Storing multiple data frames into one data structure - R

Question

Is it possible to have multiple data frames to be stored into one data structure and process it later by each data frame? i.e. example

df1 <- data.frame(c(1,2,3), c(4,5,6))
df2 <- data.frame(c(11,22,33), c(44,55,66))

.. then I would like to have them added in a data structure, such that I can loop through that data structure retrieving each data frame one at a time and process it, something like

 for ( iterate through the data structure) # this gives df1, then df2
 {
    write data frame to a file
}

I cannot find any such data structure in R. Can anyone point me to any code that illustrates the same functionality?

Answer 1

Just put the data.frames in a list. A plus is that a list works really well with apply style loops. For example, if you want to save the data.frame's, you can use mapply:

l = list(df1, df2)
mapply(write.table, x = l, file = c("df1.txt", "df2.txt"))

If you like apply style loops (and you will, trust me :)) please take a look at the epic plyr package. It might not be the fastest package (look data.table for fast), but it drips with syntactic sugar.

Answer 2

Lists can be used to hold almost anything, including data.frames:

## Versatility of lists
l <- list(file(), new.env(), data.frame(a=1:4))

For writing out multiple data objects stored in a list, lapply() is your friend:

ll <- list(df1=df1, df2=df2)
## Write out as *.csv files
lapply(names(ll), function(X) write.csv(ll[[X]], file=paste0(X, ".csv")))
## Save in *.Rdata files
lapply(names(ll), function(X) {
    assign(X, ll[[X]]) 
    save(list=X, file=paste0(X, ".Rdata"))
})