Add function that creates factor in order of case_when matches

[dchiu911]

A common workflow I do is map one vector to another using some (possibly complex) conditions, then coerce to a factor with the level order the same as parsed in dplyr::case_when(). It would be helpful if there was a wrapper that created the factor without having to manually specify the levels.
Currently, I'd do something like this:

library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union

set.seed(2022)
x <- sample(
  c("low", "intermediate", "high"),
  prob = c(0.5, 0.2, 0.3),
  size = 100,
  replace = TRUE
)
z <- rbinom(
  n = 100,
  size = 100,
  prob = 0.3
)
y <- case_when(
  x == "intermediate" | (x == "low" & z < 30) ~ "B",
  x == "low" ~ "A",
  x == "high" ~ "C",
  TRUE ~ NA_character_
) %>%
  factor(levels = c("B", "A", "C"))
str(y)
#>  Factor w/ 3 levels "B","A","C": 1 3 2 3 2 3 2 1 1 3 ...

^{Created on 2022-02-01 by the reprex package (v2.0.1)}

Can we add a function that makes y into a factor with the level order the same as specified in the case_when()? For example,

y <- fct_case(
  x == "intermediate" | (x == "low" & z < 30) ~ "B",
  x == "low" ~ "A",
  x == "high" ~ "C",
  TRUE ~ NA_character_
)

[hadley]

I think we'd need to make the syntax more limiting than case_when() because the RHS of a case_when() can itself use data values, and reasoning through how those values should interact between conditions seems hard.

Since we'd want to restrict each expression to a single character level, we could put it in the LHS of =, something like:

something(
  "B" = x == "intermediate" | (x == "low" & z < 30),
  "A" = x == "low",
  "C" = x == "high",
)

But I don't know if any existing tidyverse function uses similar syntax.

[dchiu911]

I do think removing the usage of ~ would make it more consistent as case_when() syntax is quite unique

[DavisVaughan]

But I don't know if any existing tidyverse function uses similar syntax.

FWIW this is basically how fct_recode() works (name represents new level, value was the old level), so it wouldn't be unheard to let the name represent the new level, and the value be the logical condition

[hadley]

Will wait until lower level functions are exposed by vctrs.

[brianmsm]

I would think it would be convenient to solve this from the case_when() itself:

Something like this:

set.seed(2022)
x <- sample(
  c("low", "intermediate", "high"),
  prob = c(0.5, 0.2, 0.3),
  size = 100,
  replace = TRUE
)
z <- rbinom(
  n = 100,
  size = 100,
  prob = 0.3
)
y <- case_when(
  x == "intermediate" | (x == "low" & z < 30) ~ "B",
  x == "low" ~ "A",
  x == "high" ~ "C",
  TRUE ~ NA_character_,
  .ptype = "factor"
)