Growing a list with variable names in R

Question

I am trying to grow a list in R, where both the value and name of each entry is held in a variable, but it doesn't seem to work.

my_models_names <- names(my_models)
my_rocs=list() 
for (modl in my_models_names) {

    my_probs <- testPred[[modl]]$Y1
    my_roc <- roc(Ytst, my_probs)
    c(my_rocs, modl=my_roc) # <-- modl and my_roc are both variables
    }

My list my_rocs is empty at the end, even though I know that the loop iterates (my_roc is filled in) Why?

On a related note, is there a way to do this without looping?

Answer 1

Generally in R, growing objects is bad. It increases the amount of memory used over starting with the full object and filling it in. It seems you know what the size of the list should be in advance.

For example:

my_keys <- letters[1:3]
mylist <- vector(mode="list", length=length(my_keys))
names(mylist) <- my_keys

mylist
## $a
## NULL

## $b
## NULL

## $c
## NULL

You can do assignment this way:

key <- "a"
mylist[[key]] <- 5
mylist
## $a
## [1] 5
##
## $b
## NULL
##
## $c
## NULL