R计算某些行的组中位数和最后一行

Question

I'm working with grouping and median, I'd like to have a grouping of a data.frame with the median of certain rows (not all) and the last value.
My data are something like this:

 test <- data.frame(
id = c('A','A','A','A','A','B','B','B','B','B','C','C','C','C'),
value = c(1,2,3,4,5,3,4,5,1,8,3,4,2,9))
> test
   id value
1   A     1
2   A     2
3   A     3
4   A     4
5   A     5
6   B     3
7   B     4
8   B     5
9   B     1
10  B     8
11  C     3
12  C     4
13  C     2
14  C     9

For each id , I need the median of the three (number may vary, in this case three) central rows, then the last value.
I've tried first of all with only one id .

test_a <- test[which(test$id == 'A'),]
> test_a
  id value
1  A     1
2  A     2
3  A     3
4  A     4
5  A     5

The desired output is this for this one, Having this:

median(test_a[(nrow(test_a)-3):(nrow(test_a)-1),]$value) # median of three central values
tail(test_a,1)$value                                     # last value

I used this:

library(tidyverse)

test_a %>% group_by(id) %>%
  summarise(m = median(test_a[(nrow(test_a)-3):(nrow(test_a)-1),]$value),
            last = tail(test_a,1)$value) %>%
  data.frame()
  id m last
1  A 3    5

But when I tried to generalize to all id:

test %>% group_by(id) %>%
   summarise(m = median(test[(nrow(test)-3):(nrow(test)-1),]$value),
             last = tail(test,1)$value) %>%
   data.frame()
  id m last
1  A 3    9
2  B 3    9
3  C 3    9

I think that the formulas take the full dataset to calculate last value and median, but I cannot imagine how to make it works. Thanks in advance.

Answer 1

This works:

test %>% 
  group_by(id) %>%
  summarise(m = median(value[(length(value)-3):(length(value)-1)]),
            last = value[length(value)])

# A tibble: 3 x 3
      id     m  last
  <fctr> <dbl> <dbl>
1      A     3     5
2      B     4     8
3      C     4     9

You just refer to variable value instead of the whole dataset within summarise .

---

Edit: Here's a generalized version.

test %>% 
  group_by(id) %>%
  summarise(m = ifelse(length(value) == 1, value, 
                       ifelse(length(value) == 2, median(value), 
                              median(value[(ceiling(length(value)/2)-1):(ceiling(length(value)/2)+1)])),
            last = value[length(value)])

If a group has only one row, the value itself will be stored in m . If it has only two rows, the median of these two rows will be stored in m . If it has three or more rows, the middle three rows will be chosen dynamically and the median of those will be stored in m .