[R] How Can I Concatenate Every Row in a Data Frame with Every Other Row?

[Duncan Murdoch]

On 21/03/2009 12:01 PM, Donald Macnaughton wrote:
> I have a data frame with roughly 500 rows and 120 variables.  I would like
> to generate a new data frame that will include one row for each PAIR of
> rows in the original data frame and will include all 120 + 120 = 240
> variables from the two rows.  I need only one row for each pair, not two
> rows.  Thus the new data frame will contain 500 x 499 / 2 = 124,750 rows.  
>  
> Is there an easy way to do this with R?  

Probably the easiest is to generate row indices for each pair, e.g.


n <- nrow(mydata)

row1 <- rep(1:n, n)
row2 <- rep(1:n, each=n)
keep <- row1 < row2

big <- cbind(mydata[row1[keep],], mydata[row2[keep],])


With a simple example

 > mydata <- data.frame(a=1:3, b=letters[1:3])
 > mydata
   a b
1 1 a
2 2 b
3 3 c

this produces

 > big
     a b a b
1   1 a 2 b
1.1 1 a 3 c
2   2 b 3 c