要点有两个

1. 不匹配的时候会返回 NA，而不是保持不变
2. 根据顺序进行条件判断，顺序很重要

下面这段代码，

x <- 1:50
case_when(
  x %% 35 == 0 ~ "fizz buzz",
  x %% 5 == 0 ~ "fizz",
  x %% 7 == 0 ~ "buzz",
  TRUE ~ as.character(x)
)

如果不包含TRUE ~ as.character(x)，会返回

[1] NA          NA          NA          NA          "fizz"      NA         
 [7] "buzz"      NA          NA          "fizz"      NA          NA         
[13] NA          "buzz"      "fizz"      NA          NA          NA         
[19] NA          "fizz"      "buzz"      NA          NA          NA         
[25] "fizz"      NA          NA          "buzz"      NA          "fizz"     
[31] NA          NA          NA          NA          "fizz buzz" NA         
[37] NA          NA          NA          "fizz"      NA          "buzz"     
[43] NA          NA          "fizz"      NA          NA          NA         
[49] "buzz"      "fizz"

如果不包含TRUE ~ as.character(x)，不满足前面几个条件的 数字 到了这里会被转成 character，因此不会返回 NA

x <- 1:50
case_when(
  x %% 35 == 0 ~ "fizz buzz",
  x %% 5 == 0 ~ "fizz",
  x %% 7 == 0 ~ "buzz",
  TRUE ~ as.character(x)
)
#>  [1] "1"         "2"         "3"         "4"         "fizz"      "6"        
#>  [7] "buzz"      "8"         "9"         "fizz"      "11"        "12"       
#> [13] "13"        "buzz"      "fizz"      "16"        "17"        "18"       
#> [19] "19"        "fizz"      "buzz"      "22"        "23"        "24"       
#> [25] "fizz"      "26"        "27"        "buzz"      "29"        "fizz"     
#> [31] "31"        "32"        "33"        "34"        "fizz buzz" "36"       
#> [37] "37"        "38"        "39"        "fizz"      "41"        "buzz"     
#> [43] "43"        "44"        "fizz"      "46"        "47"        "48"       
#> [49] "buzz"      "fizz"     

# Like an if statement, the arguments are evaluated in order, so you must
# proceed from the most specific to the most general. This won't work:
case_when(
  TRUE ~ as.character(x),
  x %%  5 == 0 ~ "fizz",
  x %%  7 == 0 ~ "buzz",
  x %% 35 == 0 ~ "fizz buzz"
)
#>  [1] "1"  "2"  "3"  "4"  "5"  "6"  "7"  "8"  "9"  "10" "11" "12" "13" "14" "15"
#> [16] "16" "17" "18" "19" "20" "21" "22" "23" "24" "25" "26" "27" "28" "29" "30"
#> [31] "31" "32" "33" "34" "35" "36" "37" "38" "39" "40" "41" "42" "43" "44" "45"
#> [46] "46" "47" "48" "49" "50"

可以配合verb使用

starwars %>%
  select(name:mass, gender, species) %>%
  mutate(
    type = case_when(
      height > 200 | mass > 200 ~ "large",
      species == "Droid"        ~ "robot",
      TRUE                      ~ "other"
    )
  )

case_when可以用在和tidyverse库以外的函数一起使用

case_character_type <- function(height, mass, species) {
  case_when(
    height > 200 | mass > 200 ~ "large",
    species == "Droid"        ~ "robot",
    TRUE                      ~ "other"
  )
}

case_character_type(150, 250, "Droid")
#> [1] "large"
case_character_type(150, 150, "Droid")
#> [1] "robot"