lengths {base}  R Documentation 
Get the length of each element of a list
or atomic
vector (is.atomic
) as an integer or numeric vector.
lengths(x, use.names = TRUE)
x 
a 
use.names 
logical indicating if the result should inherit the

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.
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.
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.
length
for getting the length of any R object.
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))))