# lengths

0th

Percentile

##### Lengths of List or Vector Elements

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

Keywords
attribute
##### Usage
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'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.

length for getting the length of any R object.
library(base) 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))))