Subsetting and repetition of rows in a dataframe using R

Question

Suppose we have the following data with column names "id", "time" and "x":

df<-
structure(
list(
id = c(1L, 1L, 1L, 2L, 2L, 3L, 3L),
time = c(20L, 6L, 7L, 11L, 13L, 2L, 6L),
x = c(1L, 1L, 0L, 1L, 1L, 1L, 0L)
),
.Names = c("id", "time", "x"),
class = "data.frame",
row.names = c(NA,-7L)
)

Each id has multiple observations for time and x. I want to extract the last observation for each id and form a new dataframe which repeats these observations according to the number of observations per each id in the original data. I am able to extract the last observations for each id using the following codes

library(dplyr) 
df<-df%>% 
group_by(id) %>% 
filter( ((x)==0 & row_number()==n())| ((x)==1 & row_number()==n()))

What is left unresolved is the repetition aspect. The expected output would look like

df <-
structure(
list(
id = c(1L, 1L, 1L, 2L, 2L, 3L, 3L),
time = c(7L, 7L, 7L, 13L, 13L, 6L, 6L),
x = c(0L, 0L, 0L, 1L, 1L, 0L, 0L)
),
.Names = c("id", "time", "x"),
class = "data.frame",
row.names = c(NA,-7L)
)

Thanks for your help in advance.

Answer 1

We can use ave to find the max row number for each ID and subset it from the data frame.

df[ave(1:nrow(df), df$id, FUN = max), ]

#    id time x
#3    1    7 0
#3.1  1    7 0
#3.2  1    7 0
#5    2   13 1
#5.1  2   13 1
#7    3    6 0
#7.1  3    6 0

Answer 2

You can do this by using last() to grab the last row within each id.

df %>%
    group_by(id) %>%
    mutate(time = last(time),
           x = last(x))

Because last(x) returns a single value, it gets expanded out to fill all the rows in the mutate() call.

This can also be applied to an arbitrary number of variables using mutate_at:

df %>%
    group_by(id) %>%
    mutate_at(vars(-id), ~ last(.))