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
--
Andrew Robinson
Department of Mathematics and Statistics Tel: +61-3-8344-9763
University of Melbourne, VIC 3010 Australia Fax: +61-3-8344-4599
http://www.ms.unimelb.edu.au/~andrewpr
http://blogs.mbs.edu/fishing-in-the-bay/
## The new function
my.tapply <- function (X, INDEX, FUN=NULL, ..., simplify=TRUE)
{
FUN <- if (!is.null(FUN)) match.fun(FUN)
if (!is.list(INDEX)) INDEX <- list(INDEX)
nI <- length(INDEX)
namelist <- vector("list", nI)
names(namelist) <- names(INDEX)
extent <- integer(nI)
nx <- length(X)
one <- as.integer(1)
group <- rep.int(one, nx)#- to contain the splitting vector
ngroup <- one
for (i in seq.int(INDEX)) {
index <- as.factor(INDEX[[i]])
if (length(index) != nx)
stop("arguments must have same length")
namelist[[i]] <- levels(index)#- all of them, yes !
extent[i] <- nlevels(index)
group <- group + ngroup * (as.integer(index) - one)
ngroup <- ngroup * nlevels(index)
}
if (is.null(FUN)) return(group)
ans <- lapply(split(X, group), FUN, ...)
index <- as.numeric(names(ans))
if (simplify && all(unlist(lapply(ans, length)) == 1)) {
ansmat <- array(dim=extent, dimnames=namelist)
ans <- unlist(ans, recursive = FALSE)
}
else {
ansmat <- array(vector("list", prod(extent)),
dim=extent, dimnames=namelist)
}
## old : ansmat[as.numeric(names(ans))] <- ans
names(ans) <- NULL
ansmat[index] <- ans
if (sum(table(INDEX) < 1) > 0)
ansmat[table(INDEX) < 1] <- do.call(FUN, list(c(NULL), ...))
ansmat
}
## Check its utility
group <- factor(c(1,1,3,3), levels=c("1","2","3"))
x <- c(1,2,3,4)
## Ok with mean?
tapply(x, group, mean)
my.tapply(x, group, mean)
## Ok with sum?
tapply(x, group, sum)
my.tapply(x, group, sum)
## Check that other arguments are carried through
x <- c(NA,2,3,10)
tapply(x, group, sum, na.rm=TRUE)
tapply(x, group, mean, na.rm=TRUE)
my.tapply(x, group, sum, na.rm=TRUE)
my.tapply(x, group, mean, na.rm=TRUE)
## Check that listed groups work ok also
group.2 <- factor(c(1,2,3,3), levels=c("1","2","3"))
tapply(x, list(group, group.2), sum, na.rm=TRUE)
tapply(x, list(group, group.2), mean, na.rm=TRUE)
my.tapply(x, list(group, group.2), sum, na.rm=TRUE)
my.tapply(x, list(group, group.2), mean, na.rm=TRUE)
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