`case_when` 函数中 `~` 后的条件项

Question

我想在 case_when 函数中的 ~ 之后放置一个条件项。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))

无效代码：

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))

换言之：

group_by x
在 y

b

cumsum

取该组(=b)的最后一个值(=15)和
将此值 (=15) 放入组为 a

y

期望的输出：

   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

只需在计算第二个 mutate 之前添加 ungroup() 并使用 last 和条件来获得最后一个 y 和 x == "b"

library(dplyr)

df %>% 
  group_by(x) %>% 
  mutate(y = case_when(x=="b" ~ cumsum(y),
    TRUE ~ y)) %>% 
  # add the ungroup here
  ungroup() %>%
  # and then the value is like this
  mutate(y = case_when(x=="a" ~ last(y[x == "b"]), 
    TRUE ~ y))
#> # A tibble: 12 x 2
#>    x         y
#>    <chr> <int>
#>  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

^{由 reprex package (v2.0.0)}

于 2021-04-22 创建

Answer 2

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

`case_when` 函数中 `~` 后的条件项

Conditional term after the `~` in a `case_when` function

r

dplyr

tidyverse

case-when