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You can use the .N special symbol in data.table to count the number of rows within each group. Then you can filter the groups that have a specific number of rows using the subset operator []. Here is an example:
library(data.table)
# Create a sample data.table
DT <- data.table(x = rep(c("A", "B"), each = 5), y = rnorm(10))
# Count the number of rows per group
n_rows <- DT[, .N, by = x]
# Retrieve groups that have exactly 3 rows
n_rows[n_rows$N == 3, ]
# Alternatively, you can filter the original data.table
DT[x %in% n_rows[x == "A" & N == 3]$x]
In this example, we first create a data.table DT with two columns x and y. We then use the .N symbol to count the number of rows in each group defined by the column x. We store the result in a new data.table n_rows. Finally, we filter n_rows to retrieve the groups that have exactly 3 rows. Alternatively, you can filter the original DT data.table directly using the subset operator [].