R devel - Jan 2022 - partial matching of row names in [-indexing

People are often surprised that row-indexing a data frame by [ + 
character does partial matching (and annoyed that there is no way to 
turn it off:

https://stackoverflow.com/questions/18033501/warning-when-partial-matching-rownames

https://stackoverflow.com/questions/34233235/r-returning-partial-matching-of-row-names

https://stackoverflow.com/questions/70716905/why-does-r-have-inconsistent-behaviors-when-a-non-existent-rowname-is-retrieved


?"[" says:

Character indices can in some circumstances be partially matched
      (see ?pmatch?) to the names or dimnames of the object being
      subsetted (but never for subassignment).  UNLIKE S (Becker et al_
      p. 358), R NEVER USES PARTIAL MATCHING WHEN EXTRACTING BY ?[?, and
      partial matching is not by default used by ?[[? (see argument
      ?exact?).

(EMPHASIS ADDED).

Looking through the rest of that page, I don't see any other text that 
modifies or supersedes that statement.

   Is this a documentation bug?

The example given in one of the links above:

b <- as.data.frame(matrix(4:5, ncol = 1, nrow = 2, dimnames = 
list(c("A10", "B"), "V1")))

b["A1",]  ## 4 (partial matching)
b[rownames(b) == "A1",]  ## logical(0)
b["A1", , exact=TRUE]    ## unused argument error
b$V1[["A1"]] ## subscript out of bounds error
b$V1["A1"]   ## NA

I don't think this is a bug in the documentation. The help page for
`?[.data.frame` has the following in the last paragraph of the
details:

Both [ and [[ extraction methods partially match row names. By default
neither partially match column names, but [[ will if exact = FALSE
(and with a warning if exact = NA). If you want to exact matching on
row names use match, as in the examples.

The example it refers to is

sw <- swiss[1:5, 1:4]  # select a manageable subset
sw["C", ] # partially matches
sw[match("C", row.names(sw)), ] # no exact match

Whether this is good behaviour or not is a different question, but the
documentation seems clear enough (to me, at least).

Best,
Steve

On Fri, 14 Jan 2022 at 20:40, Ben Bolker <bbolker at gmail.com>
wrote:
>
>
>    People are often surprised that row-indexing a data frame by [ +
> character does partial matching (and annoyed that there is no way to
> turn it off:
>
>
https://stackoverflow.com/questions/18033501/warning-when-partial-matching-rownames
>
>
https://stackoverflow.com/questions/34233235/r-returning-partial-matching-of-row-names
>
>
https://stackoverflow.com/questions/70716905/why-does-r-have-inconsistent-behaviors-when-a-non-existent-rowname-is-retrieved
>
>
> ?"[" says:
>
> Character indices can in some circumstances be partially matched
>       (see ?pmatch?) to the names or dimnames of the object being
>       subsetted (but never for subassignment).  UNLIKE S (Becker et al_
>       p. 358), R NEVER USES PARTIAL MATCHING WHEN EXTRACTING BY ?[?, and
>       partial matching is not by default used by ?[[? (see argument
>       ?exact?).
>
> (EMPHASIS ADDED).
>
> Looking through the rest of that page, I don't see any other text that
> modifies or supersedes that statement.
>
>    Is this a documentation bug?
>
> The example given in one of the links above:
>
> b <- as.data.frame(matrix(4:5, ncol = 1, nrow = 2, dimnames >
list(c("A10", "B"), "V1")))
>
> b["A1",]  ## 4 (partial matching)
> b[rownames(b) == "A1",]  ## logical(0)
> b["A1", , exact=TRUE]    ## unused argument error
> b$V1[["A1"]] ## subscript out of bounds error
> b$V1["A1"]   ## NA
>
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