R purrr detect 查找第一个匹配的值或位置

detect(
  .x,
  .f,
  ...,
  .dir = c("forward", "backward"),
  .right = NULL,
  .default = NULL
)

detect_index(.x, .f, ..., .dir = c("forward", "backward"), .right = NULL)

keep() 用于保留所有匹配值。

is_even <- function(x) x %% 2 == 0

3:10 |> detect(is_even)
#> [1] 4
3:10 |> detect_index(is_even)
#> [1] 2

3:10 |> detect(is_even, .dir = "backward")
#> [1] 10
3:10 |> detect_index(is_even, .dir = "backward")
#> [1] 8


# Since `.f` is passed to as_mapper(), you can supply a
# lambda-formula or a pluck object:
x <- list(
  list(1, foo = FALSE),
  list(2, foo = TRUE),
  list(3, foo = TRUE)
)

detect(x, "foo")
#> [[1]]
#> [1] 2
#> 
#> $foo
#> [1] TRUE
#> 
detect_index(x, "foo")
#> [1] 2


# If you need to find all values, use keep():
keep(x, "foo")
#> [[1]]
#> [[1]][[1]]
#> [1] 2
#> 
#> [[1]]$foo
#> [1] TRUE
#> 
#> 
#> [[2]]
#> [[2]][[1]]
#> [1] 3
#> 
#> [[2]]$foo
#> [1] TRUE
#> 
#> 

# If you need to find all positions, use map_lgl():
which(map_lgl(x, "foo"))
#> [1] 2 3