Duplicate list names in R

Question

What\'s going on here when there are duplicate list names in R?

l <- list()
l[[\"A\"]] <- 5
l[[\"B\"]] <- 7
l[[\"C\"]] <- 9
names(l) <- c(\"B\         


        

Answer 1

When you have duplicate names and you call a subset by name, only the first element is returned. In fact, [[ will only ever give you one element anyway, so let's look at [ instead.

l["B"]
# $B
# [1] 5

We can also see that trying c("B", "B") as the subset won't even give us the right result because R goes back and gets the first B again.

l[c("B", "B")]
# $B
# [1] 5
#
# $B
# [1] 5

One of the safest ways to retrieve all the B elements is to use a logical subset of the names() vector. This will give us the correct elements.

l[names(l) == "B"]
# $B
# [1] 5
#
# $B
# [1] 7

This is a great example of why duplicate names should be avoided.