r - `~` 函数中 `case_when` 之后的条件项

Question

我想在 ~ 后面放一个条件项在 case_when功能。
我的例子:
df:

df <- structure(list(x = c("a", "a", "a", "b", "b", "b", "c", "c", 
"c", "a", "a", "a"), y = 1:12), class = "data.frame", row.names = c(NA, 
-12L))

不工作的代码:

library(dplyr)
df %>% 
  group_by(x) %>% 
  mutate(y = case_when(x=="b" ~ cumsum(y),
                       TRUE ~ y)) %>% 
  mutate(y = case_when(x=="a" ~ "what I want: last value of group "b" in column y", 
                       TRUE ~ y))

用一句话来说:

group_by x

计算 cumsum组b在栏目y

取该组 (=b) 的最后一个值 (=15) 和

将此值 (=15) 放入列 y组在哪里a

所需的输出:

   x         y
   <chr> <dbl>
 1 a        15
 2 a        15
 3 a        15
 4 b         4
 5 b         9
 6 b        15
 7 c         7
 8 c         8
 9 c         9
10 a        15
11 a        15
12 a        15

非常感谢!!!

Answer 1

在这种情况下，group_by()不是必需的(尽管它有助于提高可读性等):

df %>%
 mutate(y = case_when(x == "b" ~ cumsum(y * (x == "b")),
                      x == "a" ~ max(cumsum(y[x == "b"])),
                      TRUE ~ y))

   x  y
1  a 15
2  a 15
3  a 15
4  b  4
5  b  9
6  b 15
7  c  7
8  c  8
9  c  9
10 a 15
11 a 15
12 a 15