Writing functions in R

R
intermediate
Published

June 18, 2024

Session materials

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Welcome!

  • this session is an 🌶🌶 intermediate practical designed for those with some R experience

Session outline

  • why functions?
  • basic syntax
  • adding arguments
  • vectorised functions
  • the mystery of the paired brackets
  • …

Why functions?

  • most beginners write repetitious code
  • repetitious code is hard to maintain
  • functions give you an easy way of repeating chunks of code

Basic syntax

  • think of this as a way of repeating yourself
  • in time-honoured fashion…
hi_wrld <- function(){
  "hello world"
}

hi_wrld()
[1] "hello world"

Adding arguments

  • most of the time, you’ll want to add arguments to your function
    • add a variable name inside the round bracket of function
    • use that variable name in your function body
hi_wrld_n <- function(n){
  paste(rep("hello world", n))
}

hi_wrld_n(4)
[1] "hello world" "hello world" "hello world" "hello world"

Another argument

  • you can add another argument
  • either position or name can be used in the function call
hi_name_n <- function(name, n){
  rep(paste("hello", name) , n)
}

hi_name_n("sue", 4)
[1] "hello sue" "hello sue" "hello sue" "hello sue"
hi_name_n(n = 3, name = "tango") # evil but legal
[1] "hello tango" "hello tango" "hello tango"

Defaults

hi_name_n_def <- function(n, name = "janelle"){
  rep(paste("hello", name) , n)
}

hi_name_n_def(n = 4)
[1] "hello janelle" "hello janelle" "hello janelle" "hello janelle"
hi_name_n_def(n = 2, name = "bruce")
[1] "hello bruce" "hello bruce"

Vectorised functions

round(c(1.2, 3.2, 5.4, 2.7), 0)
[1] 1 3 5 3
  • that means that mostly, our functions will end up vectorised without us doing any work at all
div_seven_n_round <- function(nums){
  round(nums / 7, 0)
}

numbers <- rnorm(4, 5, 50)

numbers
[1] 29.34742 13.74713 26.13774 21.76297
div_seven_n_round(numbers)
[1] 4 2 4 3
  • but there are a few cases where that can fail: most famously, using if/else
is_even <- function(n){
  
  if(n %% 2){
  paste(n, "is odd")
} else {
  paste(n, "is even")
}
  
}
is_even(9)
[1] "9 is odd"
is_even(10)
[1] "10 is even"
try(is_even(9:10))
Error in if (n%%2) { : the condition has length > 1

Three solutions

vectorize with Vectorize

is_even_v <- Vectorize(is_even)
is_even_v(9:10)
[1] "9 is odd"   "10 is even"

apply

  • apply with lapply / purrr::map with Vectorize
lapply(9:10, is_even)
[[1]]
[1] "9 is odd"

[[2]]
[1] "10 is even"
purrr::map(9:10, is_even)
[[1]]
[1] "9 is odd"

[[2]]
[1] "10 is even"

refactor

  • refactor to avoid scalar functions
is_even_rf <- function(n){
  ifelse(n %% 2, paste(n, "is odd"), paste(n, "is even"))
}
is_even_rf(9:10)
[1] "9 is odd"   "10 is even"

{{}}

What’s the problem?

mtcars |>
  dplyr::summarise(average = round(mean(hp)))
  average
1     147

so

carmo <- function(column){
  mtcars |>
    dplyr::summarise(average = round(mean(column)))
  }
  • but…
try(carmo(hp))
Error in dplyr::summarise(mtcars, average = round(mean(column))) : 
  ℹ In argument: `average = round(mean(column))`.
Caused by error:
! object 'hp' not found

object ‘hp’ not found

  • we get used to R (and particularly tidyverse) helping us with some sugar when selecting column by their names
    • mtcars$hp / mtcars |> select(hp)
    • effectively, we’re just able to specify hp like an object, and R figures out the scope etc for us
  • that misfires inside functions. R isn’t sure where to look for an object called hp

Enter {{}}

carmo_woo <- function(column){
  mtcars |>
    dplyr::summarise(average = round(mean({{column}})))
}

carmo_woo(hp)
  average
1     147
  • for 95% of purposes, take {{}} as a purely empirical fix
  • but, if you’re very enthusiastic:

…

  • pass arbitrary arguments into/through a function with …
dotty <- function(n, ...){
  rep(paste(..., collapse = ""), n)
}

dotty(4, letters[1:5])
[1] "abcde" "abcde" "abcde" "abcde"

Resources

  • best = home made! Refactor something simple in your code today.
  • hard to beat the treatment of functions in R4DS
  • the Rlang page on data masking is surprisingly sane for such a complicated area