sample data.table rows with different conditions - r

Question

I have a data.table with multiple columns. One of these columns currently works as a 'key' (keyb for the example). Another column (let's say A), may or may not have data in it. I would like to supply a vector that randomly sample two rows per key, -if this key appears in the vector, where 1 row contains data in A, while the other does not.
MRE:
#data.table
trys <- structure(list(keyb = c("x", "x", "x", "x", "x", "y", "y", "y",
"y", "y"), A = c("1", "", "1", "", "", "1", "", "", "1", "")), .Names = c("keyb",
"A"), row.names = c(NA, -10L), class = c("data.table", "data.frame"
))
setkey(trys,keyb)
#list with keys
list_try <- structure(list(a = "x", b = c("r", "y","x")), .Names = c("a", "b"))
I could, for instance subset the data.table based on the elements that appear in list_try:
trys[keyb %in% list_try[[2]]]
My original (and probably inefficient idea), was to try to chain a sample of two rows per key, where the A column has data or no data, and then merge. But it does not work:
#here I was trying to sample rows based on whether A has data or not
#here for rows where A has no data
trys[keyb %in% list_try[[2]]][nchar(A)==0][sample(.N, 2), ,by = keyb]
#here for rows where A has data
trys[keyb %in% list_try[[2]]][nchar(A)==1][sample(.N, 2), ,by = keyb]
In this case, my expected output would be two data.tables (one for a and one for b in list_try), of two rows per appearing element: So the data.table from a would have two rows (one with and without data in A), and the one from b, four rows (two with and two without data in A).
Please let me know if I can make this post any clearer

Answer 1

You could add A to the by statement too, while converting it to a binary vector by modifying to A != "", combine with a binary join (while adding nomatch = 0L in order to remove non-matches) you could then sample from the row index .I by those two aggregators and then subset from the original data set
For a single subset case
trys[trys[list_try[[2]], nomatch = 0L, sample(.I, 1L), by = .(keyb, A != "")]$V1]
# keyb A
# 1: y 1
# 2: y
# 3: x 1
# 4: x
For a more general case, when you want to create separate data sets according to a list of keys, you could easily embed this into lapply
lapply(list_try,
function(x) trys[trys[x, nomatch = 0L, sample(.I, 1L), by = .(keyb, A != "")]$V1])
# $a
# keyb A
# 1: x 1
# 2: x
#
# $b
# keyb A
# 1: y 1
# 2: y
# 3: x 1
# 4: x