# Nested data

    library(tidyr)
    library(dplyr)
    library(purrr)

## Basics

A nested data frame is a data frame where one (or more) columns is a list of data frames. You can create simple nested data frames by hand:

    df1 <- tibble(
      g = c(1, 2, 3),
      data = list(
        tibble(x = 1, y = 2),
        tibble(x = 4:5, y = 6:7),
        tibble(x = 10)
      )
    )

    df1
    #> # A tibble: 3 x 2
    #>       g data
    #>   <dbl> <list>
    #> 1     1 <tibble [1 × 2]>
    #> 2     2 <tibble [2 × 2]>
    #> 3     3 <tibble [1 × 1]>

(It is possible to create list-columns in regular data frames, not just in tibbles, but it’s considerably more work because the default behaviour of `data.frame()` is to treat lists as lists of columns.)

But more commonly you’ll create them with `tidyr::nest()`:

    df2 <- tribble(
      ~g, ~x, ~y,
       1,  1,  2,
       2,  4,  6,
       2,  5,  7,
       3, 10,  NA
    )
    df2 %>% nest(data = c(x, y))
    #> # A tibble: 3 x 2
    #>       g data
    #>   <dbl> <list>
    #> 1     1 <tibble [1 × 2]>
    #> 2     2 <tibble [2 × 2]>
    #> 3     3 <tibble [1 × 2]>

`nest()` specifies which variables should be nested inside; an alternative is to use `dplyr::group_by()` to describe which variables should be kept outside.

    df2 %>% group_by(g) %>% nest()
    #> # A tibble: 3 x 2
    #> # Groups:   g [3]
    #>       g data
    #>   <dbl> <list>
    #> 1     1 <tibble [1 × 2]>
    #> 2     2 <tibble [2 × 2]>
    #> 3     3 <tibble [1 × 2]>

I think nesting is easiest to understand in connection to grouped data: each row in the output corresponds to one group in the input. We’ll see shortly this is particularly convenient when you have other per-group objects.

The opposite of `nest()` is `unnest()`. You give it the name of a list-column containing data frames, and it row-binds the data frames together, repeating the outer columns the right number of times to line up.

    df1 %>% unnest(data)
    #> # A tibble: 4 x 3
    #>       g     x     y
    #>   <dbl> <dbl> <dbl>
    #> 1     1     1     2
    #> 2     2     4     6
    #> 3     2     5     7
    #> 4     3    10    NA

## Nested data and models

Nested data is a great fit for problems where you have one of something for each group. A common place this arises is when you’re fitting multiple models.

    mtcars_nested <- mtcars %>%
      group_by(cyl) %>%
      nest()

    mtcars_nested
    #> # A tibble: 3 x 2
    #> # Groups:   cyl [3]
    #>     cyl data
    #>   <dbl> <list>
    #> 1     6 <tibble [7 × 10]>
    #> 2     4 <tibble [11 × 10]>
    #> 3     8 <tibble [14 × 10]>

Once you have a list of data frames, it’s very natural to produce a list of models:

    mtcars_nested <- mtcars_nested %>%
      mutate(model = map(data, function(df) lm(mpg ~ wt, data = df)))
    mtcars_nested
    #> # A tibble: 3 x 3
    #> # Groups:   cyl [3]
    #>     cyl data               model
    #>   <dbl> <list>             <list>
    #> 1     6 <tibble [7 × 10]>  <lm>
    #> 2     4 <tibble [11 × 10]> <lm>
    #> 3     8 <tibble [14 × 10]> <lm>

And then you could even produce a list of predictions:

    mtcars_nested <- mtcars_nested %>%
      mutate(model = map(model, predict))
    mtcars_nested
    #> # A tibble: 3 x 3
    #> # Groups:   cyl [3]
    #>     cyl data               model
    #>   <dbl> <list>             <list>
    #> 1     6 <tibble [7 × 10]>  <dbl [7]>
    #> 2     4 <tibble [11 × 10]> <dbl [11]>
    #> 3     8 <tibble [14 × 10]> <dbl [14]>

This workflow works particularly well in conjunction with broom, which makes it easy to turn models into tidy data frames which can then be `unnest()`ed to get back to flat data frames. You can see a bigger example in the broom and dplyr vignette.