[Rd] rbind.data.frame reacts on levels without factor (PR#9578)

[ripley at stats.ox.ac.uk]

There is no example to reproduce here. Please do show the courtesy to 
follow the request at the bottom of every R-help posting and many other 
places and provide some reproducible evidence to support your points.

On Wed, 21 Mar 2007, prechelt at inf.fu-berlin.de wrote:

> Full_Name: Lutz Prechelt
> Version: 2.4.1
> OS: Windows XP
> Submission from: (NULL) (160.45.111.67)
>
>
> I stack a number of data.frames using rbind.
> Each of these dataframes has a column 'authorname', which is a factor
> and a column author = unclass(authorname) as piecewise pseudonyms.
> When using rbind to stack these dataframes, R warns about invalid factor levels
> and inserts all NAs in the author column.
>
> The reason appears to be that rbind.data.frame looks for the presence of levels,
> not actually for class==factor when deciding what to handle as a factor:
>                  if (!is.null(levels(xj))) {
>
> I find this behavior surprising, hence dangerous, and it is not documented.
> Rather, the documentation says:
>    "The 'rbind' data frame method takes the classes of the columns
>     from the first data frame, and matches columns by name (rather
>     than by position).  Factors have their levels expanded as
>     necessary [...]"
>
> The behavior has bitten me fairly hard, because I searched for the origin of the
> warning in all the wrong places before finding the real one after about 3
> hours.
> (Although I still have not understood _why_ it results in that warning.)
>
> I believe the behavior of rbind.data.frame should be fixed, so that it ignores
> levels attributes when there is no factor class as well.
>
> The alternative would be to just add a warning to the documentation that
> 'unclass' on factors is insufficient if users want to avoid factor handling for
> rbind.
>
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>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
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