In a data.table remove identical consecutive values over certain times by group

Question

In a data.table, if a certain column has identical values occurring consecutively over a certain number of times, I'd like to remove the corresponding rows. I also would like to do this by group.

For example, say dt is my data.table. I would like to remove rows if the same value occurs consecutively over 2 times in Petal.Width grouped by Species.

dt <- iris[c(1:3, 7:7, 51:53, 62:63), ]
setDT(dt)
#    Sepal.Length Sepal.Width Petal.Length Petal.Width    Species
# 1           5.1         3.5          1.4         0.2     setosa
# 2           4.9         3.0          1.4         0.2     setosa
# 3           4.7         3.2          1.3         0.2     setosa
# 7           4.6         3.4          1.4         0.3     setosa
# 51          7.0         3.2          4.7         1.4 versicolor
# 52          6.4         3.2          4.5         1.5 versicolor
# 53          6.9         3.1          4.9         1.5 versicolor
# 62          5.9         3.0          4.2         1.5 versicolor
# 63          6.0         2.2          4.0         1.0 versicolor

The desired outcome is a data.table with the following rows.

# 7           4.6         3.4          1.4         0.3     setosa
# 51          7.0         3.2          4.7         1.4 versicolor
# 63          6.0         2.2          4.0         1.0 versicolor

Answer 1

Here is an option:

library(data.table)
setDT(dt)[dt[,{
    rl <- rleid(Species, Petal.Width)
    rw <- rowid(rl)
    .I[!rl %in% rl[rw > 1]]
}]]

output:

   Sepal.Length Sepal.Width Petal.Length Petal.Width    Species
1:          4.6         3.4          1.4         0.3     setosa
2:          7.0         3.2          4.7         1.4 versicolor
3:          6.0         2.2          4.0         1.0 versicolor