[Rd] median and data frames

[Patrick Burns]

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(df3.2)
[1] 2 8
 > median(df4.2)
[1] 2.5 8.5
 > median(df3.3)
   NA
1  7
2  8
3  9
 > median(df4.3)
   NA
1  7
2  8
3  9
4 10
 > median(df3.4)
[1]  8 11
 > 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.

Under the current regime we can get numbers
that are correct, partially correct, or sort
of random (given the intention).

I claim that much better behavior would be
to always get exactly one of the following:

* a numeric answer (that is consistently correct)
* an error

I would think a method in analogy to
'mean.data.frame' would be a logical choice.
But I'm presuming there might be an argument
against that or 'median.data.frame' would already
exist.


 > sessionInfo()
R version 2.13.0 (2011-04-13)
Platform: i386-pc-mingw32/i386 (32-bit)

locale:
[1] LC_COLLATE=English_United Kingdom.1252
[2] LC_CTYPE=English_United Kingdom.1252
[3] LC_MONETARY=English_United Kingdom.1252
[4] LC_NUMERIC=C
[5] LC_TIME=English_United Kingdom.1252

attached base packages:
[1] graphics  grDevices utils     datasets  stats     methods   base

other attached packages:
[1] xts_0.8-0 zoo_1.6-5

loaded via a namespace (and not attached):
[1] grid_2.13.0     lattice_0.19-23 tools_2.13.0

-- 
Patrick Burns
pburns at pburns.seanet.com
twitter: @portfolioprobe
http://www.portfolioprobe.com/blog
http://www.burns-stat.com
(home of 'Some hints for the R beginner'
and 'The R Inferno')