How to select last N observation from each group in dplyr dataframe?

by

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))

More Related Contents:

  1. Use dynamic name for new column/variable in `dplyr`
  2. data.table vs dplyr: can one do something well the other can’t or does poorly?
  3. Extract a dplyr tbl column as a vector
  4. Pass arguments to dplyr functions
  5. dplyr::select function clashes with MASS::select
  6. Sum across multiple columns with dplyr
  7. dplyr: nonstandard column names (white space, punctuation, starts with numbers)
  8. dplyr filter: Get rows with minimum of variable, but only the first if multiple minima
  9. dplyr mutate/replace several columns on a subset of rows
  10. dplyr: How to use group_by inside a function?
  11. What does the dplyr period character “.” reference?
  12. Summarize all group values and a conditional subset in the same call
  13. How to add a cumulative column to an R dataframe using dplyr?
  14. ignore NA in dplyr row sum
  15. dplyr on data.table, am I really using data.table?
  16. Use filter in dplyr conditional on an if statement in R
  17. How to do range grouping on a column using dplyr?
  18. combine two data frames with all posible combinations
  19. Performing dplyr mutate on subset of columns
  20. dplyr / R cumulative sum with reset
  21. finding close match from data frame 1 in data fame 2
  22. R: How to filter/subset a sequence of dates
  23. Add a column with count of NAs and Mean
  24. select columns based on multiple strings with dplyr contains()
  25. Error with select function from dplyr
  26. Create sequential counter that restarts on a condition within panel data groups [duplicate]
  27. dplyr: put count occurrences into new variable [duplicate]
  28. Add row in each group using dplyr and add_row()
  29. How to replace certain values in a specific rows and columns with NA in R?
  30. Add margin row totals in dplyr chain

Leave a Comment