[Rd] median and data frames

[Martin Maechler]

>>>>> Martin Maechler <maechler at stat.math.ethz.ch>
>>>>>     on Fri, 29 Apr 2011 16:25:09 +0200 writes:

>>>>> Paul Johnson <pauljohn32 at gmail.com>
>>>>>     on Thu, 28 Apr 2011 00:20:27 -0500 writes:

    >> On Wed, Apr 27, 2011 at 12:44 PM, Patrick Burns
    >> <pburns at pburns.seanet.com> wrote:
    >>> Here are some data frames:
    >>> 
    >>> df3.2 <- data.frame(1:3, 7:9) df4.2 <- data.frame(1:4,
    >>> 7:10) df3.3 <- data.frame(1:3, 7:9, 10:12) df4.3 <-
    >>> data.frame(1:4, 7:10, 10:13) df3.4 <- data.frame(1:3,
    >>> 7:9, 10:12, 15:17) df4.4 <- data.frame(1:4, 7:10, 10:13,
    >>> 15:18)
    >>> 
    >>> Now here are some commands and their answers:

    >>>> median(df4.4)
    >>> [1]  8.5 11.5
    >>>> median(df3.2[c(1,2,3),])
    >>> [1] 2 8
    >>>> median(df3.2[c(1,3,2),])
    >>> [1]  2 NA Warning message: In mean.default(X[[2L]], ...)
    >>> :  argument is not numeric or logical: returning NA
    >>> 
    >>> 
    >>> 
    >>> The sessionInfo is below, but it looks to me like the
    >>> present behavior started in 2.10.0.
    >>> 
    >>> Sometimes it gets the right answer.  I'd be grateful to
    >>> hear how it does that -- I can't figure it out.
    >>> 

    > Hello, Pat.

    >> Nice poetry there!  I think I have an actual answer, as
    >> opposed to the usual crap I spew.

    >> I would agree if you said median.data.frame ought to be
    >> written to work columnwise, similar to mean.data.frame.

    >> apply and sapply always give the correct answer

    >>> apply(df3.3, 2, median)
    >> X1.3 X7.9 X10.12 2 8 11

    >     [...........]

    > exactly

    >> mean.data.frame is now implemented as

    >> mean.data.frame <- function(x, ...) sapply(x, mean, ...)

    > exactly.

    > My personal oppinion is that mean.data.frame() should
    > never have been written.  People should know, or learn, to
    > use apply functions for such a task.

    > The unfortunate fact that mean.data.frame() exists makes
    > people think that median.data.frame() should too, and then

    >   var.data.frame() sd.data.frame() mad.data.frame()
    > min.data.frame() max.data.frame() ...  ...

    > all just in order to *not* to have to know sapply() ????

    > No, rather not.

    > My vote is for deprecating mean.data.frame().
    > Martin

This has now happened -- for R 2.14.0 and later.
As raised in this thread in April, there's a similar
"extra helpful" behavior within the sd() function,
and we've also deprecated that.

In addition -- getting back to Pat Burns' original post,
I'm also proposing to change  median(<data.frame>)  
such that it produces an error instead of the current "sometimes
correct" (but mostly not!) results. 

Martin