Question

How can I get a data frame’s name from a list? Sure, get() gets the object itself, but I want to have its name for use within another function. Here’s the use case, in case you would rather suggest a work around:

lapply(somelistOfDataframes, function(X) {
    ddply(X, .(idx, bynameofX), summarise, checkSum = sum(value))
})

There is a column in each data frame that goes by the same name as the data frame within the list. How can I get this name bynameofX? names(X) would return the whole vector.

EDIT: Here’s a reproducible example:

df1 <- data.frame(value = rnorm(100), cat = c(rep(1,50),
    rep(2,50)), idx = rep(letters[1:4],25))
df2 <- data.frame(value = rnorm(100,8), cat2 = c(rep(1,50), 
    rep(2,50)), idx = rep(letters[1:4],25))

mylist <- list(cat = df1, cat2 = df2)
lapply(mylist, head, 5)

Answer 1

I’d use the names of the list in this fashion:

dat1 = data.frame()
dat2 = data.frame()
l = list(dat1 = dat1, dat2 = dat2)
> str(l)
List of 2
 $ dat1:'data.frame':   0 obs. of  0 variables
 $ dat2:'data.frame':   0 obs. of  0 variables

and then use lapply + ddply like:

lapply(names(l), function(x) {
    ddply(l[[x]], c("idx", x), summarise,checkSum = sum(value))
  })

This remains untested without a reproducible answer. But it should help you in the right direction.

EDIT (ran2): Here’s the code using the reproducible example.

l <- lapply(names(mylist), function(x) {
ddply(mylist[[x]], c("idx", x), summarise,checkSum = sum(value))
})
names(l) <- names(mylist); l