Expand two large data files and apply using data.table?

Question

I am attempting to apply a function to two data sets df1 and df2 where df1 contains (a, b) and can be 1 million rows long, and df2 contains (x, y, z) and can be very large, anywhere from ~100 to >10,000. I would like to apply a function foo over every combination of both data sets and then sum over the second data set.

foo <- function(a, b, x, y, z) a + b + x + y + z
df1 <- data.frame(a = 1:10, b = 11:20)
df2 <- data.frame(x= 1:5, y = 21:25, z = 31:35)

The code I am using to apply this function (taken from @jlhoward here How to avoid multiple loops with multiple variables in R )

foo.new <- function(p1, p2) {
  p1 = as.list(p1); p2 = as.list(p2)
  foo(p1$a, p1$b, p2$x, p2$y, p2$z)
}

indx   <- expand.grid(indx2 = seq(nrow(df2)), indx1 = seq(nrow(df1)))
result <- with(indx, foo.new(df1[indx1, ], df2[indx2, ]))
sums   <- aggregate(result, by = list(rep(seq(nrow(df1)), each = nrow(df2))), sum)

However, as df2 gets large (>1000) I quickly run out of memory to perform the result function above (running 64bit PC with 32GB RAM).

I have read about data.table quite a bit but can't evaluate whether there is a function in there that would assist in saving memory. Something that would replace with and create a smaller file at the result step, or expand.grid at the index step, which creates the largest file by far.

Answer 1

Here is a data.table solution: should be pretty fast:

library(data.table)
indx<-CJ(indx1=seq(nrow(df2)),indx2=seq(nrow(df1))) #CJ is data.table function for expand.grid
 indx[,`:=`(result=foo.new(df1[indx1, ], df2[indx2, ]),Group.1=rep(seq(nrow(df1)), each = nrow(df2)))][,.(sums=sum(result)),by=Group.1]

    Group.1 sums
 1:       1  355
 2:       2  365
 3:       3  375
 4:       4  385
 5:       5  395
 6:       6  405
 7:       7  415
 8:       8  425
 9:       9  435
10:      10  445