lmap: Apply a function to list-elements of a list

In purrr: Functional Programming Tools

Apply a function to list-elements of a list

Description

lmap(), lmap_at() and lmap_if() are similar to map(), map_at() and map_if(), except instead of mapping over .x[[i]], they instead map over .x[i].

This has several advantages:

• It makes it possible to work with functions that exclusively take a list.

• It allows .f to access the attributes of the encapsulating list, like names().

• It allows .f to return a larger or small list than it receives changing the size of the output.

Usage

lmap(.x, .f, ...)

lmap_if(.x, .p, .f, ..., .else = NULL)

lmap_at(.x, .at, .f, ...)

Arguments

.x	A list or data frame.
.f	A function that takes a length-1 list and returns a list (of any length.)
...	Additional arguments passed on to the mapped function. We now generally recommend against using ... to pass additional (constant) arguments to .f. Instead use a shorthand anonymous function: # Instead of x |> map(f, 1, 2, collapse = ",") # do: x |> map(\(x) f(x, 1, 2, collapse = ",")) This makes it easier to understand which arguments belong to which function and will tend to yield better error messages.
.p	A single predicate function, a formula describing such a predicate function, or a logical vector of the same length as .x. Alternatively, if the elements of .x are themselves lists of objects, a string indicating the name of a logical element in the inner lists. Only those elements where .p evaluates to TRUE will be modified.
.else	A function applied to elements of .x for which .p returns FALSE.
.at	A logical, integer, or character vector giving the elements to select. Alternatively, a function that takes a vector of names, and returns a logical, integer, or character vector of elements to select. : if the tidyselect package is installed, you can use vars() and tidyselect helpers to select elements.

Value

A list or data frame, matching .x. There are no guarantees about the length.

See Also

Other map variants: imap(), map(), map2(), map_depth(), map_if(), modify(), pmap()

Examples

set.seed(1014)

# Let's write a function that returns a larger list or an empty list
# depending on some condition. It also uses the input name to name the
# output
maybe_rep <- function(x) {
  n <- rpois(1, 2)
  set_names(rep_len(x, n), paste0(names(x), seq_len(n)))
}

# The output size varies each time we map f()
x <- list(a = 1:4, b = letters[5:7], c = 8:9, d = letters[10])
x |> lmap(maybe_rep) |> str()

# We can apply f() on a selected subset of x
x |> lmap_at(c("a", "d"), maybe_rep) |> str()

# Or only where a condition is satisfied
x |> lmap_if(is.character, maybe_rep) |> str()

purrr documentation built on April 3, 2025, 6:13 p.m.