R: How to Get the Last Element from Each Group

Get last row of each group in R

You might try:

a %>% 
  group_by(ID) %>% 
  arrange(NUM) %>%  
  slice(n())

R: How to get the last element from each group?

Did you try this?

val_eod <- aggregate(d$val1, by = list(d$day), FUN = tail, n = 1)

R: get last row of each group in dataframe

Package dplyr has a nice function for doing this.

library(tidyverse)

iris %>% 
    group_by(Species) %>% 
    slice_tail(n = 1)

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

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))

Get the first to the last element of each group in the form of a datatable using only data.table operations

Select rows until last "B" value in each group.

library(data.table)
datatable_example[, .SD[seq_len(max(which(b == 'B')))], a]

#   a b
#1: 1 A
#2: 1 B
#3: 1 B
#4: 2 B
#5: 2 B
#6: 3 A
#7: 3 B

R get date of first and last element in group

using dplyr and data.table:

df %>%
  mutate(Day = as.Date(Date)) %>%
  group_by(grp = rleid(Day, Element, Event), Day, Element, Event) %>%
  summarize(Begin = min(Date),
            End = max(Date)) %>%
  ungroup() %>%
  select(-grp)

Which results in:

# A tibble: 6 x 5
  Day        Element Event Begin               End                
  <date>     <chr>   <chr> <chr>               <chr>              
1 2020-01-01 A       OK    2020-01-01 00:00:00 2020-01-01 00:10:00
2 2020-01-01 A       Alarm 2020-01-01 00:15:00 2020-01-01 00:25:00
3 2020-01-01 A       OK    2020-01-01 00:30:00 2020-01-01 00:30:00
4 2020-01-01 B       OK    2020-01-01 00:00:00 2020-01-01 00:05:00
5 2020-01-01 B       Alarm 2020-01-01 00:10:00 2020-01-01 00:20:00
6 2020-01-01 B       OK    2020-01-01 00:25:00 2020-01-01 00:30:00

Select first and last row from grouped data

There is probably a faster way:

df %>%
  group_by(id) %>%
  arrange(stopSequence) %>%
  filter(row_number()==1 | row_number()==n())

Get the last row of a previous group in data.table

You could do

dt[, newcol := shift(dt[, last(Product), by = Group]$V1)[.GRP], by = Group]

This results in the following updated dt, where newcol matches your desired column with the unnecessarily long name. ;)

   Product Group LastProductOfPriorGroup newcol
1:       A     1                      NA     NA
2:       B     1                      NA     NA
3:       C     2                       B      B
4:       D     2                       B      B
5:       E     2                       B      B
6:       F     3                       E      E
7:       G     3                       E      E

Let's break the code down from the inside out. I will use ... to denote the accumulated code:

dt[, last(Product), by = Group]$V1 is getting the last values from each group as a character vector.
shift(...) shifts the character vector in the previous call
dt[, newcol := ...[.GRP], by = Group] groups by Group and uses the internal .GRP values for indexing

Update: Frank brings up a good point about my code above calculating the shift for every group over and over again. To avoid that, we can use either

shifted <- shift(dt[, last(Product), Group]$V1)
dt[, newcol := shifted[.GRP], by = Group]

so that we don't calculate the shift for every group. Or, we can take Frank's nice suggestion in the comments and do the following.

dt[dt[, last(Product), by = Group][, v := shift(V1)], on="Group", newcol := i.v]