Extract object from list using dplyr

Question

This is related to the questions 1 and 2

I have a list of objects (in my case they are also lists AFAII), as returned by running:

gof_stats <- models %>% map(gof_stats)

Where models is a list of models created by fitdistrplus and gof_stats is a function that computes goodness of fit stats for each model.

Now if I want to extract a specific stat from that list I could do something like:

gof_stats[[1]]$cvm

to get the Cramer von Mises stat. I can achieve the same over the whole list (as per the linked questions) like so:

cvms <- sapply(gof_stats, "[[", "cvm")

Is there a way to do the same using dplyr/purrr syntax?

BONUS: How would you handle the case where some of the elements in the models list are NULL?

Answer 1

If you prefer map to sapply for this, you can do

library(purrr)
map(gof_stats, ~ .x[["cvm"]])

If you just like pipes you could do

gof_stats %>% sapply("[[", "cvm")

Your question is about lists, not data frames, so dplyr doesn't really apply. You may want to look up ?magrittr::multiply_by to see a list of other aliases from the package that defines %>% as you seem to like piping. For example, magrittr::extract2 is an alias of [[ that can be easily used in the middle of a piping chain.

As for your bonus, I would pre-filter the list to remove NULL elements before attempting to extract things.

Answer 2

A solution composed completely of tidyverse functions is:

gof_stats %>%
   map(~ .x %>% pluck("cvm"))

Legend:

• map is the tidyverse function for lists, replacing the apply family
• .x is the object in each iteration
• ~ is the purrr short syntax for the anonymous function function(x) { }
• pluck extracts a list element by index or string