purrr::walk() this way

Meet the map() family

purrr’s map() family of functions are tools for iteration, performing the same action on multiple inputs. If you’re new to purrr, the Iteration chapter of R for Data Science is a good place to get started.

One of the benefits of using map() is that the function has variants (e.g.  map2(), pmap(), etc.) all of which work the same way. To borrow from Jennifer Thompson’s excellent Intro to purrr,the arguments can be broken into two groups: what we’re iterating over, and what we’re doing each time. The adapted figure below shows what this looks like for map(), map2(), and pmap().

In addition to handling different input arguments, the map family of functions has variants that create different outputs. The following table from the Map-variants section of Advanced R shows how the orthogonal inputs and outputs can be used to organise the variants into a matrix:

What’s up with walk()?

Based on the table above, you might think that walk() isn’t very useful. Indeed, walk(), walk2(), and pwalk() all invisibly return .x. However, they come in handy when you want to call a function for its side effects rather than its return value.

Here, we’ll go through two common use cases: saving multiple CSVs, and multiple plots. We’ll also make use of the fs package, a cross-platform interface to file system operations, to inspect our outputs.

If you want to try this out but don’t want to save files locally, there’s a companion project on Posit Cloud where you can follow along.

Writing (and deleting) multiple CSVs

To get started, we’ll need some data. Let’s use the gapminder example Sheet built into googlesheets4. Because there are multiple worksheets (one for each continent), we’ll use map() to apply read_sheet()¹ to each one, and get back a list of data frames.

library(tidyverse)
library(googlesheets4)

ss <- gs4_example("gapminder") # get sheet id
sheets <- sheet_names(ss) # get the names of individual sheets
gap_dfs <- map(sheets, .f = \(x) read_sheet(ss, sheet = x))
#> ✔ Reading from gapminder.
#> ✔ Range ''Africa''.
#> ✔ Reading from gapminder.
#> ✔ Range ''Americas''.
#> ✔ Reading from gapminder.
#> ✔ Range ''Asia''.
#> ✔ Reading from gapminder.
#> ✔ Range ''Europe''.
#> ✔ Reading from gapminder.
#> ✔ Range ''Oceania''.

Note that the backslash syntax for anonymous functions (e.g. \(x) x + 1) was introduced in base R version 4.1.0 as a shorthand for function(x) x + 1. If you’re using an earlier version of R, you can use purrr’s shorthand: a formula (e.g. ~ .x + 1).

Typically, you’d want to combine these data frames into one to make it easier to work with your data. To do so, we’ll use list_rbind() on gap_dfs. I’ve kept the intermediary object, since we’ll use it in a moment with walk(), but could have just as easily piped the output directly. The combination of purrr::map() and list_rbind() is a handy one that you can learn more about in the R for Data Science.

gap_combined <- gap_dfs |>
  list_rbind()

Now let’s say that, for whatever reason, you’d like to save the data from these sheets as individual CSVs. This is where walk() comes into play—writing out the file with write_csv() is a “side effect.” We’ll use fs::dir_create() to create a data folder to put our files into², and build a vector of paths/file names. Since we have two arguments, the list of data frames, and the paths, we’ll use walk2().

fs::dir_create("data")
paths <- str_glue("data/gapminder_{tolower(sheets)}.csv")
walk2(
  gap_dfs, 
  paths,
  \(df, name) write_csv(df, name)
  )

To see what we’ve done, we can use fs::dir_tree() to see the contents of the directory as a tree, or fs::dir_ls() to return the paths as a vector. These functions also take glob and regexp arguments, allowing you to filter paths by file type with globbing patterns (e.g. *.csv) or using a regular expression passed on to grep().

fs::dir_tree("data")
#> data
#> ├── gapminder_africa.csv
#> ├── gapminder_americas.csv
#> ├── gapminder_asia.csv
#> ├── gapminder_europe.csv
#> └── gapminder_oceania.csv
fs::dir_ls("data")
#> data/gapminder_africa.csv   data/gapminder_americas.csv 
#> data/gapminder_asia.csv     data/gapminder_europe.csv   
#> data/gapminder_oceania.csv

If you’re having regrets, or want to return your example project to its previous state, it’s just as easy to walk() fs::file_delete() along those same paths.³

walk(paths, \(paths) fs::file_delete(paths))

Saving multiple plots

Now, let’s say you want to create and save a bunch of plots. We’ll use a modified version of the conditional_bars()⁴ function from the R for Data Science chapter on writing functions, and the built-in diamonds dataset.

# modified conditional bars function from R4DS
conditional_bars <- function(df, condition, var) {
  df |>
    filter({{ condition }}) |>
    ggplot(aes(x = {{ var }})) +
    geom_bar() +
    ggtitle(rlang::englue("Count of diamonds by {{var}} where {{condition}}"))
}

It’s easy enough to run this for one condition, for example for the diamonds with cut == "Good".

diamonds |> conditional_bars(cut == "Good", clarity)

But what if we want to make and save a plot for each cut? Again, it’s map() and walk() to the rescue.

Because we’re using the same data (diamonds) and conditioning on the same variable (cut), we’ll only need to map() across the levels of cut, and can hard code the rest into the anonymous function.

# get the levels
cuts <- levels(diamonds$cut)

# make the plots
plots <- map(
  cuts,
  \(x) conditional_bars(
    df = diamonds,
    cut == {{ x }},
    clarity
    )
)

The plots are now saved in a list—a fine format for storing ggplots. As we did when saving our CSVs, we’ll use fs to create a directory to store them in, and make a vector of paths for file names.

# make the folder to put them it (if exists, {fs} does nothing)
fs::dir_create("plots")
# make the file names
plot_paths <- str_glue("plots/{tolower(cuts)}_clarity.png")

Now we can use the paths and plots with walk2() to pass them as arguments to ggsave().

walk2(
  plot_paths,
  plots,
  \(path, plot) ggsave(path, plot, width = 6, height = 6)
)

Again, we can use fs to see what we’ve done:

fs::dir_tree("plots")
#> plots
#> ├── fair_clarity.png
#> ├── good_clarity.png
#> ├── ideal_clarity.png
#> ├── premium_clarity.png
#> └── very good_clarity.png

And, clean up after ourselves if we didn’t really want those plots after all.

walk(plot_paths, \(paths) fs::file_delete(paths))

Fin

Hopefully this gave you a taste for some of what walk() can do. To learn more, see Saving multiple outputs in the Iteration chapter of R for Data Science.