Create nested data.tables by folding lines into new data.tables

Question

How to create a data table containing nested data.tables?

Example

set.seed(7908)
dt <- data.table(x=1:5)[,list(y=letters[1:x],z=sample(1:100,x)),by=x]

dt
##     x y  z
##  1: 1 a 13
##  2: 2 a 27
##  3: 2 b 87
##  4: 3 a 85
##  5: 3 b 98
##  6: 3 c  1
##  7: 4 a 53
##  8: 4 b 81
##  9: 4 c 64
## 10: 4 d 45
## 11: 5 a 28
## 12: 5 b 26
## 13: 5 c 52
## 14: 5 d 55
## 15: 5 e 12

Output required

For each unique x value in dt, collapse the rows and create a data table with the y and z columns contained in the same dt column. The result should look like this:

##    x        dt.yz
## 1: 1 <data.table>
## 2: 2 <data.table>
## 3: 3 <data.table>
## 4: 4 <data.table>
## 5: 5 <data.table>

In my actual example, I have several data tables with different columns that I want to keep in the same metadata table.

Answer 1

, y z , , "" . , . by=x x.

dt2 <- dt[, list(dt.yz=list(data.table(y, z))), by=x]
dt2
##    x        dt.yz
## 1: 1 <data.table>
## 2: 2 <data.table>
## 3: 3 <data.table>
## 4: 4 <data.table>
## 5: 5 <data.table>

, .SD :

dt2 <- dt[, list(dt.yz=list(.SD)), by=x]
## dt.yz will include all columns not in the `by=`;
## Use `.SDcols=` to select specific columns

, meta data.table(dt2) x, ( ) dt.yz.

dt2[x==5,dt.yz[[1]]]
##    y  z
## 1: a 28
## 2: b 26
## 3: c 52
## 4: d 55
## 5: e 12