| lapply | R Documentation |
lapply returns a list of the same length as X, each
element of which is the result of applying FUN to the
corresponding element of X.
sapply is a user-friendly version and wrapper of lapply
by default returning a vector, matrix or, if simplify = "array", an
array if appropriate, by applying simplify2array().
sapply(x, f, simplify = FALSE, USE.NAMES = FALSE) is the same as
lapply(x, f).
vapply is similar to sapply, but has a pre-specified
type of return value, so it can be safer (and sometimes faster) to
use.
replicate is a wrapper for the common use of sapply for
repeated evaluation of an expression (which will usually involve
random number generation).
simplify2array() is the utility called from sapply()
when simplify is not false and is similarly called from
mapply().
lapply(X, FUN, ...) sapply(X, FUN, ..., simplify = TRUE, USE.NAMES = TRUE) vapply(X, FUN, FUN.VALUE, ..., USE.NAMES = TRUE) replicate(n, expr, simplify = "array") simplify2array(x, higher = TRUE)
X |
a vector (atomic or list) or an |
FUN |
the function to be applied to each element of |
... |
optional arguments to |
simplify |
logical or character string; should the result be
simplified to a vector, matrix or higher dimensional array if
possible? For |
USE.NAMES |
logical; if |
FUN.VALUE |
a (generalized) vector; a template for the return value from FUN. See ‘Details’. |
n |
integer: the number of replications. |
expr |
the expression (a language object, usually a call) to evaluate repeatedly. |
x |
a list, typically returned from |
higher |
logical; if true, |
FUN is found by a call to match.fun and typically
is specified as a function or a symbol (e.g., a backquoted name) or a
character string specifying a function to be searched for from the
environment of the call to lapply.
Function FUN must be able to accept as input any of the
elements of X. If the latter is an atomic vector, FUN
will always be passed a length-one vector of the same type as X.
Arguments in ... cannot have the same name as any of the
other arguments, and care may be needed to avoid partial matching to
FUN. In general-purpose code it is good practice to name the
first two arguments X and FUN if ... is passed
through: this both avoids partial matching to FUN and ensures
that a sensible error message is given if arguments named X or
FUN are passed through ....
Simplification in sapply is only attempted if X has
length greater than zero and if the return values from all elements
of X are all of the same (positive) length. If the common
length is one the result is a vector, and if greater than one is a
matrix with a column corresponding to each element of X.
Simplification is always done in vapply. This function
checks that all values of FUN are compatible with the
FUN.VALUE, in that they must have the same length and type.
(Types may be promoted to a higher type within the ordering logical
< integer < double < complex, but not demoted.)
Users of S4 classes should pass a list to lapply and
vapply: the internal coercion is done by the as.list in
the base namespace and not one defined by a user (e.g., by setting S4
methods on the base function).
For lapply, sapply(simplify = FALSE) and
replicate(simplify = FALSE), a list.
For sapply(simplify = TRUE) and replicate(simplify =
TRUE): if X has length zero or n = 0, an empty list.
Otherwise an atomic vector or matrix or list of the same length as
X (of length n for replicate). If simplification
occurs, the output type is determined from the highest type of the
return values in the hierarchy NULL < raw < logical < integer < double <
complex < character < list < expression, after coercion of pairlists
to lists.
vapply returns a vector or array of type matching the
FUN.VALUE. If length(FUN.VALUE) == 1 a
vector of the same length as X is returned, otherwise
an array. If FUN.VALUE is not an array, the
result is a matrix with length(FUN.VALUE) rows and
length(X) columns, otherwise an array a with
dim(a) == c(dim(FUN.VALUE), length(X)).
The (Dim)names of the array value are taken from the FUN.VALUE
if it is named, otherwise from the result of the first function call.
Column names of the matrix or more generally the names of the last
dimension of the array value or names of the vector value are set from
X as in sapply.
sapply(*, simplify = FALSE, USE.NAMES = FALSE) is
equivalent to lapply(*).
For historical reasons, the calls created by lapply are
unevaluated, and code has been written (e.g., bquote) that
relies on this. This means that the recorded call is always of the
form FUN(X[[i]], ...), with i replaced by the current
(integer or double) index. This is not normally a problem, but it can
be if FUN uses sys.call or
match.call or if it is a primitive function that makes
use of the call. This means that it is often safer to call primitive
functions with a wrapper, so that e.g. lapply(ll, function(x)
is.numeric(x)) is required to ensure that method dispatch for
is.numeric occurs correctly.
If expr is a function call, be aware of assumptions about where
it is evaluated, and in particular what ... might refer to.
You can pass additional named arguments to a function call as
additional named arguments to replicate: see ‘Examples’.
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
apply, tapply,
mapply for applying a function to multiple
arguments, and rapply for a recursive version of
lapply(), eapply for applying a function to each
entry in an environment.
require(stats); require(graphics)
x <- list(a = 1:10, beta = exp(-3:3), logic = c(TRUE,FALSE,FALSE,TRUE))
# compute the list mean for each list element
lapply(x, mean)
# median and quartiles for each list element
lapply(x, quantile, probs = 1:3/4)
sapply(x, quantile)
i39 <- sapply(3:9, seq) # list of vectors
sapply(i39, fivenum)
vapply(i39, fivenum,
c(Min. = 0, "1st Qu." = 0, Median = 0, "3rd Qu." = 0, Max. = 0))
## sapply(*, "array") -- artificial example
(v <- structure(10*(5:8), names = LETTERS[1:4]))
f2 <- function(x, y) outer(rep(x, length.out = 3), y)
(a2 <- sapply(v, f2, y = 2*(1:5), simplify = "array"))
a.2 <- vapply(v, f2, outer(1:3, 1:5), y = 2*(1:5))
stopifnot(dim(a2) == c(3,5,4), all.equal(a2, a.2),
identical(dimnames(a2), list(NULL,NULL,LETTERS[1:4])))
hist(replicate(100, mean(rexp(10))))
## use of replicate() with parameters:
foo <- function(x = 1, y = 2) c(x, y)
# does not work: bar <- function(n, ...) replicate(n, foo(...))
bar <- function(n, x) replicate(n, foo(x = x))
bar(5, x = 3)
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