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Get first element of nested data

I was cleaning some data today and found myself wanting to grab just the first row of a set of rows belonging to a certain group. Here’s how to do that using some tidyverse packages in R.

library(tidyr); packageVersion("tidyr")
#> [1] '0.8.3'
library(dplyr); packageVersion("dplyr")
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union
#> [1] '0.8.0.1'
library(purrr); packageVersion("purrr")
#> [1] '0.3.2'

iris %>%
    group_by(Species) %>%
    nest() %>%
    mutate(sample = map(data, function(x) head(x, 1))) %>%
    select(-data) %>%
    unnest()
#> # A tibble: 3 x 5
#>   Species    Sepal.Length Sepal.Width Petal.Length Petal.Width
#>   <fct>             <dbl>       <dbl>        <dbl>       <dbl>
#> 1 setosa              5.1         3.5          1.4         0.2
#> 2 versicolor          7           3.2          4.7         1.4
#> 3 virginica           6.3         3.3          6           2.5

Created on 2019-06-18 by the reprex package (v0.2.1)

So here, I’ve take the ubiquitous iris dataset and grouped the rows by species and took the first row. You can probably do a random sample in the map() function for stratified sampling.