Extract elements from nested list only using functions from purrr package

Question

How do I extract elements from a nested list only using the purrr package? In this case I want to get a vector of intercepts after splitting a data.frame. I have accomplished what I need using lapply(), but I would like to use only functions purrr package.

library(purrr)
mtcars %>% 
split(.$cyl) %>%
map(  ~lm(mpg ~ wt, data = .)) %>%        # shorthand  NOTE: ~ lm  
lapply(function(x) x[[1]] [1]) %>% # extract intercepts  <==is there a purrr function for this line?
as_vector()                               # convert to vector

I have tried map() and at_depth() but nothing seemed to work for me.

How did you try map? If you delete the function name lapply and replace it with map - exact same arguments - it works just fine. — Gregor Thomas
I found the help page for map useful here. It looks like you can do some short-cut coding compared to lapply. Like map_dbl(c(1, 1)) for indexing nested lists. — aosmith

aosmith aosmith · Accepted Answer · 2016-01-12T19:24:24

The map functions have some shorthand coding for indexing nested lists. A helpful snippet from the help page:

To index deeply into a nested list, use multiple values; c("x", "y") is equivalent to z[["x"]][["y"]].

So using code for nested indexes along with map_dbl, which reduces to a vector, you can simply do:

mtcars %>%
    split(.$cyl) %>%
    map(~lm(mpg ~ wt, data = .)) %>%
    map_dbl(c(1, 1))

       4        6        8 
39.57120 28.40884 23.86803

I also found this blog post introducing purrr 0.1.0 useful, as it gave a few more example of the shorthand coding that I ended up using.

Extract elements from nested list only using functions from purrr package

2 Answers