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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