Lengths of List or Vector Elements

Description

Get the length of each element of a list or atomic vector (is.atomic) as an integer or numeric vector.

Usage

lengths(x, use.names = TRUE)

Arguments

Details

This function loops over x and returns a compatible vector containing the length of each element in x. Effectively, length(x[[i]]) is called for all i, so any methods on length are considered.

lengths is generic: you can write methods to handle specific classes of objects, see InternalMethods.

Value

A non-negative integer of length length(x), except when any element has a length of more than 2^{31}-1 elements, when it returns a double vector. When use.names is true, the names are taken from the names on x, if any. The result has the same dimensions as x.

Note

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 lengths(x) is trivial in that case.

See Also

length for getting the length of any R object.

Examples

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
## or l has dimensions
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))))