lengths
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 anexpressionor an atomic vector (for which the result is trivial).- use.names
logical indicating if the result should inherit the
namesfromx.
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'<U+00EA>tre 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
library(base)
# NOT RUN {
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))))
# }