R Language Using built-in functionals


Example

Built-in functionals: lapply(), sapply(), and mapply()

R comes with built-in functionals, of which perhaps the most well-known are the apply family of functions. Here is a description of some of the most common apply functions:

  • lapply() = takes a list as an argument and applies the specified function to the list.
  • sapply() = the same as lapply() but attempts to simplify the output to a vector or a matrix.
    • vapply() = a variant of sapply() in which the output object's type must be specified.
  • mapply() = like lapply() but can pass multiple vectors as input to the specified function. Can be simplified like sapply().
    • Map() is an alias to mapply() with SIMPLIFY = FALSE.

lapply()

lapply() can be used with two different iterations:

  • lapply(variable, FUN)
  • lapply(seq_along(variable), FUN)
# Two ways of finding the mean of x
set.seed(1)
df <- data.frame(x = rnorm(25), y = rnorm(25))
lapply(df, mean)
lapply(seq_along(df), function(x) mean(df[[x]))

sapply()

sapply() will attempt to resolve its output to either a vector or a matrix.

# Two examples to show the different outputs of sapply()
sapply(letters, print)  ## produces a vector
x <- list(a = 1:10, beta = exp(-3:3), logic = c(TRUE,FALSE,FALSE,TRUE))
sapply(x, quantile)  ## produces a matrix

mapply()

mapply() works much like lapply() except it can take multiple vectors as input (hence the m for multivariate).

mapply(sum, 1:5, 10:6, 3) # 3 will be "recycled" by mapply