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

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lengths(x, use.names = TRUE)

Arguments

x

a list, list-like such as an expression or an atomic vector (for which the result is trivial).

use.names

logical indicating if the result should inherit the names from x.

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.

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.

Note

One raison d'ĂȘtre 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

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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))))

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