R data.table subsetting within a group and splitting a data table into two -

i have following data.table.

ts,id 1,a 2,a 3,a 4,a 5,a 6,a 7,a 1,b 2,b 3,b 4,b 

i want subset data.table two. criteria have approximately first half each group (in case column "id") in 1 data table , remaining in data.table. expected result 2 data.tables follows

ts,id 1,a 2,a 3,a 4,a 1,b 2,b 

and

 ts,id  5,a  6,a  7,a  3,b  4,b 

i tried following,

z1 = x[,.sd[.i < .n/2,],by=dev] z1 

and got following

id ts  1  2  3 

somehow, .i within .sd isn't working way think should. appreciated. in advance.

.i gives row locations respect whole data.table. can't used within .sd.

something like

dt[, subset := seq_len(.n) > .n/2,by='id']  subset1 <- dt[(subset)][,subset:=null] subset2 <- dt[!(subset)][,subset:=null]  subset1 #    ts id # 1:  4  # 2:  5  # 3:  6  # 4:  7  # 5:  3  b # 6:  4  b subset2 #   ts id # 1:  1  # 2:  2  # 3:  3  # 4:  1  b # 5:  2  b 

should work

for more 2 groups, use cut create factor appropriate number of levels

something like

 dt[, subset := cut(seq_len(.n), 3, labels= false),by='id']  # copy global environment subset each,   # not memory efficient!  list2env(setattr(split(dt, dt[['subset']]),'names', paste0('s',1:3)), .globalenv)