如何计算列表列中的向量长度（嵌套）

Question

I have the following code

library(tidyverse)
dat <- iris %>% 
    group_by(Species) %>% 
    summarise(summary = list(fivenum(Petal.Width))) 

dat
#> # A tibble: 3 x 2
#>   Species    summary  
#>   <fct>      <list>   
#> 1 setosa     <dbl [5]>
#> 2 versicolor <dbl [5]>
#> 3 virginica  <dbl [5]>

Basically I used the Iris data, grouped it by Species and then calculated fivenum() .

What I want to do is to simply calculate the length of the summary values: this is what I have tried but it doesn't produce what I expect:

dat %>% 
  mutate(nof_value = length(summary))

# A tibble: 3 x 3
#  Species    summary   nof_values
#  <fct>      <list>         <int>
#1 setosa     <dbl [5]>          3
#2 versicolor <dbl [5]>          3
#3 virginica  <dbl [5]>          3

The nof_values should all be equal to 5. What's the right way to do it?

Answer 1

We can use lengths to calculate the length of nested list

library(tidyverse)
dat %>%
   mutate(nof_values = lengths(summary))

#  Species    summary   nof_values
#  <fct>      <list>         <int>
#1 setosa     <dbl [5]>          5
#2 versicolor <dbl [5]>          5
#3 virginica  <dbl [5]>          5

whose equivalent in base R is

dat$nof_values <- lengths(dat$summary)

---

Side note : length is different from lengths

length(dat$summary)
#[1] 3

lengths(dat$summary)
#[1] 5 5 5

Answer 2

You can use the map_int command from the purrr package (which is part of the tidyverse)

dat <- iris %>% 
  group_by(Species) %>% 
  summarise(summary = list(fivenum(Petal.Width))) %>% 
  mutate(nof_value = map_int(summary, length))