R data.table grouping for lagged regression

Question

table with data (its a data.table object) that looks like the following :

      date         stock_id logret
   1: 2011-01-01        1  0.001
   2: 2011-01-02        1  0.003
   3: 2011-01-03        1  0.005
   4: 2011-01-04        1  0.007
   5: 2011-01-05        1  0.009
   6: 2011-01-06        1  0.011
   7: 2011-01-01        2  0.013
   8: 2011-01-02        2  0.015
   9: 2011-01-03        2  0.017
  10: 2011-01-04        2  0.019
  11: 2011-01-05        2  0.021
  12: 2011-01-06        2  0.023
  13: 2011-01-01        3  0.025
  14: 2011-01-02        3  0.027
  15: 2011-01-03        3  0.029
  16: 2011-01-04        3  0.031
  17: 2011-01-05        3  0.033
  18: 2011-01-06        3  0.035

The above can be created as :

DT = data.table(
   date=rep(as.Date('2011-01-01')+0:5,3) , 
   stock_id=c(1,1,1,1,1,1,2,2,2,2,2,2,3,3,3,3,3,3),
  logret=seq(0.001, by=0.002, len=18));

setkeyv(DT,c('stock_id','date'))

Of course the real table is larger with many more stock_ids and dates. The aim to to reshape this data table such that I can run a regression of all stockid log_returns with their corresponding log_returns with a lag of 1 day (or prior traded day in case of weekends).

The final results would look like :

      date         stock_id logret lagret
   1: 2011-01-01        1  0.001    NA
   2: 2011-01-02        1  0.003    0.001
   3: 2011-01-03        1  0.005    0.003
   ....
  16: 2011-01-04        3  0.031  0.029
  17: 2011-01-05        3  0.033  0.031
  18: 2011-01-06        3  0.035  0.033

I'm finding this data structure really tricky to build without mixing up my stockid.

Answer 1

Just some additional notes due to Alex's comment. The reason you have difficulties understanding what's going on here is that a lot of things are done within one line. So it's always a good idea to break things down.

What do we actually want? We want a new column lagret and the syntax to add a new column in data.table is the following:

DT[, lagret := xxx]

where xxx has to be filled up with whatever you want to have in column lagret. So if we just want a new column that gives us the rows, we could just call

DT[, lagret := seq(from=1, to=nrow(DT))]

Here, we actually want the lagged value of logret, but we have to consider that there are many stocks in here. That's why we do a self-join, i.e. we join the data.table DT with itself by the columns stock_id and date, but since we want the previous value of each stock, we use date-1. Note that we have to set the keys first to do such a join:

setkeyv(DT,c('stock_id','date'))
DT[list(stock_id,date-1)]
    stock_id       date logret
 1:        1 2010-12-31     NA
 2:        1 2011-01-01  0.001
 3:        1 2011-01-02  0.003
 4:        1 2011-01-03  0.005
 5:        1 2011-01-04  0.007
 6:        1 2011-01-05  0.009
...

As you can see, we now have what we want. logret is now lagged by one period. But we actually want that in a new column lagret in DT, so we just get that column by calling [[3L]] (this means nothing else then get me the third column) and name this new column lagret:

DT[,lagret:=DT[list(stock_id,date-1),logret][[3L]]]
          date stock_id logret lagret
 1: 2011-01-01        1  0.001     NA
 2: 2011-01-02        1  0.003  0.001
 3: 2011-01-03        1  0.005  0.003
 4: 2011-01-04        1  0.007  0.005
 5: 2011-01-05        1  0.009  0.007
...

This is already the correct solution. In this simple case, we do not need roll=TRUE because there are no gaps in the dates. However, in a more realistic example (as mentioned above, for instance when we have weekends), there might be gaps. So let's make such a realistic example by just deleting two days in the DT for the first stock:

DT <- DT[-c(4, 5)]
setkeyv(DT,c('stock_id','date'))
DT[,lagret:=DT[list(stock_id,date-1),logret][[3L]]]
          date stock_id logret lagret
 1: 2011-01-01        1  0.001     NA
 2: 2011-01-02        1  0.003  0.001
 3: 2011-01-03        1  0.005  0.003
 4: 2011-01-06        1  0.011     NA
 5: 2011-01-01        2  0.013     NA
...

As you can see, the problem is now that we don't have a value for the 6th of January. That's why we use roll=TRUE:

DT[,lagret:=DT[list(stock_id,date-1),logret,roll=TRUE][[3L]]]
          date stock_id logret lagret
 1: 2011-01-01        1  0.001     NA
 2: 2011-01-02        1  0.003  0.001
 3: 2011-01-03        1  0.005  0.003
 4: 2011-01-06        1  0.011  0.005
 5: 2011-01-01        2  0.013     NA
...

Just have a look on the documentation on how roll=TRUE works exactly. In a nutshell: If it can't find the previous value (here logret for the 5th of January), it just takes the last available one (here from the 3rd of January).