Given a dataframe:
df <- structure(list(a = c(1, 1, 1, 2, 2, 2, 3, 3, 4, 4), b = c(34,
343, 54, 11, 55, 62, 59, -9, 0, -0.5)), row.names = c(NA, -10L
), class = c("tbl_df", "tbl", "data.frame"))
I want to take last N observations / rows from each group:
df %>%
dplyr::group_by(a) %>%
dplyr::last(2)
Gives me wrong results.
I want it to be:
a b
1 343
1 54
2 55
2 62
3 59
3 -9
4 0
4 -0.5
Please advise what is wrong here?
The error I get is:
Error in order(order_by)[[n]] : subscript out of bounds
As it is a specific question based on dplyr
1) after the group_by, use slice on the row_number()
library(tidyverse)
df %>%
group_by(a) %>%
slice(tail(row_number(), 2))
# A tibble: 8 x 2
# Groups: a [4]
# a b
# <dbl> <dbl>
#1 1 343
#2 1 54
#3 2 55
#4 2 62
#5 3 59
#6 3 -9
#7 4 0
#8 4 -0.5
2) Or use filter from dplyr
df %>%
group_by(a) %>%
filter(row_number() >= (n() - 1))
3) or with do and tail
df %>%
group_by(a) %>%
do(tail(., 2))
4) In addition to the tidyverse, methods, we can also use compact data.table
library(data.table)
setDT(df)[df[, .I[tail(seq_len(.N), 2)], a]$V1]
5) Or by from base R
by(df, df$a, FUN = tail, 2)
6) or with aggregate from base R
df[aggregate(c ~ a, transform(df, c = seq_len(nrow(df))), FUN = tail, 2)$c,]
7) or with split from base R
do.call(rbind, lapply(split(df, df$a), tail, 2))