Summarize the lowest values in a Dataframe?

Question

My data frame looks like this:

View(df)
Product     Value
  a           2
  b           4 
  c           3
  d           10
  e           15
  f           5
  g           6
  h           4
  i           50
  j           20
  k           35
  l           25
  m           4
  n           6
  o           30
  p           4
  q           40
  r           5
  s           3
  t           40

I want to find the 9 most expensive products and summaries the rest. It should look like this:

Product     Value 
  d           10
  e           15
  i           50
  j           20
  k           35
  l           25
  o           30
  q           40
  t           40
 rest         46

Rest is the sum of the other 11 products. I tried it with summaries, but it didn't work:

new <- df %>%
  group_by(Product)%>%
summarise((Value > 10) = sum(Value)) %>%
  ungroup()

Answer 1

We can use dplyr::row_number to effectively rank the observations after using arrange to order the data by Value. Then, we augment the Product column so that any values that aren't in the top 9 are coded as Rest. Finally, we group by the updated Product and take the sum using summarise

dat %>%
    arrange(desc(Value)) %>%
    mutate(RowNum = row_number(),
           Product = ifelse(RowNum <= 9, Product, 'Rest')) %>%
    group_by(Product) %>%
    summarise(Value = sum(Value))

# A tibble: 10 × 2
   Product Value
     <chr> <int>
1        d    10
2        e    15
3        i    50
4        j    20
5        k    35
6        l    25
7        o    30
8        q    40
9     Rest    46
10       t    40

data

dat <- structure(list(Product = c("a", "b", "c", "d", "e", "f", "g", 
"h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t"
), Value = c(2L, 4L, 3L, 10L, 15L, 5L, 6L, 4L, 50L, 20L, 35L, 
25L, 4L, 6L, 30L, 4L, 40L, 5L, 3L, 40L)), .Names = c("Product", 
"Value"), class = "data.frame", row.names = c(NA, -20L))