Would it be appropriate to add a Which higher-order function?
Which <- function(f, x) {
ind <- vapply(x, f, logical(1))
!is.na(ind) & ind
}
Then Filter would be:
Filter <- function(f, x) x[Which(f, x)]
I think the use of vapply is slightly better than the current
as.logical(sapply(x, f)) because it will return a more informative
error message if the predicate doesn't return a logical vector of the
correct length.
Hadley
--
Assistant Professor / Dobelman Family Junior Chair
Department of Statistics / Rice University
http://had.co.nz/
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