[Rd] partial matching of row names in [-indexing

[Ben Bolker]

   Makes sense if you realize that ?"[" only applies to *vector*, 
*list*, and *matrix* indexing and that data frames follow their own 
rules that are documented elsewhere ...

   So yes, not a bug but I claim it's an infelicity. I might submit a 
doc patch.

  FWIW

b["A1",]
as.matrix(b)["A1",]

  illustrates the difference.

  thanks
    Ben


On 1/14/22 9:19 PM, Steve Martin wrote:
> 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 using 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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>> https://stat.ethz.ch/mailman/listinfo/r-devel

-- 
Dr. Benjamin Bolker
Professor, Mathematics & Statistics and Biology, McMaster University
Director, School of Computational Science and Engineering
(Acting) Graduate chair, Mathematics & Statistics