Creating nested lists in a loop in R - r

Question

This bit of code does what I want it to do, but generates a warning for every iteration of the loop:
library(epiR)
cccList <- list()
for (i in 3:ncol(dfData)){
tmpvar <- paste("cccIntactVs.", i, sep = "")
assign(
tmpvar,
epi.ccc(
dfData[2:nrow(dfData),2],
dfData[2:nrow(dfData),i],
ci = "z-transform",
conf.level = 0.95,
rep.measure = FALSE
)
)
cccList[i] <- get(paste0("cccIntactVs.", i))
}
I get this warning every time the output of epi.ccc() is added to cccList():
Warning in cccList[i] <- get(paste0("cccIntactVs.", i)) :
number of items to replace is not a multiple of replacement length
Is there a more proper way of accomplishing this? The output of epi.ccc() is a list of 7 elements. Since the output is the same length each time and I'm only adding to the list, why is it complaining about mismatched lengths or replacement?

Answer 1

You want to use [[i]] instead of [i]
Basically, [ means you want to replace a certain part of a list with different content, and the replacement needs to have just as many items as the number of slots you are trying to replace.
OTOH, using [[ means you want to put everything you are assigning into one slot, which it seems you want to do.
An example of what happens:
myList <- list(1,2,3,4,5,6,7)
myList[3:5] <- c(11, 12, 13)
myList[[6]] <- c(14, 15, 16)
Here, 11-13 are distributed among slots 3 through 5: 3 replacement items in 3 slots.
And 14-16 are placed in one slot: This slot now contains a length-3 vector.
Now what happens if we try this?
myList[1] <- c(17,18,19)
We tell R it should distribute 3 items over one slot. It tries the best it can, which is not much: it discards everything but the first item. But luckily it warns you. If you really just want to assign just the first item, you can use
myList[1] <- c(17,18,19)[1]
But that's not really useful, there's no use in supplying 18 and 19.
You could make it a list of length one:
myList[1] <- list(c(17,18,19))
But generally, if you want to put it in one slot, using [[ is the way to go.
And as a sidetrack: why was it build this way?
The reason is that [ can give you access to multiple slots, and you might not know which ones or how many beforehand. What should happen if I try this?
someVar <- readLines(somefile) # someVar happens to be c(1, 2) instead of having length 1
myList[someVar] <- 21:23
Put 21:23 in both slots? Put 21 in the first slot and the rest (22, 23) in slot 2?
Using [[ means you are sure only one slot is used, and you're not unexpectedly overwriting anything.

Answer 2

cccList[i] <- get(paste0("cccIntactVs.", i)) will trigger this warning if get(paste0("cccIntactVs.", i)) is not of length 1.
Using get(paste0("cccIntactVs.", i))[1] should solve it but if you didn't expect get(paste0("cccIntactVs.", i)) to have a length superior to 1 you likely have a mistake somewhere else in your code, despite the result looking fine to you now.