R Why does listing an element in a data frame by row name and component name return NA?

Question

I create a data frame dfrm with one column, and set the row names as so:

v1 = c(1,2,3)
dfrm <- data.frame(v1)
row.names(dfrm) <- c("AD","BP","CD")

dfrm
   v1
AD  1
BP  2
CD  3

I can access elements by row name and index:

dfrm$v1[1]
[1] 1

I can access elements by row name and component name in quotes:

dfrm["AD","v1"]
[1] 1

But why can't I access elements by row name and component name?

dfrm$v1["AD"]
[1] NA

Answer 1

The answer is that vectors don't have rownames although they can have names.

When you access a column as a list item, R does not take the additional step of passing along the rownames to the names of the vector:

> dfrm$v1
[1] 1 2 3
> dfrm[["v1"]]
[1] 1 2 3
> dfrm[,"v1"]
[1] 1 2 3
> dfrm[,1]
[1] 1 2 3
> names(dfrm$v1)
NULL

Note that this is probably a good thing, as the cases where this is helpful are limited, and the overhead to copy the names every time a data.frame has a column pulled out likely not worth it.

If you want to copy them yourself:

> vone <- dfrm$v1
> names(vone) <- rownames(dfrm)
> vone
AD BP CD 
 1  2  3