r - Is there a simple way of pairing unique data points in a data frame? -

i want extract pairs of data data frame, paired data not in own column. each number in column 1 paired numbers right of column. likewise numbers in column 2 paired numbers in columns 3 or above.

i have created script using bird's nest of 'for' loops feel there should more elegant way it.

example data:

structure(list(a = 1:3, b = 4:6, c = 7:9), .names = c("a", "b",            "c"), class = "data.frame", row.names = c(na, -3l)) 

desired output:

structure(list(x1 = c(1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 3,            3, 3, 3, 3, 3, 4, 4, 4, 5, 5, 5, 6, 6, 6), x2 = c(4, 5, 6, 7,            8, 9, 4, 5, 6, 7, 8, 9, 4, 5, 6, 7, 8, 9, 7, 8, 9, 7, 8, 9, 7,            8, 9)), .names = c("x1", "x2"), row.names = c(na, 27l), class = "data.frame") 

here's approach using data.table package , efficient cj , rbindlist functions (assuming data set called df)

library(data.table) res <- rbindlist(lapply(seq_len(length(df) - 1),          function(i) cj(df[, i], unlist(df[, -(seq_len(i))])))) 

you set column names reference (if insist on "x1" , "x2") using setnames

setnames(res, 1:2, c("x1", "x2")) 

you can convert data.frame reference (if want match desired output "exactly") using setdf()

setdf(res)