Q:

R: Keep values below the 99 quantile for groups in data frame

I have a dataframe with groups and values. Firstly, I calculate 99% quantile per group. Now, I want to remove the values above the 99% quantile for every group. 
df<-data.frame(group = rep(c("A", "B"), each = 4),
               value = c(c(6,5,80,4,60)*10,3,5,4))

# data
  group value
1     A    60
2     A    50
3     A   800
4     A    40
5     B   600
6     B     3
7     B     5
8     B     4

Calculate quantils for individual groups
quant<-aggregate(df$value, by = list(df$group), FUN = quantile, probs  = 0.99)

> quant
  Group.1      x
1       A 777.80
2       B 582.15

I tried to apply the vector of quantiles to select lower values. However, it miss the group specification..  
df[df$value < quant$x,]

Expected result:
  group value
1     A    60
2     A    50
4     A    40
5     B     3
6     B     5
7     B     4

How to apply vector of quantiles to keep only values below 99% by group in data frame?

A:

We can do a filter after grouping
library(dplyr)
df %>%
   group_by(group) %>%
   filter(value < quantile(value, probs = 0.99))
# A tibble: 6 x 2
# Groups:   group [2]
#   group value
#  <fctr> <dbl>
#1      A    60
#2      A    50
#3      A    40
#4      B     3
#5      B     5
#6      B     4

Or similar syntax with data.table
library(data.table)
setDT(df)[, .(value = value[value < quantile(value, probs = 0.99)]), by = group]

Or using base R using ave
df[with(df, as.logical(ave(value, group, FUN= function(x) x <quantile(x, probs = 0.99)))), ]

