[Rd] Misleading sentence in documentation of data.frame (PR#13850)

[groemping at bht-berlin.de]

Full_Name: Ulrike Groemping
Version: 2.9.0
OS: Windows
Submission from: (NULL) (84.190.173.190)


The documentation for data.frame contains the sentence "A data frame is a list
of variables of the same length with unique row names, given class
"data.frame"." This of course does describe the standard use of data frames, but
other versions are possible (e.g. df <- swiss; df$proof <- as.matrix(swiss);
ncol(df); df; length(df$proof)), which is important to know at least for package
developers and should be documented (it is documented with the extractor methods
for data.frame, but I don't think that this is sufficient).

Regards, Ulrike