R Language *apply family of functions (functionals) Using user-defined functionals
Example
User-defined functionals
Users can create their own functionals to varying degrees of complexity. The following examples are from Functionals by Hadley Wickham:
randomise <- function(f) f(runif(1e3))
lapply2 <- function(x, f, ...) {
out <- vector("list", length(x))
for (i in seq_along(x)) {
out[[i]] <- f(x[[i]], ...)
}
out
}
In the first case, randomise accepts a single argument f, and calls it on a sample of Uniform random variables. To demonstrate equivalence, we call set.seed below:
set.seed(123)
randomise(mean)
#[1] 0.4972778
set.seed(123)
mean(runif(1e3))
#[1] 0.4972778
set.seed(123)
randomise(max)
#[1] 0.9994045
set.seed(123)
max(runif(1e3))
#[1] 0.9994045
The second example is a re-implementation of base::lapply, which uses functionals to apply an operation (f) to each element in a list (x). The ... parameter allows the user to pass additional arguments to f, such as the na.rm option in the mean function:
lapply(list(c(1, 3, 5), c(2, NA, 6)), mean)
# [[1]]
# [1] 3
#
# [[2]]
# [1] NA
lapply2(list(c(1, 3, 5), c(2, NA, 6)), mean)
# [[1]]
# [1] 3
#
# [[2]]
# [1] NA
lapply(list(c(1, 3, 5), c(2, NA, 6)), mean, na.rm = TRUE)
# [[1]]
# [1] 3
#
# [[2]]
# [1] 4
lapply2(list(c(1, 3, 5), c(2, NA, 6)), mean, na.rm = TRUE)
# [[1]]
# [1] 3
#
# [[2]]
# [1] 4
