Access and preserve list names in lapply function

Question

I need to access list names inside the lapply function. I've found some threads online where it's said I should iterate through the names of the list to be able to fetch each list element name in my function:

> n = names(mylist) > mynewlist = lapply(n, function(nameindex, mylist) { return(mylist[[nameindex]]) }, mylist) > names(mynewlist) NULL > names(mynewlist) = n 

The problem is that mynewlist loses the original mylist indexes and I have to add that last names() assignment to restore them.

Is there a way to give an explicit index name to each element returned by the lapply function? Or a different way to make sure mynewlist elements have the correct index names set? I feel mynewlist index names could be wrong if lapply does not return the list elements in the same order than mylist.

Answer 1

I believe that lapply by default keeps the names attribute of whatever you are iterating over. When you store the names of myList in n, that vector no longer has any "names". So if you add that back in via,

names(n) <- names(myList) 

and the use lapply as before, you should get the desired result.

Edit

My brains a bit foggy this morning. Here's another, perhaps more convenient, option:

sapply(n,FUN = ...,simplify = FALSE,USE.NAMES = TRUE) 

I was groping about, confused that lapply didn't have a USE.NAMES argument, and then I actually looked at the code for sapply and realized I was being silly, and this was probably a better way to go.

Answer 2

the setNames function is a useful shortcut here

mylist <- list(a = TRUE, foo = LETTERS[1:3], baz = 1:5) n <- names(mylist) mynewlist <- lapply(setNames(n, n), function(nameindex) {mylist[[nameindex]]}) 

which preserves the names

> mynewlist $a [1] TRUE  $foo [1] "A" "B" "C"  $baz [1] 1 2 3 4 5