R help - Sep 2004 - Efficient Cartesian product of data.frames

Hello List,

I am looking for efficient code to produce the Cartesian product of two 
or more data.frames. I'd like to be able to do this without resorting to 
looping. I have searched the FAQ, web, etc without luck. That being 
said, the help page for merge says that the function can produce what 
I'm looking for if the by vectors are of zero length. Would someone be 
so kind as to provide me with a quick example of exactly how to do this?

Cheers,

Tim Victor

Timothy W. Victor <tvictor <at> dolphin.upenn.edu> writes:
> I am looking for efficient code to produce the Cartesian product of two 
> or more data.frames. 

First create some test data consisting of a list of n=2 data frames.

   data(iris)
   L <- list(iris1 = iris[1:3,1:2], iris2 = iris[1:3,3:4])

Now calculate the cartesian product of the row indices, grid,
and, in the second line, cbind together the corresponding rows:

   grid <- expand.grid(1:nrow(L[[1]]), 1:nrow(L[[2]]))
   cbind(L[[1]][grid[,1],], L[[2]][grid[,2],])

Now generalize that to n >= 2 data frames:

   grid <- do.call("expand.grid", lapply(L, function(x) 1:nrow(x)))
   do.call("cbind", lapply(seq(L), function(i)L[[i]][grid[,i],]))