dplyr 1.1.4 breaks on empty dataframes constructed from matrices

[john-b-edwards]

Previous versions of dplyr would allow you to manipulate empty dataframes that had been constructed from empty matrices, e.g.

packageurl <- "https://cran.r-project.org/src/contrib/Archive/dplyr/dplyr_1.1.3.tar.gz"
install.packages(packageurl, repos=NULL, type="source")
#> Installing package into 'C:/Users/edwar/Documents/R/win-library/4.1'
#> (as 'lib' is unspecified)
packageVersion('dplyr')
#> [1] '1.1.3'
as.data.frame(matrix(nrow = 0, ncol = 0)) |> dplyr::slice(1)
#> data frame with 0 columns and 0 rows

In dplyr 1.1.4, this code causes an internal error:

packageurl <- "https://cran.r-project.org/src/contrib/dplyr_1.1.4.tar.gz"
install.packages(packageurl, repos=NULL, type="source")
#> Installing package into 'C:/Users/edwar/Documents/R/win-library/4.1'
#> (as 'lib' is unspecified)
packageVersion('dplyr')
#> [1] '1.1.4'
as.data.frame(matrix(nrow = 0, ncol = 0)) |> dplyr::slice(1)
#> Error: Internal error: `template` must have a `names` attribute.

Both versions are able to reliably manipulate empty dataframes that were not constructed from matrices (e.g. data.frame() |> dplyr::slice(1) works just fine).

[DavisVaughan]

I think this may actually be a base R bug. I believe that all data.frames should have a names attribute. If there are 0 columns then it should be set to character(). The fact that that isn't the case here is odd.

attributes(as.data.frame(matrix(nrow = 0, ncol = 0)))
#> $class
#> [1] "data.frame"
#> 
#> $row.names
#> integer(0)

attributes(data.frame())
#> $names
#> character(0)
#> 
#> $row.names
#> integer(0)
#> 
#> $class
#> [1] "data.frame"

Not having a names attribute can seriously break other parts of base R, making my guess that it is a base R bug feel more validated:

# Good OOB error
df <- data.frame()
df[1]
#> Error in `[.data.frame`(df, 1): undefined columns selected

# WTF is this
df <- as.data.frame(matrix(nrow = 0, ncol = 0))
df[1]
#> NULL
#> <0 rows> (or 0-length row.names)

In general dplyr works fine with the 2nd case, i.e. a well formed data frame

---

I think we should consider switching from names2() to names() in DataMask to try and catch this earlier on:

dplyr/R/data-mask.R

Line 14 in d698022

names_bindings <- chr_unserialise_unicode(names2(data))

i.e. something like

names <- names(data)

if (is.null(names)) {
  abort("Can't transform a data frame with `NULL` names.")
}

which would go nicely with our other names related checks there.

It did use names() in the past, but I switched to names2() here to supposedly fix this exact case. Supposedly not supporting NULL column names broke 2 packages and that was what I was out to fix, but that seems like a false alarm because current CRAN dplyr obviously already doesn't support NULL column names based on this issue. So the change above would just be giving a better error message for this invalid data frame.
#4896