Insert a row of NAs after each group of data using data.table

Question

I am trying to add a row of NAs after each group of data in R.

A similar question has been asked earlier. Insert a blank row after each group of data.

Answer 1

One way I could think of is to construct a vector first as follows:

foo <- function(x) {
    o = order(rep.int(seq_along(x), 2L))
    c(x, rep.int(NA, length(x)))[o]
}
join_values = head(foo(unique(df_new$group)), -1L)
# [1] "a" NA  "b" NA  "c" NA  "d"

And then setkey() and join.

setkey(setDT(df_new), group)
df_new[.(join_values), allow.cartesian=TRUE]
#     group xvalue yvalue
#  1:     a     16      1
#  2:    NA     NA     NA
#  3:     b     17      2
#  4:     b     18      3
#  5:    NA     NA     NA
#  6:     c     19      4
#  7:     c     20      5
#  8:     c     21      6
#  9:    NA     NA     NA
# 10:     d     22      7
# 11:     d     23      8
# 12:     d     24      9
# 13:     d     25     10

Answer 2

You could try

df$group <- as.character(df$group)
setDT(df)[, .SD[1:(.N+1)], by=group][is.na(xvalue), group:=NA][!.N]
#     group xvalue yvalue
#1:     a     16      1
#2:    NA     NA     NA
#3:     b     17      2
#4:     b     18      3
#5:    NA     NA     NA
#6:     c     19      4
#7:     c     20      5
#8:     c     21      6
#9:    NA     NA     NA
#10:    d     22      7
#11:    d     23      8
#12:    d     24      9
#13:    d     25     10

Or as suggested by @David Arenburg

 setDT(df)[, indx := group][, .SD[1:(.N+1)], indx][,indx := NULL][!.N]

Or

 setDT(df)[df[,.I[1:(.N+1)], group]$V1][!.N]

Or it could be further simplified based on @eddi's comments

 setDT(df)[df[, c(.I, NA), group]$V1][!.N]