lengths {base}  R Documentation 
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
x 
a 
use.names 
logical indicating if the result should inherit the

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 nonnegative 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
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 "listlike" 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))))