keep: Keep/discard elements based on their values

In purrr: Functional Programming Tools

Keep/discard elements based on their values

Description

keep() selects all elements where .p evaluates to TRUE; discard() selects all elements where .p evaluates to FALSE. compact() discards elements where .p evaluates to an empty vector.

Usage

keep(.x, .p, ...)

discard(.x, .p, ...)

compact(.x, .p = identity)

Arguments

.x	A list or vector.
.p	A predicate function (i.e. a function that returns either TRUE or FALSE) specified in one of the following ways: A named function, e.g. is.character. An anonymous function, e.g. ⁠\(x) all(x < 0)⁠ or function(x) all(x < 0). A formula, e.g. ~ all(.x < 0). Use .x to refer to the first argument. No longer recommended.
...	Additional arguments passed on to .p.

Details

In other languages, keep() and discard() are often called select()/ filter() and reject()/ drop(), but those names are already taken in R. keep() is similar to Filter(), but the argument order is more convenient, and the evaluation of the predicate function .p is stricter.

See Also

keep_at()/discard_at() to keep/discard elements by name.

Examples

rep(10, 10) |>
  map(sample, 5) |>
  keep(function(x) mean(x) > 6)

# Or use shorthand form
rep(10, 10) |>
  map(sample, 5) |>
  keep(\(x) mean(x) > 6)

# Using a string instead of a function will select all list elements
# where that subelement is TRUE
x <- rerun(5, a = rbernoulli(1), b = sample(10))
x
x |> keep("a")
x |> discard("a")

# compact() discards elements that are NULL or that have length zero
list(a = "a", b = NULL, c = integer(0), d = NA, e = list()) |>
  compact()

purrr documentation built on Nov. 5, 2025, 7:27 p.m.