How to get the value in the largest date of each name?

Question

I need to use dplyr mutate to create a new column that can show me (perhaps repeated values) of each maximum date what the respective value is.

library(dplyr)

set.seed(1234)
df <- data.frame(
  dates = seq.Date(as.Date("2022-08-23"), 
                   as.Date("2022-09-11"),
                   by = "days"),
  names = c(
    "a", 
    "a",
    "b",
    "c",
    "a",
    "a",
    "c",
    "b",
    "a",
    "a",
    "b",
    "c",
    "a",
    "b",
    "c",
    "d",
    "b",
    "a",
    "b",
    "c"   
    ),
  value = rnorm(20,0,1)
)

glimpse(df)

df

How could I use mutate to insert a column that repeats the largest date value for each name until the name changes?

Answer 1

library(dplyr)
df %>%
  arrange(names, dates) %>% 
  group_by(names) %>% 
  mutate(last_val = last(value))

# A tibble: 20 × 4
# Groups:   names [4]
   dates      names   value max_val
   <date>     <chr>   <dbl>   <dbl>
 1 2022-08-23 a     -1.21    -0.911
 2 2022-08-24 a      0.277   -0.911
 3 2022-08-27 a      0.429   -0.911
 4 2022-08-28 a      0.506   -0.911
 5 2022-08-31 a     -0.564   -0.911
 6 2022-09-01 a     -0.890   -0.911
 7 2022-09-04 a     -0.776   -0.911
 8 2022-09-09 a     -0.911   -0.911
 9 2022-08-25 b      1.08    -0.837
10 2022-08-30 b     -0.547   -0.837
11 2022-09-02 b     -0.477   -0.837
12 2022-09-05 b      0.0645  -0.837
13 2022-09-08 b     -0.511   -0.837
14 2022-09-10 b     -0.837   -0.837
15 2022-08-26 c     -2.35     2.42 
16 2022-08-29 c     -0.575    2.42 
17 2022-09-03 c     -0.998    2.42 
18 2022-09-06 c      0.959    2.42 
19 2022-09-11 c      2.42     2.42 
20 2022-09-07 d     -0.110   -0.110

Or if you don't care about the ordering

df %>% 
  group_by(names) %>% 
  mutate(last_val = last(value, order_by = dates))