copy data from one row to a new row in R data.table

Question

I have a table like follows:

library(data.table)
dt <- data.table(t0.id=1:2,t0.V1=c("a","e"),t0.V2=c("b","f"),t1.id=3:4,t1.V1=c("c","g"),t1.V2=c("d","h"))

dt
   t0.id t0.V1 t0.V2 t1.id t1.V1 t1.V2
1:     1     a     b     3     c     d
2:     2     e     f     4     g     h

and I want to copy data from row one to a new row, like this:

   t0.id t0.V1 t0.V2 t1.id t1.V1 t1.V2
1:     1     a     b                  
2:                       3     c     d
3:     2     e     f     4     g     h

I know how to duplicate the row (I've seen this post), but I don't know how to clear columns by condition (eg. t0.id==1) since both rows would be equal.

I guess it could be done by row index, but my real table has thousands of rows and I don't think that's the best way to do it.

Thank you

EDIT:

• The final order of rows doesn't matter, that is, final rows no 1 and 2 don't need to be next to each other.
• I 'manually' (by looking at some variables) identify which rows need to be split. So, the only condition to be applied is based on 't0.id'.

Answer 1

library(data.table)
splitids <- 1L # t0.id
out <- rbindlist(list(
    dt[t0.id %in% splitids, .SD, .SDcols = patterns("^t0")],
    dt[t0.id %in% splitids, .SD, .SDcols = patterns("^t1")],
    dt[!t0.id %in% splitids,]),
  use.names = TRUE, fill = TRUE)
out
#    t0.id  t0.V1  t0.V2 t1.id  t1.V1  t1.V2
#    <int> <char> <char> <int> <char> <char>
# 1:     1      a      b    NA   <NA>   <NA>
# 2:    NA   <NA>   <NA>     3      c      d
# 3:     2      e      f     4      g      h

It may make more sense if you look at each of them:

dt[t0.id %in% splitids, .SD, .SDcols = patterns("^t0")]
#    t0.id  t0.V1  t0.V2
#    <int> <char> <char>
# 1:     1      a      b
dt[t0.id %in% splitids, .SD, .SDcols = patterns("^t1")]
#    t1.id  t1.V1  t1.V2
#    <int> <char> <char>
# 1:     3      c      d
dt[!t0.id %in% splitids,]
#    t0.id  t0.V1  t0.V2 t1.id  t1.V1  t1.V2
#    <int> <char> <char> <int> <char> <char>
# 1:     2      e      f     4      g      h

If you need the blank "" instead of NA, then that can be done to your character columns, but not to the t*.id columns since that would convert them from integer to character.

ischr <- which(sapply(dt, inherits, "character"))
ischr
# t0.V1 t0.V2 t1.V1 t1.V2 
#     2     3     5     6 
out[, (ischr) := lapply(.SD, fcoalesce, ""), .SDcols = ischr][]
#    t0.id  t0.V1  t0.V2 t1.id  t1.V1  t1.V2
#    <int> <char> <char> <int> <char> <char>
# 1:     1      a      b    NA              
# 2:    NA                   3      c      d
# 3:     2      e      f     4      g      h