ctapply: Fast tapply() replacement functions

Description

ctapply is a fast replacement of tapply that assumes contiguous input, i.e. unique values in the index are never speparated by any other values. This avoids an expensive split step since both value and the index chungs can be created on the fly. It also cuts a few corners to allow very efficient copying of values. This makes it many orders of magnitude faster than the classical lapply(split(), ...) implementation.

Usage

ctapply(X, INDEX, FUN, ..., MERGE=c)

Arguments

X: an atomic object, typically a vector
INDEX: numeric or character vector of the same length as X
FUN: the function to be applied
...: additional arguments to FUN. They are passed as-is, i.e., without replication or recycling
MERGE: function to merge the resulting vector or NULL if the arguments to such a functiona re to be returned instead

Author

Simon Urbanek

Details

Note that ctapply supports either integer, real or character vectors as indices (note that factors are integer vectors and thus supported, but you do not need to convert character vectors). Unlike tapply it does not take a list of factors - if you want to use a cross-product of factors, create the product first, e.g. using paste(i1, i2, i3, sep='\01') or multiplication - whetever method is convenient for the input types.

ctapply requires the INDEX to contiguous. One (slow) way to achieve that is to use sort or order.

Examples

Run this code

i = rnorm(4e6)
names(i) = as.integer(rnorm(1e6))
i = i[order(names(i))]
system.time(tapply(i, names(i), sum))
system.time(ctapply(i, names(i), sum))