In R, how do I merge two groups of the result of the split function?-CodePudding

I have a dataframe, with 4 groups of grades of students in one column, and the responses of a question in other column. I am using the split function to create a boxplot and, after, to test the groups. I want to create only two groups (merging the 2 lowest groups and the 2 highest). However, I am struggling with that. The split function gives a dataframe, a matrix, vectors?

    df <- data.frame(
  groups = c("group 1", "group 2", "group 3", "group 4", "group 1", "group 2", "group 3", "group 4"),
  responses = c(1, 4, 5, 9, 3, 4, 6, 9)
)
splittedDF <- split(x = df$responses, f = df$groups)

THis gives this output:

      $`group 1`
[1] 1 3
$`group 2`
[1] 4 4
$`group 3`
[1] 5 6
$`group 4`
[1] 9 9

And I want to merge to something like this:

    $`Lowest groups`
[1] 1 3 4 4

$`Highest groups`
[1] 5 6 9 9

DO you have some ideas?

CodePudding user response：

We may do an arrange first and then create the grouping before we do the split

library(dplyr)
df %>% 
 arrange(responses, groups) %>% 
 mutate(groups = c("Lowest groups", "Highest groups")[gl(n(), 4, n())]) %>%
 {split(.$responses, .$groups)}

-output

$`Highest groups`
[1] 5 6 9 9

$`Lowest groups`
[1] 1 3 4 4

If the splitted list is already ordered,

list(`Lowest groups` = unname(unlist(head(splittedDF, 2))),
    `Highest groups` = unname(unlist(tail(splittedDF, 2))))

-output

$`Lowest groups`
[1] 1 3 4 4

$`Highest groups`
[1] 5 6 9 9

If it is not ordered, then get the max value from each of the list element, order

i1 <- order(sapply(splittedDF, max))
splittedDF2 <- splittedDF[i1]
list(`Lowest groups` = unname(unlist(head(splittedDF2, 2))),
    `Highest groups` = unname(unlist(tail(splittedDF2, 2))))