doesn't work. Oddly enough when I go case by case, for example: ``` > dt[id == 1,j = list(list(letter)), by = id] id V1 1: 1 a,a,b,c ``` the result is fine... I feel like I'm missing an `.SD` somewhere or something like that... Can anybody point me in the right direction? Thanks!