[Rd] xtabs to dataframe fails (PR#3754)

[Kurt Hornik]

>>>>> daniel frey writes:

> Full_Name: Daniel Frey
> Version: 1.7.1
> OS: Windows 2000
> Submission from: (NULL) (80.254.164.242)


> Generating a data frame out of a xtabs result acts unusual. Take the following
> sample to reproduce it:

>> a.a <- c("a","a","a","b","b")

>> a.b <- c("c","c","d","e","f")

>> a.df <- data.frame(list("A"=a.a,"B"=a.b))

>> a.x <- xtabs(~A+B,a.df)

>> a.x
>        B
>     A   c d e f
>       a 2 1 0 0
>       b 0 0 1 1

>> data.frame(a.x)
>       A B Freq
>     1 a c    2
>     2 b c    0
>     3 a d    1
>     4 b d    0
>     5 a e    0
>     6 b e    1
>     7 a f    0
>     8 b f    1

> I would expect something like a.x itself. Instead I have to give the exact
> bounds, otherwise it doesn't convert properly:

>> data.frame(a.x[1:2,1:4])
>       c d e f
>     a 2 1 0 0
>     b 0 0 1 1

> I consider this a bug, as consistency of the handling breaks here.

See ?table:

     The 'as.data.frame' method for objects inheriting from class
     '"table"' can be used to convert the array-based representation of
     a contingency table to a data frame containing the classifying
     factors and the corresponding counts (the latter as component
     'Freq').  This is the inverse of 'xtabs'.

and

R> class(a.x)
[1] "xtabs" "table"

in your example.  Use data.frame(I(x)) to prevent conversion to the data
frame representation of the contingency table.

-k