data.table with two string columns of set elements, extract unique rows with each row unsorted

Question

Suppose I have a data.table like this:

Table:

V1 V2
 A  B
 C  D
 C  A
 B  A
 D  C

I want each row to be regarded as a set, which means that B A and A B are the same. So after the process, I want to get:

V1 V2
 A  B
 C  D
 C  A

In order to do that, I have to first sort the table row-by-row and then use unique to remove the duplicates. The sorting process is quite slow if I have millions of rows. So is there an easy way to remove the duplicates without sorting?

Answer 1

For just two columns you can use the following trick:

dt = data.table(a = letters[1:5], b = letters[5:1])
#   a b
#1: a e
#2: b d
#3: c c
#4: d b
#5: e a

dt[dt[, .I[1], by = list(pmin(a, b), pmax(a, b))]$V1]
#   a b
#1: a e
#2: b d
#3: c c

Answer 2

Borrowing (probably unrealistic) data from a dupe:

library(data.table)
size <- 118000000
key1 <- sample( LETTERS, size, replace=TRUE, prob=runif(length(LETTERS), 0.0, 5.0) )
key2 <- sample( LETTERS, size, replace=TRUE, prob=runif(length(LETTERS), 0.0, 5.0) )
val <- runif(size, 0.0, 5.0)

dt <- data.table(key1, key2, val, stringsAsFactors=FALSE)

Here's a fast way if your data looks like this:

# eddi's answer
system.time(res1 <- dt[dt[, .I[1], by=.(pmin(key1, key2), pmax(key1, key2))]$V1])
#    user  system elapsed 
#  101.79    3.01  107.98 

# optimized for this data
system.time({
  dt2 <- unique(dt, by=c("key1", "key2"))[key1 > key2, c("key1", "key2") := .(key2, key1)]
  res2 <- unique(dt2, by=c("key1", "key2")) 
})
#    user  system elapsed 
#    8.50    1.16    4.93 

fsetequal(copy(res1)[key1 > key2, c("key1", "key2") := .(key2, key1)], res2)
# [1] TRUE

Data like this seems unlikely if it pertains to covariances, since you should have at most one duplicate (ie, A-B with B-A).