The purrr package in R

R
intermediate
Published

July 29, 2024

Session materials

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  • The session was a useful introduction to purrr, though I struggled a bit to identify how I might use it in my daily work
  • .

Welcome

  • this session is for 🌶🌶 intermediate users
  • you’ll need R + Rstudio / Posit Workbench / posit.cloud to follow along

Session outline

  • a digression about Linnaeus
  • functionals
  • base-R functional programming
  • map and walk
  • map2 and pmap
  • niceties and add-ons

A digression about Linnaeus

1758 edition of the Systema Naturae
  • used an existing system of binomial classification
  • Homo sapiens
    • Homo = generic name, which applies to similar species
    • sapiens = specific name, for that species and that species only
  • Pan troglodytes and Pan paniscus = two similar species in a genus
  • Elephas maximus and Loxodonta africana = two similar species in different genera

Functionals

Here are some numbers:

n1 <- 7:9

Let’s find their average. We’d usually do this by passing those numbers to a function:

mean(n1)
[1] 8

But in R, interestingly, we can also do this the other way round by passing a function name:

my_num_f <- function(funct = mean) funct(n1)
my_num_f(mean)
[1] 8
my_num_f(sum)
[1] 24

We’d describe this as a functional. It’s fun, but a bit messy and annoying (e.g. how to change the numbers you’re averaging??).

Functional programming in base R

Say we’ve got a function we want to apply:

round_root <- function(n) round(n ^ 0.5, 1)

There are several ways of applying functions to stuff in base R. + we could use a loop: that’s another session + we could just exploit the vectorised nature of most functions in R

round_root(n1)
[1] 2.6 2.8 3.0
  • or we could use some of the apply family of functions, like lapply and sapply:
lapply(n1, round_root) # returns a list
[[1]]
[1] 2.6

[[2]]
[1] 2.8

[[3]]
[1] 3
sapply(n1, round_root) # simplifies that list to a vector
[1] 2.6 2.8 3.0

There’s no real reason to use these functions when things are this simple, but when our applications become more complicated…

n2 <- 11:13

lapply(list(n1, n2), round_root)
[[1]]
[1] 2.6 2.8 3.0

[[2]]
[1] 3.3 3.5 3.6
sapply(list(n1, n2), round_root) # oddball output
     [,1] [,2]
[1,]  2.6  3.3
[2,]  2.8  3.5
[3,]  3.0  3.6
lapply(list(n1, n2[1:2]), round_root) # quirky
[[1]]
[1] 2.6 2.8 3.0

[[2]]
[1] 3.3 3.5

purrr

map

map is our purrr type specimen

library(purrr)
map(n1, round_root)
[[1]]
[1] 2.6

[[2]]
[1] 2.8

[[3]]
[1] 3

Pleasingly, map will handle all kinds of odd inputs without fuss:

map(c(n1, n2), round_root)
[[1]]
[1] 2.6

[[2]]
[1] 2.8

[[3]]
[1] 3

[[4]]
[1] 3.3

[[5]]
[1] 3.5

[[6]]
[1] 3.6
map(dplyr::tibble(n1 = n1, n2 = n2), round_root)
$n1
[1] 2.6 2.8 3.0

$n2
[1] 3.3 3.5 3.6
map(rbind(n1, n2), round_root) 
[[1]]
[1] 2.6

[[2]]
[1] 3.3

[[3]]
[1] 2.8

[[4]]
[1] 3.5

[[5]]
[1] 3

[[6]]
[1] 3.6

map will always return a list - that’s because, no matter what the output, you can always cram it into a list. If you want different output, you can have it. You just need to find the right species:

map_vec(n1, round_root)
[1] 2.6 2.8 3.0
try(map_int(n1, round_root)) # surly and strict
Error in map_int(n1, round_root) : ℹ In index: 1.
Caused by error:
! Can't coerce from a number to an integer.
round_root_int <- function(n) as.integer(n ^ 0.5)
map_int(n1, round_root_int) 
[1] 2 2 3
round_root_lgl <- function(n) as.integer(n ^ 0.5) %% 2 == 0
map_lgl(n1, round_root_lgl)
[1]  TRUE  TRUE FALSE

anonymous functions

If you’re comfortable with the new anonymous function syntax, you can build an anonymous function in place:

map_lgl(1:4, \(x) x %% 2 == 0)
[1] FALSE  TRUE FALSE  TRUE

walk

walk is intended for code where the side-effect is the point: graphs, pipes, and Rmarkdown especially. Otherwise, it’s as map:

walk(n1, round_root) # wtf?
round_root_print <- function(n) print(n ^ 0.5)
walk(n1, round_root_print) 
[1] 2.645751
[1] 2.828427
[1] 3
round_root_cat <- function(n) cat(n ^ 0.5, "  \n")
walk(n1, round_root_cat)
2.645751   
2.828427   
3

map2

map2 is for 2-argument functions:

map2_int(n1, n2, `+`) # the best terrible way of adding I know
[1] 18 20 22
round_root_places <- function(n, dp = 1) round(n ^ 0.5, dp)
round_root_places(n1, 0)
[1] 3 3 3
map2(n1, 0, round_root_places)
[[1]]
[1] 3

[[2]]
[1] 3

[[3]]
[1] 3

Beware of recycling rules

You’ll be unable to use map2 if your inputs are different lengths:

try(map2(1:3, 0:3, round_root_places))
Error in map2(1:3, 0:3, round_root_places) : 
  Can't recycle `.x` (size 3) to match `.y` (size 4).

This makes expand.grid valuable if you’re looking to try out all the combinations of two vectors, for example.

dat <- expand.grid(nums = 1:3, dplaces = 0:3) 

map2(dat$nums, dat$dplaces, round_root_places)
[[1]]
[1] 1

[[2]]
[1] 1

[[3]]
[1] 2

[[4]]
[1] 1

[[5]]
[1] 1.4

[[6]]
[1] 1.7

[[7]]
[1] 1

[[8]]
[1] 1.41

[[9]]
[1] 1.73

[[10]]
[1] 1

[[11]]
[1] 1.414

[[12]]
[1] 1.732

Or (probably more usefully) this could be done inside dplyr::mutate to add to a tibble:

expand.grid(nums = n1, dplaces = 0:3) |>
  dplyr::as_tibble() |>
  dplyr::mutate(rr = map2_vec(nums, dplaces, round_root_places)) |>
  knitr::kable()
nums dplaces rr
7 0 3.000
8 0 3.000
9 0 3.000
7 1 2.600
8 1 2.800
9 1 3.000
7 2 2.650
8 2 2.830
9 2 3.000
7 3 2.646
8 3 2.828
9 3 3.000

pmap

pmap is for n argument functions.

round_roots_places <- function(n, root = 2, places = 1) round(n ^ 1/root, places)

round_roots_places(n1, root = 4, places = 2) # use named arguments to avoid misery
[1] 1.75 2.00 2.25
pmap(list(n = n1, root = 4, places = 2), round_roots_places)
[[1]]
[1] 1.75

[[2]]
[1] 2

[[3]]
[1] 2.25

Niceties and addons

imap allows you to work with indicies, and list_c converts simple lists back to vectors:

imap(list("a", "b", "c"), \(x, y) paste0(y, ": ", x)) |> # index map where y is the name or index
  list_c()
[1] "1: a" "2: b" "3: c"

If your function returns a tibble, you can use list_rbind or list_cbind to row- or column-bind the results into a tibble:

map(n1, \(x) dplyr::tibble("Val" = x, "sq_val" = x^2)) |>
  list_rbind() |>
  knitr::kable()
Val sq_val
7 49
8 64
9 81

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