# lengths: Lengths of List or Vector Elements

 lengths 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 `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.

`lengths` is generic: you can write methods to handle specific classes of objects, see InternalMethods.

### 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.

### 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 "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))))
```