Why does 'out of bounds' indexing differ between a matrix and a data.frame?

Question

I'm sure this is kind of basic, but I'd just like to really understand the logic of R data structures here.

If I subset a matrix by index out of bounds, I get exactly that error:

m <- matrix(data = c("foo", "bar"), nrow = 1)
m[2,]
# Error in m[2, ] : subscript out of bounds

If I do the same do a data frame, however, I get all NA rows:

df <- data.frame(foo = "foo", bar = "bar")
df[2,]
#    foo  bar
# NA <NA> <NA>

If I subset into a non-existent data frame column I get the familiar

df[, 3]
# Error in `[.data.frame`(df, , 3) : undefined columns selected

I know (roughly) that data frame rows are weird and to be treated carefully, but I don't quite see the connection to the above behavior.

Can someone explain why R behaves in this way for non-existent df rows?

Update

To be sure, giving NA on out-of-bounds subsets, is normal R behavior for 1D vectors:

vec <- c("foo", "bar")
vec[3]
# [1] NA

So in a way, the weird one out here is matrix subsetting, not dataframe subsetting, depending from where you're starting out. Still the different 2D subsetting behavior (m[2, ] vs df[2, ]) might strike a dense user (as I am right now) as inconsistent.

Answer 1

Can someone explain why R behaves in this way[?]

Short answer: No, probably not.

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Longer answer: Once upon a time I was thinking about something similar and read this thread on R-devel: Definition of [[. Basically it boils down to:

The semantics of [ and [[ don't seem to be fully specified in the Reference manual. [...] I assume that these are features, not bugs, but I can't find documentation for them

Duncan Murdoch, a former member of the R core team gives a very nice reply:

There is more documentation in the man page for Extract, but I think it is incomplete. The most complete documentation is of course the source code*, but it may not answer the question of what's intentional and what's accidental

As mentioned in the R-devel thread, the only description in the manual is 3.4.1 Indexing by vectors:

If i is positive and exceeds length(x) then the corresponding selection is NA

But, this applies to "indexing of simple vectors". Similar out of bounds indexing for "non-simple" vectors does not seem to be described. Duncan Murdoch again:

So what is a simple vector? That is not explicitly defined, and it probably should be.

Thus, it may seem like no one knows the answer to your why question.

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See also "8.2.13 nonexistent value in subscript" in the excellent R Inferno by Patrick Burns, and the section "Missing/out of bounds indices" in Hadley's book.

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*Source code for the [ subset operator. A search for R_MSG_subs_o_b (which corresponds to error message "subscript out of bounds") provides no obvious clue why OOB [ indexing of matrices and when using [[ give an error, whereas OOB [ indexing of "simple vectors" results in NA.