data.table in r : subset using column index

Question

DT - data.table with column \"A\"(column index==1), \"B\"(column index 2), \"C\" and etc

for example next code makes subset DT1 which consists rows where A==2:

Answer 1

We can get the row index with .I and use that to subset the DT

DT[DT[, .I[.SD==2], .SDcols = 1]]
#   A B C
#1: 2 3 4

data

DT <- data.table(A = 1:5, B = 2:6, C = 3:7)