Apply a Function over a List or Vector

Description

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 of lapply by default returning a vector or matrix if appropriate.

replicate is a wrapper for the common use of sapply for repeated evaluation of an expression (which will usually involve random number generation).

Usage

lapply(X, FUN, ...)

sapply(X, FUN, ..., simplify = TRUE, USE.NAMES = TRUE)

replicate(n, expr, simplify = TRUE)

Arguments

Details

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.

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.

Users of S4 classes should pass a list to lapply: the internal coercion is done by the system as.list in the base namespace and not one defined by a user (e.g. by setting S4 methods on the system function).

Value

For lapply and sapply(simplify=FALSE), a list.

For sapply(simplify=TRUE) and replicate: 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 < real < complex < character < list < expression, after coercion of pairlists tolists.

Note

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[[0L]], ...), with 0L replaced by the current integer index. This 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 in R 2.7.1 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’.

References

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

See Also

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.

Examples

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)

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)