问使用R按另一列有条件地选择一组中的最后N个值

EN

do.call(rbind, lapply(split(df, df$Name), function(a)
    a[tail(sequence(NROW(a)), c(3,1)[(NROW(a) < 3) + 1]),]))

do.call(rbind, lapply(split(df, df$Name), function(a)
    a[tail(sequence(NROW(a)), ifelse(NROW(a) < 3, 1, 3)),]))
#    Name Value
#x.2    x     2
#x.3    x     3
#x.4    x     4
#y.5    y     5
#y.6    y     6
#y.7    y     7
#z      z     9

do.call(rbind, lapply(split(df, df$Name), function(a)
      a[tail(sequence(NROW(a)), ifelse(NROW(a) >= 6, 6, ifelse(NROW(a) >= 3, 3, 1))),]))

df %>% group_by(Name) %>% slice(tail(row_number(), 
  if (n_distinct(Value) < 3) 1 else 3
))

# A tibble: 7 x 2
# Groups:   Name [3]
    Name Value
  <fctr> <dbl>
1      x     2
2      x     3
3      x     4
4      y     5
5      y     6
6      y     7
7      z     9

library(data.table)
setDT(df)
df[, tail(.SD, if (uniqueN(Value) < 3) 1 else 3), by=Name]

with(df, {
  len = tapply(Value, Name, FUN = length)
  nv  = tapply(Value, Name, FUN = function(x) length(unique(x)))
  df[ sequence(len) > rep(nv - ifelse(nv < 3, 1, 3), len), ]
})

library(tidyverse)

df %>%
  split(.$Name) %>%
  map_df(~ if (n_distinct(.x) >= 3) tail(.x, 3) else tail(.x, 1))

#  Name Value
#1    x     2
#2    x     3
#3    x     4
#4    y     5
#5    y     6
#6    y     7
#7    z     9