R data.table generate random pairings in data table -

i have following sample data table.

   id val 1:    1 2:  b   3 3:  c   2 4:  d   1 

i make random pairings amongst id columns, not want id paired itself. efficient way data.tables? 1 approach have tried first find random rows in data table follows

x = x[sample(nrow(x),1),] 

but hit block because have run check make sure current index not present in 1 returned. expensive computationally. example possible output result be

  id val id.pair val.pair 1:  1  b  3 2: b  3  c  2 3: c  2   1 4: d  1   1 

thanks in advance

you use combn , sample.int this:

df <- read.table(text="id val  1 b  3 c  2 d  1", header=true, stringsasfactors=false)  library(data.table) dt <- data.table(df)  set.seed(42) combis <- combn(dt[,id], 2)[,sample.int(choose(nrow(dt),2), nrow(dt))]  setkey(dt, "id") cbind(dt[combis[1,],], dt[combis[2,],])  #    id val id val # 1:  c   2  d   1 # 2:  b   3  d   1 # 3:    1  c   2 # 4:    1  d   1 

however, if number of ids big need something function avoid calculating possible combinations.