Using lists to change columns in multiple dataframes in R - r

Question

I am using a list of variables to download and create dataframes in R. I'd like to be able to use this list to make changes to different columns in each dataframe, but I am having trouble calling particular columns using the list of variables.
countries= c("USA","CHN")
for (i in 1:length(countries)){
download.file(url[i],savedata[i])
assign(countries[i],xmlToDataFrame(savedata[i]))
}
Now I have dataframes that look like this:
head(USA)
indicator country date value decimal
1 GDP (current US$) United States 2012 15684800000000 0
2 GDP (current US$) United States 2011 14991300000000 0
3 GDP (current US$) United States 2010 14419400000000 0
4 GDP (current US$) United States 2009 13898300000000 0
5 GDP (current US$) United States 2008 14219300000000 0
6 GDP (current US$) United States 2007 13961800000000 0
And I would like to go through and make several changes, such as formatting the date column with the as.date() function, or changing the units of the value column, but I want to be able to do the same to both dataframe (or an arbitrary number in case I increase the length of countries.
However, whenever I try to do this I can seem to use the list of countries in the countries variable to get 'inside' each data frame. My initial guess was putting something like this in a loop:
assign(paste(countries[i],"date",sep="$"),
as.date(get(paste(countries[i],"date",sep="$")))
In particular, I get confused about how the get(paste(countries[i])) works if I am not trying to get the particular column date, and how the paste(countries[i],"date",sep="$") prints the correct name, but I can't seem to get just the one column I'd like to manipulate.
Additionally, I realize loops are not the ideal way of doing this, but I've been having the same problem with the apply functions, though I am likely having trouble with them due to my lack of experience. Suggestions for either how to do it in a loop, or with out, would be much appreciated. Super R novice here, just trying to learn. Also, if you've come across a clear explanation/answer for this somewhere else, I'd appreciate you pointing me towards it.

Answer 1

It's much easier if you use lists. Start with an empty one:
mylist = list()
Then change this:
assign(countries[i],xmlToDataFrame(savedata[i]))
to this:
mylist[[i]] <- xmlToDataFrame(savedata[i])
Then make a function that does your formatting, for instance:
f <- function(df){
within(df, date <- as.date(date))
}
And use lapply to apply it to all dataframes:
mylist2 <- lapply(mylist, f)
If you want to access dataframes by name, use this:
names(mylist2) <- countries
And test:
mylist2[["USA"]]