A suggestion for an amendment to tapply

On Tue, 6 Nov 2007, Bill.Venables at csiro.au wrote:

Unfortunately I think it would break too much existing code.  tapply()
is an old function and many people have gotten used to the way it works
now.

It is also not necessarily desirable: FUN(numeric(0)) might be an error.
For example:

Z <- data.frame(x=rnorm(10), f=rep(c("a", "b"), each=5))[1:5, ]
tapply(Z$x, Z$f, sd)

but sd(numeric(0)) is an error.  (Similar things involving var are 'in the 
wild' and so would be broken.)

This is not to suggest there could not be another argument added at the
end to indicate that you want the new behaviour, though.  e.g.

tapply <- function (X, INDEX, FUN=NULL, ..., simplify=TRUE,
handle.empty.levels = FALSE)

but this raises the question of what sort of time penalty the
modification might entail.  Probably not much for most situations, I
suppose.  (I know this argument name looks long, but you do need a
fairly specific argument name, or it will start to impinge on the ...
argument.)

Just some thoughts.

Bill Venables.

Bill Venables
CSIRO Laboratories
PO Box 120, Cleveland, 4163
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-----Original Message-----
From: r-devel-bounces at r-project.org
[mailto:r-devel-bounces at r-project.org] On Behalf Of Andrew Robinson
Sent: Tuesday, 6 November 2007 3:10 PM
To: R-Devel
Subject: [Rd] A suggestion for an amendment to tapply

Dear R-developers,

when tapply() is invoked on factors that have empty levels, it returns
NA.  This behaviour is in accord with the tapply documentation, and is
reasonable in many cases.  However, when FUN is sum, it would also
seem reasonable to return 0 instead of NA, because "the sum of an
empty set is zero, by definition."

I'd like to raise a discussion of the possibility of an amendment to
tapply.

The attached patch changes the function so that it checks if there are
any empty levels, and if there are, replaces the corresponding NA
values with the result of applying FUN to the empty set.  Eg in the
case of sum, it replaces the NA with 0, whereas with mean, it replaces
the NA with NA, and issues a warning.

This change has the following advantage: tapply and sum work better
together.  Arguably, tapply and any other function that has a non-NA
response to the empty set will also work better together.
Furthermore, tapply shows a warning if FUN would normally show a
warning upon being evaluated on an empty set.  That deviates from
current behaviour, which might be bad, but also provides information
that might be useful to the user, so that would be good.

The attached script provides the new function in full, and
demonstrates its application in some simple test cases.

Best wishes,

Andrew

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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