Length of an Object

Get or set the length of vectors (including lists) and factors, and of any other R object for which a method has been defined.

length(x) <- value
an R object. For replacement, a vector or factor.
a non-negative integer or double (which will be rounded down).

Both functions are generic: you can write methods to handle specific classes of objects, see InternalMethods. length<- has a "factor" method. The replacement form can be used to reset the length of a vector. If a vector is shortened, extra values are discarded and when a vector is lengthened, it is padded out to its new length with NAs (nul for raw vectors). Both are primitive functions.


The default method for length currently returns a non-negative integer of length 1, except for vectors of more than \(2^{31}-1\) elements, when it returns a double. For vectors (including lists) and factors the length is the number of elements. For an environment it is the number of objects in the environment, and NULL has length 0. For expressions and pairlists (including language objects and dotlists) it is the length of the pairlist chain. All other objects (including functions) have length one: note that for functions this differs from S. The replacement form removes all the attributes of x except its names, which are adjusted (and if necessary extended by "").


Package authors have written methods that return a result of length other than one (https://CRAN.R-project.org/package=Formula) and that return a vector of type double (https://CRAN.R-project.org/package=Matrix), even with non-integer values (earlier versions of https://CRAN.R-project.org/package=sets). Where a single double value is returned that can be represented as an integer it is returned as a length-one integer vector.


Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

See Also

nchar for counting the number of characters in character vectors, lengths for getting the length of every element in a list.

  • length
  • length<-
  • length<-.factor
library(base) length(diag(4)) # = 16 (4 x 4) length(options()) # 12 or more length(y ~ x1 + x2 + x3) # 3 length(expression(x, {y <- x^2; y+2}, x^y)) # 3 ## from example(warpbreaks) require(stats) fm1 <- lm(breaks ~ wool * tension, data = warpbreaks) length(fm1$call) # 3, lm() and two arguments. length(formula(fm1)) # 3, ~ lhs rhs
Documentation reproduced from package base, version 3.4.0, License: Part of R 3.4.0

Community examples

richie@datacamp.com at Jan 17, 2017 base v3.3.2

The length of an atomic vector is simply the number of elements in it. ```{r} (v <- runif(10)) length(v) ``` The same is true of matrices. ```{r} (m <- matrix(1:30, 5, 6)) length(m) identical(length(m), nrow(m) * ncol(m)) ``` … and `array`s. ```{r} (a <- array(1:24, 2:4)) length(a) all.equal(length(a), prod(dim(a))) ``` For `list`s, the length is the number of top-level elements ```{r} (l <- list(a = 1, b = 2:4, c = 5:9, d = 10:16, e = 17:25)) length(l) ``` Be careful: for `data.frame`s, the length refers to the number of columns, not the total number of elements. ```{r} (d <- data.frame(x = letters[1:7], y = runif(7) > 0.5)) length(d) identical(length(d), ncol(d)) ``` formulae also have a length: 3 if there is a left-hand side, and 2 if not. ```{r} length(y ~ x1 + x2 + x3 + x4) length(~ x1 + x2 + x3 + x4) ``` Coerce the formulae to lists to see why. ```{r} as.list(y ~ x1 + x2 + x3 + x4) as.list(~ x1 + x2 + x3 + x4) ``` Functions always have length 1. ```{r} length(var) length(`c`) length(`if`) ``` So do names. ```{r} length(as.name("xyz")) ``` The length of a function `call` is 1 plus the number of arguments passed to the function. (Again, coerce to `list` to see why.) ```{r} (cl <- quote(mean(1:10, na.rm = TRUE))) length(cl) ``` The length of an expression is the number of calls, symbols or constants that you pass into it. If you pass multiple objects in braces, they only count as one item. ```{r} (ex <- expression( x, {y <- x ^ 2; y + 2}, mean(1:10, na.rm = TRUE), 1:5 )) length(ex) ``` External pointers always have length 1. ```{r} xptr <- xml2::read_xml("<foo><bar /></foo>")$node length(xptr) ```