作为 dplyr 过滤功能的一部分，如何循环遍历列表的元素并最终在 R 中打印差异？

Question

I have a dataframe called diamondsData that I would like to manipulate in such a was that allows me to calculate the the variance in carat size for each type of cut in R.

I can easily do it manually as follows:

diamondsData %>% 
  filter(cut=="Fair") %>% 
  select(carat) %>% 
  var()

I can change "Fair in filter(cut=="Fair") to Ideal and so on, but I would like to automate this. I tried creating a list and looping through like:

y <- list("Ideal", "Premium", "Good", "Very Good", "Fair")

for(x in y){
  print(
    diamondsData %>% 
      filter(cut==x) %>% 
      select(carat) %>% 
      var()
  )
}

but this doesn't work. This resulted in:

[

I am working in R. Any suggestions??

Answer 1

Use group_by instead of looping:

diamondsData %>%
group_by(cut) %>%
select(carat) %>%
summarize(variability = var(carat))

And it should give you a dataframe containing the variance of carat within each cut.

Edited to replace %>% var() with %>% summarise(....) .

Answer 2

The issue is that var expects a vector while select returns a data.frame/tibble as output (even when one column is selected). We can use pull to get the column as vector

for(x in y){
  print(
   diamondsData %>% 
   filter(cut==x) %>% 
    pull(carat) %>% 
     var(., na.rm = TRUE)
 )
}