Hello,
I found what looks to me like an odd edge case for duplicated(), unique() etc.
on data frames with zero columns, due to duplicated() returning a zero-length
vector for them, regardless of the number of rows:
df <- data.frame(a = 1:5)df$a <- NULLnrow(df) # 5 (row count preserved by
row.names)duplicated(df) # logical(0), should be c(FALSE, TRUE, TRUE, TRUE,
TRUE)anyDuplicated(df) # 0, should be 2nrow(unique(df)) # 0, should be 1
This behaviour isn't mentioned in the documentation; is there a reason for it
to work like this?I'm struggling to see this as anything other than unintended
behaviour, as a consequence of the do.call(Map, `names<-(c(list, x), NULL)`)
expression in duplicated.data.frame returning an empty list instead of a list
of empty lists.
Other data frame libraries have similar behaviour: tibble does the same;
data.table, Python's pandas and Rust's polars drop all the rows as soon as
there are zero columns, because they don't preserve the row count via the row
names.
---
I admit this is a case we rarely care about.However, for an example of this
being an issue, I've been running into it when treating data frames as database
relations, where they have one or more candidate keys (irreducible subsets of
the columns for which every row must have a unique value set).Sometimes, a
generated relation can have an empty candidate key, which limits it to only
having zero or one rows.Usually, I can check a relation contains no duplicated
key values by using anyDuplicated:
df2 <- unique(ChickWeight[, c("Chick", "Diet")])keycols <- "Chick" # Each chick
only has one diet (Chick -> Diet)!anyDuplicated(df2[, keycols, drop = FALSE]) #
TRUE, so Chick values are unique
When the key is empty, any row after the first must be a duplicate, but
anyDuplicated doesn't detect these because of the above edge case, so I have to
add special handling:
df3 <- data.frame(a = rep(1, 5)) # relations shouldn't have duplicate
rowskeycols <- character(0) # a is constant, so key is
empty!anyDuplicated(df3[, keycols, drop = FALSE]) # TRUE because equivalent to
!any(logical(0)) by above, should be FALSE
---
Best Regards,Mark
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