using lists in R

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June 27, 2022

One of my goals while on long service leave is to learn some new R things that have been on my radar for a while… the first of these is purrr. The purrr package allows you to iterate a function across different elements in a list or dataframe.

I have started to try and learn purr before (see a list of resources here). I have copied other people’s purrr code a couple of times too…

how to use map to read in a LOT of .csv files and
using pwalk to output plots in different colour schemes

… but when you copy purrr code from someone else and adjust it to suit you own problem… you can’t really say you know how to use purrr.

The first thing I think I need to get my head around in order to understand purrr is lists. Dataframes are the bread and butter of the tidyverse and up until now I have avoided them, or tried desperately to use as.dataframe() or unnest() to turn them into a data structure that i understand. Lists allow you to bundle together different kinds of data elements together, so now is the time to get my head around them.

how to make a list

This is a example I copied from a tutorial

myfirstlist <-  list(2, 
                     "hello", 
                     c(3,5,4), 
                     1:5, 
                     list(FALSE, 
                          c("this", "is","a","list"),
                          c(FALSE,TRUE,TRUE,TRUE,FALSE)))

The list() function lets you put elements of all different types (and lengths) into a listy bundle; characters and numbers and logicals. One of the elements in this list of 5 items when others have 3 items or even 1. Another item is ANOTHER list made up of 3 items. Lists within lists— eeeekk. We can use the class() function to check that our list is a list and the str() function to get our head around what we are dealing with.

class(myfirstlist)

## [1] "list"

str(myfirstlist)

## List of 5
##  $ : num 2
##  $ : chr "hello"
##  $ : num [1:3] 3 5 4
##  $ : int [1:5] 1 2 3 4 5
##  $ :List of 3
##   ..$ : logi FALSE
##   ..$ : chr [1:4] "this" "is" "a" "list"
##   ..$ : logi [1:5] FALSE TRUE TRUE TRUE FALSE

This iconic image (which I think comes from a Jenny Bryan talk) is supposed to help me understand how to access elements of a list.

Lets see if I can unpack it.

TBC…