Find the value or position of the first match

Find the value or position of the first match

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

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

Arguments

.x

A list or vector.

.f

A function, specified in one of the following ways:

• A named function, e.g. mean.

• An anonymous function, e.g. \(x) x + 1 or function(x) x + 1.

• A formula, e.g. ~ .x + 1. You must use .x to refer to the first argument. Only recommended if you require backward compatibility with older versions of R.

• A string, integer, or list, e.g. "idx", 1, or list("idx", 1) which are shorthand for \(x) pluck(x, "idx"), \(x) pluck(x, 1), and \(x) pluck(x, "idx", 1) respectively. Optionally supply .default to set a default value if the indexed element is NULL or does not exist.

...

Additional arguments passed on to .p.

.dir

If "forward", the default, starts at the beginning of the vector and move towards the end; if "backward", starts at the end of the vector and moves towards the beginning.

.right

Please use .dir instead.

.default

The value returned when nothing is detected.

Value

detect the value of the first item that matches the predicate; detect_index the position of the matching item. If not found, detect returns NULL and detect_index returns 0.

See also

keep() for keeping all matching values.

Examples

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