Lengths of List or Vector Elements
lengths(x, use.names = TRUE)
This function loops over
x and returns a compatible vector
containing the length of each element in
length(x[[i]]) is called for all
i, so any methods on
length are considered.
length(x), except when any element has a length of more than $2^31 - 1$ elements, when it returns a double vector. When
use.namesis true, the names are taken from the names on
x, if any.
One raison d'être of
lengths(x) is its use as a
more efficient version of
sapply(x, length) and similar
*apply calls to
length. This is the reason why
x may be an atomic vector, even though
trivial in that case.
length for getting the length of any R object.
require(stats) ## summarize by month l <- split(airquality$Ozone, airquality$Month) avgOz <- lapply(l, mean, na.rm=TRUE) ## merge result airquality$avgOz <- rep(unlist(avgOz, use.names=FALSE), lengths(l)) ## but this is safer and cleaner, but can be slower airquality$avgOz <- unsplit(avgOz, airquality$Month) ## should always be true, except when a length does not fit in 32 bits stopifnot(identical(lengths(l), vapply(l, length, integer(1L)))) ## empty lists are not a problem x <- list() stopifnot(identical(lengths(x), integer())) ## nor are "list-like" expressions: lengths(expression(u, v, 1+ 0:9)) ## and we should dispatch to length methods f <- c(rep(1, 3), rep(2, 6), 3) dates <- split(as.POSIXlt(Sys.time() + 1:10), f) stopifnot(identical(lengths(dates), vapply(dates, length, integer(1L))))