Use dynamic names for CJ in data.table

Question

I want to use CJ with a flexible number of columns with flexible names, for example

J = 3
temp = CJ(paste0('Q', 1) = 0:100)
if(J > 1){
  for(j in 2:J){
    temp = CJ(temp,paste0('Q', j) = 0:100))
  }
}

For arbitrary J.

Where the output is:

CJ(Q1 = 0:100, Q2 = 0:100, Q3 = 0:100)


          Q1  Q2  Q3
      1:   0   0   0
      2:   0   0   1
      3:   0   0   2
      4:   0   0   3
      5:   0   0   4
     ---            
1030297: 100 100  96
1030298: 100 100  97
1030299: 100 100  98
1030300: 100 100  99
1030301: 100 100 100

Answer 1

This is a classic case for do.call:

temp = do.call(CJ, replicate(J, 0:100, simplify = FALSE))
setnames(temp, paste0('Q', 1:J))
temp
#           Q1  Q2  Q3
#       1:   0   0   0
#       2:   0   0   1
#       3:   0   0   2
#       4:   0   0   3
#       5:   0   0   4
#      ---            
# 1030297: 100 100  96
# 1030298: 100 100  97
# 1030299: 100 100  98
# 1030300: 100 100  99
# 1030301: 100 100 100

It's possible to construct this in one line using setNames and lapply instead of replicate, but much less readable IMO. setnames is almost instantaneous so there's no efficiency concern.