Naming list elements in R

问题:

I've been doing some work with some large, complex lists lately and I've seen some behaviour which was surprising (to me, at least), mainly to do with assigning names to a list. A simple example:

Fil <- list(
a = list(A=seq(1, 5, 1), B=rnorm(5), C=runif(5)), 
b = list(A="Cat", B=c("Dog", "Bird"), C=list("Squirrel", "Cheetah", "Lion")),
c = list(A=rep(TRUE, 5), B=rep(FALSE, 5), C=rep(NA, 5)))

filList <- list()

for(i in 1:3){
  filList[i] <- Fil[i]
  names(filList)[i] <- names(Fil[i])
}
identical(Fil,filList)
[1] TRUE

but:

for(i in 1:3){
  filList[i] <- Fil[i]
  names(filList[i]) <- names(Fil[i])
}
identical(Fil,filList)
[1] FALSE

I think the main reason it confuses me is because the form of the left-hand side of first names line in the first for loop needs to be different from that of the right-hand side to work; I would have thought that these should be the same. Could anybody please explain this to me?

回答1:

The first case is the correct usage. In the second case you are sending filList[i] to names<- which is only exists as a temporary subsetted object.

Alternatively, you could just do everything outside the loop with:

names(filList) <- names(Fil)