Package 'purrr' reference manual

Title:	Functional Programming Tools
Description:	A complete and consistent functional programming toolkit for R.
Authors:	Hadley Wickham [aut, cre] , Lionel Henry [aut], Posit Software, PBC [cph, fnd]
Maintainer:	Hadley Wickham <[email protected]>
License:	MIT + file LICENSE
Version:	1.0.2.9000
Built:	2024-11-19 05:41:15 UTC
Source:	https://github.com/tidyverse/purrr

Accumulate intermediate results of a vector reduction

Description

accumulate() sequentially applies a 2-argument function to elements of a vector. Each application of the function uses the initial value or result of the previous application as the first argument. The second argument is the next value of the vector. The results of each application are returned in a list. The accumulation can optionally terminate before processing the whole vector in response to a done() signal returned by the accumulation function.

By contrast to accumulate(), reduce() applies a 2-argument function in the same way, but discards all results except that of the final function application.

accumulate2() sequentially applies a function to elements of two lists, .x and .y.

Usage

accumulate(
  .x,
  .f,
  ...,
  .init,
  .dir = c("forward", "backward"),
  .simplify = NA,
  .ptype = NULL
)

accumulate2(.x, .y, .f, ..., .init, .simplify = NA, .ptype = NULL)
accumulate(
  .x,
  .f,
  ...,
  .init,
  .dir = c("forward", "backward"),
  .simplify = NA,
  .ptype = NULL
)

accumulate2(.x, .y, .f, ..., .init, .simplify = NA, .ptype = NULL)

Arguments

`.x`	A list or atomic vector.
`.f`	For `accumulate()` `.f` is 2-argument function. The function will be passed the accumulated result or initial value as the first argument. The next value in sequence is passed as the second argument. For `accumulate2()`, a 3-argument function. The function will be passed the accumulated result as the first argument. The next value in sequence from `.x` is passed as the second argument. The next value in sequence from `.y` is passed as the third argument. The accumulation terminates early if `.f` returns a value wrapped in a `done()`.
`...`	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.
`.init`	If supplied, will be used as the first value to start the accumulation, rather than using `.x[[1]]`. This is useful if you want to ensure that `reduce` returns a correct value when `.x` is empty. If missing, and `.x` is empty, will throw an error.
`.dir`	The direction of accumulation as a string, one of `"forward"` (the default) or `"backward"`. See the section about direction below.
`.simplify`	If `NA`, the default, the accumulated list of results is simplified to an atomic vector if possible. If `TRUE`, the result is simplified, erroring if not possible. If `FALSE`, the result is not simplified, always returning a list.
`.ptype`	If `simplify` is `NA` or `TRUE`, optionally supply a vector prototype to enforce the output type.
`.y`	For `accumulate2()` `.y` is the second argument of the pair. It needs to be 1 element shorter than the vector to be accumulated (`.x`). If `.init` is set, `.y` needs to be one element shorted than the concatenation of the initial value and `.x`.

Value

A vector the same length of .x with the same names as .x.

If .init is supplied, the length is extended by 1. If .x has names, the initial value is given the name ".init", otherwise the returned vector is kept unnamed.

If .dir is "forward" (the default), the first element is the initial value (.init if supplied, or the first element of .x) and the last element is the final reduced value. In case of a right accumulation, this order is reversed.

The accumulation terminates early if .f returns a value wrapped in a done(). If the done box is empty, the last value is used instead and the result is one element shorter (but always includes the initial value, even when terminating at the first iteration).

Life cycle

accumulate_right() is soft-deprecated in favour of the .dir argument as of rlang 0.3.0. Note that the algorithm has slightly changed: the accumulated value is passed to the right rather than the left, which is consistent with a right reduction.

Direction

When .f is an associative operation like + or c(), the direction of reduction does not matter. For instance, reducing the vector 1:3 with the binary function + computes the sum ((1 + 2) + 3) from the left, and the same sum (1 + (2 + 3)) from the right.

In other cases, the direction has important consequences on the reduced value. For instance, reducing a vector with list() from the left produces a left-leaning nested list (or tree), while reducing list() from the right produces a right-leaning list.

Examples

# With an associative operation, the final value is always the
# same, no matter the direction. You'll find it in the first element for a
# backward (left) accumulation, and in the last element for forward
# (right) one:
1:5 |> accumulate(`+`)
1:5 |> accumulate(`+`, .dir = "backward")

# The final value is always equal to the equivalent reduction:
1:5 |> reduce(`+`)

# It is easier to understand the details of the reduction with
# `paste()`.
accumulate(letters[1:5], paste, sep = ".")

# Note how the intermediary reduced values are passed to the left
# with a left reduction, and to the right otherwise:
accumulate(letters[1:5], paste, sep = ".", .dir = "backward")

# By ignoring the input vector (nxt), you can turn output of one step into
# the input for the next. This code takes 10 steps of a random walk:
accumulate(1:10, \(acc, nxt) acc + rnorm(1), .init = 0)

# `accumulate2()` is a version of `accumulate()` that works with
# 3-argument functions and one additional vector:
paste2 <- function(acc, nxt, sep = ".") paste(acc, nxt, sep = sep)
letters[1:4] |> accumulate(paste2)
letters[1:4] |> accumulate2(c("-", ".", "-"), paste2)

# You can shortcircuit an accumulation and terminate it early by
# returning a value wrapped in a done(). In the following example
# we return early if the result-so-far, which is passed on the LHS,
# meets a condition:
paste3 <- function(out, input, sep = ".") {
  if (nchar(out) > 4) {
    return(done(out))
  }
  paste(out, input, sep = sep)
}
letters |> accumulate(paste3)

# Note how we get twice the same value in the accumulation. That's
# because we have returned it twice. To prevent this, return an empty
# done box to signal to accumulate() that it should terminate with the
# value of the last iteration:
paste3 <- function(out, input, sep = ".") {
  if (nchar(out) > 4) {
    return(done())
  }
  paste(out, input, sep = sep)
}
letters |> accumulate(paste3)

# Here the early return branch checks the incoming inputs passed on
# the RHS:
paste4 <- function(out, input, sep = ".") {
  if (input == "f") {
    return(done())
  }
  paste(out, input, sep = sep)
}
letters |> accumulate(paste4)


# Simulating stochastic processes with drift
## Not run: 
library(dplyr)
library(ggplot2)

map(1:5, \(i) rnorm(100)) |>
  set_names(paste0("sim", 1:5)) |>
  map(\(l) accumulate(l, \(acc, nxt) .05 + acc + nxt)) |>
  map(\(x) tibble(value = x, step = 1:100)) |>
  list_rbind(names_to = "simulation") |>
  ggplot(aes(x = step, y = value)) +
    geom_line(aes(color = simulation)) +
    ggtitle("Simulations of a random walk with drift")

## End(Not run)
# With an associative operation, the final value is always the
# same, no matter the direction. You'll find it in the first element for a
# backward (left) accumulation, and in the last element for forward
# (right) one:
1:5 |> accumulate(`+`)
1:5 |> accumulate(`+`, .dir = "backward")

# The final value is always equal to the equivalent reduction:
1:5 |> reduce(`+`)

# It is easier to understand the details of the reduction with
# `paste()`.
accumulate(letters[1:5], paste, sep = ".")

# Note how the intermediary reduced values are passed to the left
# with a left reduction, and to the right otherwise:
accumulate(letters[1:5], paste, sep = ".", .dir = "backward")

# By ignoring the input vector (nxt), you can turn output of one step into
# the input for the next. This code takes 10 steps of a random walk:
accumulate(1:10, \(acc, nxt) acc + rnorm(1), .init = 0)

# `accumulate2()` is a version of `accumulate()` that works with
# 3-argument functions and one additional vector:
paste2 <- function(acc, nxt, sep = ".") paste(acc, nxt, sep = sep)
letters[1:4] |> accumulate(paste2)
letters[1:4] |> accumulate2(c("-", ".", "-"), paste2)

# You can shortcircuit an accumulation and terminate it early by
# returning a value wrapped in a done(). In the following example
# we return early if the result-so-far, which is passed on the LHS,
# meets a condition:
paste3 <- function(out, input, sep = ".") {
  if (nchar(out) > 4) {
    return(done(out))
  }
  paste(out, input, sep = sep)
}
letters |> accumulate(paste3)

# Note how we get twice the same value in the accumulation. That's
# because we have returned it twice. To prevent this, return an empty
# done box to signal to accumulate() that it should terminate with the
# value of the last iteration:
paste3 <- function(out, input, sep = ".") {
  if (nchar(out) > 4) {
    return(done())
  }
  paste(out, input, sep = sep)
}
letters |> accumulate(paste3)

# Here the early return branch checks the incoming inputs passed on
# the RHS:
paste4 <- function(out, input, sep = ".") {
  if (input == "f") {
    return(done())
  }
  paste(out, input, sep = sep)
}
letters |> accumulate(paste4)


# Simulating stochastic processes with drift
## Not run: 
library(dplyr)
library(ggplot2)

map(1:5, \(i) rnorm(100)) |>
  set_names(paste0("sim", 1:5)) |>
  map(\(l) accumulate(l, \(acc, nxt) .05 + acc + nxt)) |>
  map(\(x) tibble(value = x, step = 1:100)) |>
  list_rbind(names_to = "simulation") |>
  ggplot(aes(x = step, y = value)) +
    geom_line(aes(color = simulation)) +
    ggtitle("Simulations of a random walk with drift")

## End(Not run)

Coerce array to list

Description

array_branch() and array_tree() enable arrays to be used with purrr's functionals by turning them into lists. The details of the coercion are controlled by the margin argument. array_tree() creates an hierarchical list (a tree) that has as many levels as dimensions specified in margin, while array_branch() creates a flat list (by analogy, a branch) along all mentioned dimensions.

Usage

array_branch(array, margin = NULL)

array_tree(array, margin = NULL)
array_branch(array, margin = NULL)

array_tree(array, margin = NULL)

Arguments

`array`	An array to coerce into a list.
`margin`	A numeric vector indicating the positions of the indices to be to be enlisted. If `NULL`, a full margin is used. If `numeric(0)`, the array as a whole is wrapped in a list.

Details

When no margin is specified, all dimensions are used by default. When margin is a numeric vector of length zero, the whole array is wrapped in a list.

Examples

# We create an array with 3 dimensions
x <- array(1:12, c(2, 2, 3))

# A full margin for such an array would be the vector 1:3. This is
# the default if you don't specify a margin

# Creating a branch along the full margin is equivalent to
# as.list(array) and produces a list of size length(x):
array_branch(x) |> str()

# A branch along the first dimension yields a list of length 2
# with each element containing a 2x3 array:
array_branch(x, 1) |> str()

# A branch along the first and third dimensions yields a list of
# length 2x3 whose elements contain a vector of length 2:
array_branch(x, c(1, 3)) |> str()

# Creating a tree from the full margin creates a list of lists of
# lists:
array_tree(x) |> str()

# The ordering and the depth of the tree are controlled by the
# margin argument:
array_tree(x, c(3, 1)) |> str()
# We create an array with 3 dimensions
x <- array(1:12, c(2, 2, 3))

# A full margin for such an array would be the vector 1:3. This is
# the default if you don't specify a margin

# Creating a branch along the full margin is equivalent to
# as.list(array) and produces a list of size length(x):
array_branch(x) |> str()

# A branch along the first dimension yields a list of length 2
# with each element containing a 2x3 array:
array_branch(x, 1) |> str()

# A branch along the first and third dimensions yields a list of
# length 2x3 whose elements contain a vector of length 2:
array_branch(x, c(1, 3)) |> str()

# Creating a tree from the full margin creates a list of lists of
# lists:
array_tree(x) |> str()

# The ordering and the depth of the tree are controlled by the
# margin argument:
array_tree(x, c(3, 1)) |> str()

Convert an object into a mapper function

Description

as_mapper is the powerhouse behind the varied function specifications that most purrr functions allow. It is an S3 generic. The default method forwards its arguments to rlang::as_function().

Usage

as_mapper(.f, ...)

## S3 method for class 'character'
as_mapper(.f, ..., .null, .default = NULL)

## S3 method for class 'numeric'
as_mapper(.f, ..., .null, .default = NULL)

## S3 method for class 'list'
as_mapper(.f, ..., .null, .default = NULL)
as_mapper(.f, ...)

## S3 method for class 'character'
as_mapper(.f, ..., .null, .default = NULL)

## S3 method for class 'numeric'
as_mapper(.f, ..., .null, .default = NULL)

## S3 method for class 'list'
as_mapper(.f, ..., .null, .default = NULL)

Arguments

.f

A function, formula, or vector (not necessarily atomic).

If a function, it is used as is.

If a formula, e.g. ~ .x + 2, it is converted to a function. There are three ways to refer to the arguments:

For a single argument function, use .
For a two argument function, use .x and .y
For more arguments, use ..1, ..2, ..3 etc

This syntax allows you to create very compact anonymous functions. Note that formula functions conceptually take dots (that's why you can use ..1 etc). They silently ignore additional arguments that are not used in the formula expression.

If character vector, numeric vector, or list, it is converted to an extractor function. Character vectors index by name and numeric vectors index by position; use a list to index by position and name at different levels. If a component is not present, the value of .default will be returned.

...

Additional arguments passed on to methods.

.default, .null

Optional additional argument for extractor functions (i.e. when .f is character, integer, or list). Returned when value is absent (does not exist) or empty (has length 0). .null is deprecated; please use .default instead.

Examples

as_mapper(\(x) x + 1)
as_mapper(1)

as_mapper(c("a", "b", "c"))
# Equivalent to function(x) x[["a"]][["b"]][["c"]]

as_mapper(list(1, "a", 2))
# Equivalent to function(x) x[[1]][["a"]][[2]]

as_mapper(list(1, attr_getter("a")))
# Equivalent to function(x) attr(x[[1]], "a")

as_mapper(c("a", "b", "c"), .default = NA)
as_mapper(\(x) x + 1)
as_mapper(1)

as_mapper(c("a", "b", "c"))
# Equivalent to function(x) x[["a"]][["b"]][["c"]]

as_mapper(list(1, "a", 2))
# Equivalent to function(x) x[[1]][["a"]][[2]]

as_mapper(list(1, attr_getter("a")))
# Equivalent to function(x) attr(x[[1]], "a")

as_mapper(c("a", "b", "c"), .default = NA)

Create an attribute getter function

Description

attr_getter() generates an attribute accessor function; i.e., it generates a function for extracting an attribute with a given name. Unlike the base R attr() function with default options, it doesn't use partial matching.

Usage

attr_getter(attr)
attr_getter(attr)

Arguments

attr

An attribute name as string.

Examples

# attr_getter() takes an attribute name and returns a function to
# access the attribute:
get_rownames <- attr_getter("row.names")
get_rownames(mtcars)

# These getter functions are handy in conjunction with pluck() for
# extracting deeply into a data structure. Here we'll first
# extract by position, then by attribute:
obj1 <- structure("obj", obj_attr = "foo")
obj2 <- structure("obj", obj_attr = "bar")
x <- list(obj1, obj2)

pluck(x, 1, attr_getter("obj_attr"))  # From first object
pluck(x, 2, attr_getter("obj_attr"))  # From second object
# attr_getter() takes an attribute name and returns a function to
# access the attribute:
get_rownames <- attr_getter("row.names")
get_rownames(mtcars)

# These getter functions are handy in conjunction with pluck() for
# extracting deeply into a data structure. Here we'll first
# extract by position, then by attribute:
obj1 <- structure("obj", obj_attr = "foo")
obj2 <- structure("obj", obj_attr = "bar")
x <- list(obj1, obj2)

pluck(x, 1, attr_getter("obj_attr"))  # From first object
pluck(x, 2, attr_getter("obj_attr"))  # From second object

Wrap a function so it will automatically `browse()` on error

Description

A function wrapped with auto_browse() will automatically enter an interactive debugger using browser() when ever it encounters an error.

Usage

auto_browse(.f)
auto_browse(.f)

Arguments

`.f`	A function to modify, 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`. Only recommended if you require backward compatibility with older versions of R.

Value

A function that takes the same arguments as .f, but returns a different value, as described above.

Adverbs

This function is called an adverb because it modifies the effect of a function (a verb). If you'd like to include a function created an adverb in a package, be sure to read faq-adverbs-export.

Examples

# For interactive usage, auto_browse() is useful because it automatically
# starts a browser() in the right place.
f <- function(x) {
  y <- 20
  if (x > 5) {
    stop("!")
  } else {
    x
  }
}
if (interactive()) {
  map(1:6, auto_browse(f))
}

# For interactive usage, auto_browse() is useful because it automatically
# starts a browser() in the right place.
f <- function(x) {
  y <- 20
  if (x > 5) {
    stop("!")
  } else {
    x
  }
}
if (interactive()) {
  map(1:6, auto_browse(f))
}

Get an element deep within a nested data structure, failing if it doesn't exist

Description

chuck() implements a generalised form of [[ that allow you to index deeply and flexibly into data structures. If the index you are trying to access does not exist (or is NULL), it will throw (i.e. chuck) an error.

Usage

chuck(.x, ...)
chuck(.x, ...)

Arguments

.x

A vector or environment

...

A list of accessors for indexing into the object. Can be an positive integer, a negative integer (to index from the right), a string (to index into names), or an accessor function (except for the assignment variants which only support names and positions). If the object being indexed is an S4 object, accessing it by name will return the corresponding slot.

Dynamic dots are supported. In particular, if your accessors are stored in a list, you can splice that in with ⁠!!!⁠.

Examples

x <- list(a = 1, b = 2)

# When indexing an element that doesn't exist `[[` sometimes returns NULL:
x[["y"]]
# and sometimes errors:
try(x[[3]])

# chuck() consistently errors:
try(chuck(x, "y"))
try(chuck(x, 3))
x <- list(a = 1, b = 2)

# When indexing an element that doesn't exist `[[` sometimes returns NULL:
x[["y"]]
# and sometimes errors:
try(x[[3]])

# chuck() consistently errors:
try(chuck(x, "y"))
try(chuck(x, 3))

Compose multiple functions together to create a new function

Description

Create a new function that is the composition of multiple functions, i.e. compose(f, g) is equivalent to function(...) f(g(...)).

Usage

compose(..., .dir = c("backward", "forward"))
compose(..., .dir = c("backward", "forward"))

Arguments

...

Functions to apply in order (from right to left by default). Formulas are converted to functions in the usual way.

Dynamic dots are supported. In particular, if your functions are stored in a list, you can splice that in with ⁠!!!⁠.

.dir

If "backward" (the default), the functions are called in the reverse order, from right to left, as is conventional in mathematics. If "forward", they are called from left to right.

Value

A function

Adverbs

This function is called an adverb because it modifies the effect of a function (a verb). If you'd like to include a function created an adverb in a package, be sure to read faq-adverbs-export.

Examples

not_null <- compose(`!`, is.null)
not_null(4)
not_null(NULL)

add1 <- function(x) x + 1
compose(add1, add1)(8)

fn <- compose(\(x) paste(x, "foo"), \(x) paste(x, "bar"))
fn("input")

# Lists of functions can be spliced with !!!
fns <- list(
  function(x) paste(x, "foo"),
  \(x) paste(x, "bar")
)
fn <- compose(!!!fns)
fn("input")
not_null <- compose(`!`, is.null)
not_null(4)
not_null(NULL)

add1 <- function(x) x + 1
compose(add1, add1)(8)

fn <- compose(\(x) paste(x, "foo"), \(x) paste(x, "bar"))
fn("input")

# Lists of functions can be spliced with !!!
fns <- list(
  function(x) paste(x, "foo"),
  \(x) paste(x, "bar")
)
fn <- compose(!!!fns)
fn("input")

Find the value or position of the first match

Description

Find the value or position of the first match

Usage

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

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

Examples

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

3:10 |> detect(is_even)
3:10 |> detect_index(is_even)

3:10 |> detect(is_even, .dir = "backward")
3:10 |> detect_index(is_even, .dir = "backward")


# 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")
detect_index(x, "foo")


# If you need to find all values, use keep():
keep(x, "foo")

# If you need to find all positions, use map_lgl():
which(map_lgl(x, "foo"))
is_even <- function(x) x %% 2 == 0

3:10 |> detect(is_even)
3:10 |> detect_index(is_even)

3:10 |> detect(is_even, .dir = "backward")
3:10 |> detect_index(is_even, .dir = "backward")


# 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")
detect_index(x, "foo")


# If you need to find all values, use keep():
keep(x, "foo")

# If you need to find all positions, use map_lgl():
which(map_lgl(x, "foo"))

Do every, some, or none of the elements of a list satisfy a predicate?

Description

some() returns TRUE when .p is TRUE for at least one element.
every() returns TRUE when .p is TRUE for all elements.
none() returns TRUE when .p is FALSE for all elements.

Usage

every(.x, .p, ...)

some(.x, .p, ...)

none(.x, .p, ...)
every(.x, .p, ...)

some(.x, .p, ...)

none(.x, .p, ...)

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). You must use .x to refer to the first argument). Only recommended if you require backward compatibility with older versions of R.

...

Additional arguments passed on to .p.

Value

A logical vector of length 1.

Examples

x <- list(0:10, 5.5)
x |> every(is.numeric)
x |> every(is.integer)
x |> some(is.integer)
x |> none(is.character)

# Missing values are propagated:
some(list(NA, FALSE), identity)

# If you need to use these functions in a context where missing values are
# unsafe (e.g. in `if ()` conditions), make sure to use safe predicates:
if (some(list(NA, FALSE), rlang::is_true)) "foo" else "bar"
x <- list(0:10, 5.5)
x |> every(is.numeric)
x |> every(is.integer)
x |> some(is.integer)
x |> none(is.character)

# Missing values are propagated:
some(list(NA, FALSE), identity)

# If you need to use these functions in a context where missing values are
# unsafe (e.g. in `if ()` conditions), make sure to use safe predicates:
if (some(list(NA, FALSE), rlang::is_true)) "foo" else "bar"

Does a list contain an object?

Description

Does a list contain an object?

Usage

has_element(.x, .y)
has_element(.x, .y)

Arguments

`.x`	A list or atomic vector.
`.y`	Object to test for

Examples

x <- list(1:10, 5, 9.9)
x |> has_element(1:10)
x |> has_element(3)
x <- list(1:10, 5, 9.9)
x |> has_element(1:10)
x |> has_element(3)

Find head/tail that all satisfies a predicate.

Description

Find head/tail that all satisfies a predicate.

Usage

head_while(.x, .p, ...)

tail_while(.x, .p, ...)
head_while(.x, .p, ...)

tail_while(.x, .p, ...)

Arguments

.x

A list or atomic vector.

.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.

...

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.

Value

A vector the same type as .x.

Examples

pos <- function(x) x >= 0
head_while(5:-5, pos)
tail_while(5:-5, negate(pos))

big <- function(x) x > 100
head_while(0:10, big)
tail_while(0:10, big)
pos <- function(x) x >= 0
head_while(5:-5, pos)
tail_while(5:-5, negate(pos))

big <- function(x) x > 100
head_while(0:10, big)
tail_while(0:10, big)

Apply a function to each element of a vector, and its index

Description

imap(x, ...), an indexed map, is short hand for map2(x, names(x), ...) if x has names, or map2(x, seq_along(x), ...) if it does not. This is useful if you need to compute on both the value and the position of an element.

Usage

imap(.x, .f, ...)

imap_lgl(.x, .f, ...)

imap_chr(.x, .f, ...)

imap_int(.x, .f, ...)

imap_dbl(.x, .f, ...)

imap_vec(.x, .f, ...)

iwalk(.x, .f, ...)
imap(.x, .f, ...)

imap_lgl(.x, .f, ...)

imap_chr(.x, .f, ...)

imap_int(.x, .f, ...)

imap_dbl(.x, .f, ...)

imap_vec(.x, .f, ...)

iwalk(.x, .f, ...)

Arguments

.x

A list or atomic vector.

.f

A function, specified in one of the following ways:

A named function, e.g. paste.
An anonymous function, e.g. ⁠\(x, idx) x + idx⁠ or function(x, idx) x + idx.
A formula, e.g. ~ .x + .y. You must use .x to refer to the current element and .y to refer to the current index. Only recommended if you require backward compatibility with older versions of R.

...

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.

Value

A vector the same length as .x.

Examples

imap_chr(sample(10), paste)

imap_chr(sample(10), \(x, idx) paste0(idx, ": ", x))

iwalk(mtcars, \(x, idx) cat(idx, ": ", median(x), "\n", sep = ""))
imap_chr(sample(10), paste)

imap_chr(sample(10), \(x, idx) paste0(idx, ": ", x))

iwalk(mtcars, \(x, idx) cat(idx, ": ", median(x), "\n", sep = ""))

Transform a function to wait then retry after an error

Description

insistently() takes a function and modifies it to retry after given amount of time whenever it errors.

Usage

insistently(f, rate = rate_backoff(), quiet = TRUE)
insistently(f, rate = rate_backoff(), quiet = TRUE)

Arguments

f

A function to modify, 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. Only recommended if you require backward compatibility with older versions of R.

rate

A rate object. Defaults to jittered exponential backoff.

quiet

Hide errors (TRUE, the default), or display them as they occur?

Value

A function that takes the same arguments as .f, but returns a different value, as described above.

Adverbs

This function is called an adverb because it modifies the effect of a function (a verb). If you'd like to include a function created an adverb in a package, be sure to read faq-adverbs-export.

Examples

# For the purpose of this example, we first create a custom rate
# object with a low waiting time between attempts:
rate <- rate_delay(0.1)

# insistently() makes a function repeatedly try to work
risky_runif <- function(lo = 0, hi = 1) {
  y <- runif(1, lo, hi)
  if(y < 0.9) {
    stop(y, " is too small")
  }
  y
}

# Let's now create an exponential backoff rate with a low waiting
# time between attempts:
rate <- rate_backoff(pause_base = 0.1, pause_min = 0.005, max_times = 4)

# Modify your function to run insistently.
insistent_risky_runif <- insistently(risky_runif, rate, quiet = FALSE)

set.seed(6) # Succeeding seed
insistent_risky_runif()

set.seed(3) # Failing seed
try(insistent_risky_runif())

# You can also use other types of rate settings, like a delay rate
# that waits for a fixed amount of time. Be aware that a delay rate
# has an infinite amount of attempts by default:
rate <- rate_delay(0.2, max_times = 3)
insistent_risky_runif <- insistently(risky_runif, rate = rate, quiet = FALSE)
try(insistent_risky_runif())

# insistently() and possibly() are a useful combination
rate <- rate_backoff(pause_base = 0.1, pause_min = 0.005)
possibly_insistent_risky_runif <- possibly(insistent_risky_runif, otherwise = -99)

set.seed(6)
possibly_insistent_risky_runif()

set.seed(3)
possibly_insistent_risky_runif()
# For the purpose of this example, we first create a custom rate
# object with a low waiting time between attempts:
rate <- rate_delay(0.1)

# insistently() makes a function repeatedly try to work
risky_runif <- function(lo = 0, hi = 1) {
  y <- runif(1, lo, hi)
  if(y < 0.9) {
    stop(y, " is too small")
  }
  y
}

# Let's now create an exponential backoff rate with a low waiting
# time between attempts:
rate <- rate_backoff(pause_base = 0.1, pause_min = 0.005, max_times = 4)

# Modify your function to run insistently.
insistent_risky_runif <- insistently(risky_runif, rate, quiet = FALSE)

set.seed(6) # Succeeding seed
insistent_risky_runif()

set.seed(3) # Failing seed
try(insistent_risky_runif())

# You can also use other types of rate settings, like a delay rate
# that waits for a fixed amount of time. Be aware that a delay rate
# has an infinite amount of attempts by default:
rate <- rate_delay(0.2, max_times = 3)
insistent_risky_runif <- insistently(risky_runif, rate = rate, quiet = FALSE)
try(insistent_risky_runif())

# insistently() and possibly() are a useful combination
rate <- rate_backoff(pause_base = 0.1, pause_min = 0.005)
possibly_insistent_risky_runif <- possibly(insistent_risky_runif, otherwise = -99)

set.seed(6)
possibly_insistent_risky_runif()

set.seed(3)
possibly_insistent_risky_runif()

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)
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). You must use .x to refer to the first argument). Only recommended if you require backward compatibility with older versions of R.

...

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.

Examples

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

# Or use a formula
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()
rep(10, 10) |>
  map(sample, 5) |>
  keep(function(x) mean(x) > 6)

# Or use a formula
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()

Keep/discard elements based on their name/position

Description

Keep/discard elements based on their name/position

Usage

keep_at(x, at)

discard_at(x, at)
keep_at(x, at)

discard_at(x, at)

Arguments

x

A list or atomic vector.

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.

Examples

x <- c(a = 1, b = 2, cat = 10, dog = 15, elephant = 5, e = 10)
x |> keep_at(letters)
x |> discard_at(letters)

# Can also use a function
x |> keep_at(~ nchar(.x) == 3)
x |> discard_at(~ nchar(.x) == 3)
x <- c(a = 1, b = 2, cat = 10, dog = 15, elephant = 5, e = 10)
x |> keep_at(letters)
x |> discard_at(letters)

# Can also use a function
x |> keep_at(~ nchar(.x) == 3)
x |> discard_at(~ nchar(.x) == 3)

Modify a list

Description

list_assign() modifies the elements of a list by name or position.
list_modify() modifies the elements of a list recursively.
list_merge() merges the elements of a list recursively.

list_modify() is inspired by utils::modifyList().

Usage

list_assign(.x, ..., .is_node = NULL)

list_modify(.x, ..., .is_node = NULL)

list_merge(.x, ..., .is_node = NULL)
list_assign(.x, ..., .is_node = NULL)

list_modify(.x, ..., .is_node = NULL)

list_merge(.x, ..., .is_node = NULL)

Arguments

.x

List to modify.

...

New values of a list. Use zap() to remove values.

These values should be either all named or all unnamed. When inputs are all named, they are matched to .x by name. When they are all unnamed, they are matched by position.

Dynamic dots are supported. In particular, if your replacement values are stored in a list, you can splice that in with ⁠!!!⁠.

.is_node

A predicate function that determines whether an element is a node (by returning TRUE) or a leaf (by returning FALSE). The default value, NULL, treats simple lists as nodes and everything else (including richer objects like data frames and linear models) as leaves, using vctrs::obj_is_list(). To recurse into all objects built on lists use is.list().

Examples

x <- list(x = 1:10, y = 4, z = list(a = 1, b = 2))
str(x)

# Update values
str(list_assign(x, a = 1))

# Replace values
str(list_assign(x, z = 5))
str(list_assign(x, z = NULL))
str(list_assign(x, z = list(a = 1:5)))

# Replace recursively with list_modify(), leaving the other elements of z alone
str(list_modify(x, z = list(a = 1:5)))

# Remove values
str(list_assign(x, z = zap()))

# Combine values with list_merge()
str(list_merge(x, x = 11, z = list(a = 2:5, c = 3)))

# All these functions support dynamic dots features. Use !!! to splice
# a list of arguments:
l <- list(new = 1, y = zap(), z = 5)
str(list_assign(x, !!!l))
x <- list(x = 1:10, y = 4, z = list(a = 1, b = 2))
str(x)

# Update values
str(list_assign(x, a = 1))

# Replace values
str(list_assign(x, z = 5))
str(list_assign(x, z = NULL))
str(list_assign(x, z = list(a = 1:5)))

# Replace recursively with list_modify(), leaving the other elements of z alone
str(list_modify(x, z = list(a = 1:5)))

# Remove values
str(list_assign(x, z = zap()))

# Combine values with list_merge()
str(list_merge(x, x = 11, z = list(a = 2:5, c = 3)))

# All these functions support dynamic dots features. Use !!! to splice
# a list of arguments:
l <- list(new = 1, y = zap(), z = 5)
str(list_assign(x, !!!l))

Combine list elements into a single data structure

Description

list_c() combines elements into a vector by concatenating them together with vctrs::vec_c().
list_rbind() combines elements into a data frame by row-binding them together with vctrs::vec_rbind().
list_cbind() combines elements into a data frame by column-binding them together with vctrs::vec_cbind().

Usage

list_c(x, ..., ptype = NULL)

list_cbind(
  x,
  ...,
  name_repair = c("unique", "universal", "check_unique"),
  size = NULL
)

list_rbind(x, ..., names_to = rlang::zap(), ptype = NULL)
list_c(x, ..., ptype = NULL)

list_cbind(
  x,
  ...,
  name_repair = c("unique", "universal", "check_unique"),
  size = NULL
)

list_rbind(x, ..., names_to = rlang::zap(), ptype = NULL)

Arguments

`x`	A list. For `list_rbind()` and `list_cbind()` the list must only contain only data frames or `NULL`.
`...`	These dots are for future extensions and must be empty.
`ptype`	An optional prototype to ensure that the output type is always the same.
`name_repair`	One of `"unique"`, `"universal"`, or `"check_unique"`. See `vctrs::vec_as_names()` for the meaning of these options.
`size`	An optional integer size to ensure that every input has the same size (i.e. number of rows).
`names_to`	By default, `names(x)` are lost. To keep them, supply a string to `names_to` and the names will be saved into a column with that name. If `names_to` is supplied and `x` is not named, the position of the elements will be used instead of the names.

Examples

x1 <- list(a = 1, b = 2, c = 3)
list_c(x1)

x2 <- list(
  a = data.frame(x = 1:2),
  b = data.frame(y = "a")
)
list_rbind(x2)
list_rbind(x2, names_to = "id")
list_rbind(unname(x2), names_to = "id")

list_cbind(x2)
x1 <- list(a = 1, b = 2, c = 3)
list_c(x1)

x2 <- list(
  a = data.frame(x = 1:2),
  b = data.frame(y = "a")
)
list_rbind(x2)
list_rbind(x2, names_to = "id")
list_rbind(unname(x2), names_to = "id")

list_cbind(x2)

Flatten a list

Description

Flattening a list removes a single layer of internal hierarchy, i.e. it inlines elements that are lists leaving non-lists alone.

Usage

list_flatten(
  x,
  ...,
  name_spec = "{outer}_{inner}",
  name_repair = c("minimal", "unique", "check_unique", "universal")
)
list_flatten(
  x,
  ...,
  name_spec = "{outer}_{inner}",
  name_repair = c("minimal", "unique", "check_unique", "universal")
)

Arguments

`x`	A list.
`...`	These dots are for future extensions and must be empty.
`name_spec`	If both inner and outer names are present, control how they are combined. Should be a glue specification that uses variables `inner` and `outer`.
`name_repair`	One of `"minimal"`, `"unique"`, `"universal"`, or `"check_unique"`. See `vctrs::vec_as_names()` for the meaning of these options.

Value

A list of the same type as x. The list might be shorter if x contains empty lists, the same length if it contains lists of length 1 or no sub-lists, or longer if it contains lists of length > 1.

Examples

x <- list(1, list(2, 3), list(4, list(5)))
x |> list_flatten() |> str()
x |> list_flatten() |> list_flatten() |> str()

# Flat lists are left as is
list(1, 2, 3, 4, 5) |> list_flatten() |> str()

# Empty lists will disappear
list(1, list(), 2, list(3)) |> list_flatten() |> str()

# Another way to see this is that it reduces the depth of the list
x <- list(
  list(),
  list(list())
)
x |> pluck_depth()
x |> list_flatten() |> pluck_depth()

# Use name_spec to control how inner and outer names are combined
x <- list(x = list(a = 1, b = 2), y = list(c = 1, d = 2))
x |> list_flatten() |> names()
x |> list_flatten(name_spec = "{outer}") |> names()
x |> list_flatten(name_spec = "{inner}") |> names()
x <- list(1, list(2, 3), list(4, list(5)))
x |> list_flatten() |> str()
x |> list_flatten() |> list_flatten() |> str()

# Flat lists are left as is
list(1, 2, 3, 4, 5) |> list_flatten() |> str()

# Empty lists will disappear
list(1, list(), 2, list(3)) |> list_flatten() |> str()

# Another way to see this is that it reduces the depth of the list
x <- list(
  list(),
  list(list())
)
x |> pluck_depth()
x |> list_flatten() |> pluck_depth()

# Use name_spec to control how inner and outer names are combined
x <- list(x = list(a = 1, b = 2), y = list(c = 1, d = 2))
x |> list_flatten() |> names()
x |> list_flatten(name_spec = "{outer}") |> names()
x |> list_flatten(name_spec = "{inner}") |> names()

Simplify a list to an atomic or S3 vector

Description

Simplification maintains a one-to-one correspondence between the input and output, implying that each element of x must contain a one element vector or a one-row data frame. If you don't want to maintain this correspondence, then you probably want either list_c()/list_rbind() or list_flatten().

Usage

list_simplify(x, ..., strict = TRUE, ptype = NULL)
list_simplify(x, ..., strict = TRUE, ptype = NULL)

Arguments

`x`	A list.
`...`	These dots are for future extensions and must be empty.
`strict`	What should happen if simplification fails? If `TRUE` (the default) it will error. If `FALSE` and `ptype` is not supplied, it will return `x` unchanged.
`ptype`	An optional prototype to ensure that the output type is always the same.

Value

A vector the same length as x.

Examples

list_simplify(list(1, 2, 3))

# Only works when vectors are length one and have compatible types:
try(list_simplify(list(1, 2, 1:3)))
try(list_simplify(list(1, 2, "x")))

# Unless you strict = FALSE, in which case you get the input back:
list_simplify(list(1, 2, 1:3), strict = FALSE)
list_simplify(list(1, 2, "x"), strict = FALSE)
list_simplify(list(1, 2, 3))

# Only works when vectors are length one and have compatible types:
try(list_simplify(list(1, 2, 1:3)))
try(list_simplify(list(1, 2, "x")))

# Unless you strict = FALSE, in which case you get the input back:
list_simplify(list(1, 2, 1:3), strict = FALSE)
list_simplify(list(1, 2, "x"), strict = FALSE)

Transpose a list

Description

list_transpose() turns a list-of-lists "inside-out". For instance it turns a pair of lists into a list of pairs, or a list of pairs into a pair of lists. For example, if you had a list of length n where each component had values a and b, list_transpose() would make a list with elements a and b that contained lists of length n.

It's called transpose because x[["a"]][["b"]] is equivalent to list_transpose(x)[["b"]][["a"]], i.e. transposing a list flips the order of indices in a similar way to transposing a matrix.

Usage

list_transpose(
  x,
  ...,
  template = NULL,
  simplify = NA,
  ptype = NULL,
  default = NULL
)
list_transpose(
  x,
  ...,
  template = NULL,
  simplify = NA,
  ptype = NULL,
  default = NULL
)

Arguments

`x`	A list of vectors to transpose.
`...`	These dots are for future extensions and must be empty.
`template`	A "template" that describes the output list. Can either be a character vector (where elements are extracted by name), or an integer vector (where elements are extracted by position). Defaults to the union of the names of the elements of `x`, or if they're not present, the union of the integer indices.
`simplify`	Should the result be simplified? `TRUE`: simplify or die trying. `NA`: simplify if possible. `FALSE`: never try to simplify, always leaving as a list. Alternatively, a named list specifying the simplification by output element.
`ptype`	An optional vector prototype used to control the simplification. Alternatively, a named list specifying the prototype by output element.
`default`	A default value to use if a value is absent or `NULL`. Alternatively, a named list specifying the default by output element.

Examples

# list_transpose() is useful in conjunction with safely()
x <- list("a", 1, 2)
y <- x |> map(safely(log))
y |> str()
# Put all the errors and results together
y |> list_transpose() |> str()
# Supply a default result to further simplify
y |> list_transpose(default = list(result = NA)) |> str()

# list_transpose() will try to simplify by default:
x <- list(list(a = 1, b = 2), list(a = 3, b = 4), list(a = 5, b = 6))
x |> list_transpose()
# this makes list_tranpose() not completely symmetric
x |> list_transpose() |> list_transpose()

# use simplify = FALSE to always return lists:
x |> list_transpose(simplify = FALSE) |> str()
x |>
  list_transpose(simplify = FALSE) |>
  list_transpose(simplify = FALSE) |> str()

# Provide an explicit template if you know which elements you want to extract
ll <- list(
  list(x = 1, y = "one"),
  list(z = "deux", x = 2)
)
ll |> list_transpose()
ll |> list_transpose(template = c("x", "y", "z"))
ll |> list_transpose(template = 1)

# And specify a default if you want to simplify
ll |> list_transpose(template = c("x", "y", "z"), default = NA)
# list_transpose() is useful in conjunction with safely()
x <- list("a", 1, 2)
y <- x |> map(safely(log))
y |> str()
# Put all the errors and results together
y |> list_transpose() |> str()
# Supply a default result to further simplify
y |> list_transpose(default = list(result = NA)) |> str()

# list_transpose() will try to simplify by default:
x <- list(list(a = 1, b = 2), list(a = 3, b = 4), list(a = 5, b = 6))
x |> list_transpose()
# this makes list_tranpose() not completely symmetric
x |> list_transpose() |> list_transpose()

# use simplify = FALSE to always return lists:
x |> list_transpose(simplify = FALSE) |> str()
x |>
  list_transpose(simplify = FALSE) |>
  list_transpose(simplify = FALSE) |> str()

# Provide an explicit template if you know which elements you want to extract
ll <- list(
  list(x = 1, y = "one"),
  list(z = "deux", x = 2)
)
ll |> list_transpose()
ll |> list_transpose(template = c("x", "y", "z"))
ll |> list_transpose(template = 1)

# And specify a default if you want to simplify
ll |> list_transpose(template = c("x", "y", "z"), default = NA)

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, ...)
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.

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()
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()

Apply a function to each element of a vector

Description

The map functions transform their input by applying a function to each element of a list or atomic vector and returning an object of the same length as the input.

map() always returns a list. See the modify() family for versions that return an object of the same type as the input.
map_lgl(), map_int(), map_dbl() and map_chr() return an atomic vector of the indicated type (or die trying). For these functions, .f must return a length-1 vector of the appropriate type.
map_vec() simplifies to the common type of the output. It works with most types of simple vectors like Date, POSIXct, factors, etc.
walk() calls .f for its side-effect and returns the input .x.

Usage

map(.x, .f, ..., .progress = FALSE)

map_lgl(.x, .f, ..., .progress = FALSE)

map_int(.x, .f, ..., .progress = FALSE)

map_dbl(.x, .f, ..., .progress = FALSE)

map_chr(.x, .f, ..., .progress = FALSE)

map_vec(.x, .f, ..., .ptype = NULL, .progress = FALSE)

walk(.x, .f, ..., .progress = FALSE)
map(.x, .f, ..., .progress = FALSE)

map_lgl(.x, .f, ..., .progress = FALSE)

map_int(.x, .f, ..., .progress = FALSE)

map_dbl(.x, .f, ..., .progress = FALSE)

map_chr(.x, .f, ..., .progress = FALSE)

map_vec(.x, .f, ..., .ptype = NULL, .progress = FALSE)

walk(.x, .f, ..., .progress = FALSE)

Arguments

`.x`	A list or atomic 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 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.
`.progress`	Whether to show a progress bar. Use `TRUE` to turn on a basic progress bar, use a string to give it a name, or see progress_bars for more details.
`.ptype`	If `NULL`, the default, the output type is the common type of the elements of the result. Otherwise, supply a "prototype" giving the desired type of output.

Value

The output length is determined by the length of the input. The output names are determined by the input names. The output type is determined by the suffix:

No suffix: a list; .f() can return anything.
⁠_lgl()⁠, ⁠_int()⁠, ⁠_dbl()⁠, ⁠_chr()⁠ return a logical, integer, double, or character vector respectively; .f() must return a compatible atomic vector of length 1.
⁠_vec()⁠ return an atomic or S3 vector, the same type that .f returns. .f can return pretty much any type of vector, as long as its length 1.
walk() returns the input .x (invisibly). This makes it easy to use in a pipe. The return value of .f() is ignored.

Any errors thrown by .f will be wrapped in an error with class purrr_error_indexed.

Examples

# Compute normal distributions from an atomic vector
1:10 |>
  map(rnorm, n = 10)

# You can also use an anonymous function
1:10 |>
  map(\(x) rnorm(10, x))

# Simplify output to a vector instead of a list by computing the mean of the distributions
1:10 |>
  map(rnorm, n = 10) |>  # output a list
  map_dbl(mean)           # output an atomic vector

# Using set_names() with character vectors is handy to keep track
# of the original inputs:
set_names(c("foo", "bar")) |> map_chr(paste0, ":suffix")

# Working with lists
favorite_desserts <- list(Sophia = "banana bread", Eliott = "pancakes", Karina = "chocolate cake")
favorite_desserts |> map_chr(\(food) paste(food, "rocks!"))

# Extract by name or position
# .default specifies value for elements that are missing or NULL
l1 <- list(list(a = 1L), list(a = NULL, b = 2L), list(b = 3L))
l1 |> map("a", .default = "???")
l1 |> map_int("b", .default = NA)
l1 |> map_int(2, .default = NA)

# Supply multiple values to index deeply into a list
l2 <- list(
  list(num = 1:3,     letters[1:3]),
  list(num = 101:103, letters[4:6]),
  list()
)
l2 |> map(c(2, 2))

# Use a list to build an extractor that mixes numeric indices and names,
# and .default to provide a default value if the element does not exist
l2 |> map(list("num", 3))
l2 |> map_int(list("num", 3), .default = NA)

# Working with data frames
# Use map_lgl(), map_dbl(), etc to return a vector instead of a list:
mtcars |> map_dbl(sum)

# A more realistic example: split a data frame into pieces, fit a
# model to each piece, summarise and extract R^2
mtcars |>
  split(mtcars$cyl) |>
  map(\(df) lm(mpg ~ wt, data = df)) |>
  map(summary) |>
  map_dbl("r.squared")
# Compute normal distributions from an atomic vector
1:10 |>
  map(rnorm, n = 10)

# You can also use an anonymous function
1:10 |>
  map(\(x) rnorm(10, x))

# Simplify output to a vector instead of a list by computing the mean of the distributions
1:10 |>
  map(rnorm, n = 10) |>  # output a list
  map_dbl(mean)           # output an atomic vector

# Using set_names() with character vectors is handy to keep track
# of the original inputs:
set_names(c("foo", "bar")) |> map_chr(paste0, ":suffix")

# Working with lists
favorite_desserts <- list(Sophia = "banana bread", Eliott = "pancakes", Karina = "chocolate cake")
favorite_desserts |> map_chr(\(food) paste(food, "rocks!"))

# Extract by name or position
# .default specifies value for elements that are missing or NULL
l1 <- list(list(a = 1L), list(a = NULL, b = 2L), list(b = 3L))
l1 |> map("a", .default = "???")
l1 |> map_int("b", .default = NA)
l1 |> map_int(2, .default = NA)

# Supply multiple values to index deeply into a list
l2 <- list(
  list(num = 1:3,     letters[1:3]),
  list(num = 101:103, letters[4:6]),
  list()
)
l2 |> map(c(2, 2))

# Use a list to build an extractor that mixes numeric indices and names,
# and .default to provide a default value if the element does not exist
l2 |> map(list("num", 3))
l2 |> map_int(list("num", 3), .default = NA)

# Working with data frames
# Use map_lgl(), map_dbl(), etc to return a vector instead of a list:
mtcars |> map_dbl(sum)

# A more realistic example: split a data frame into pieces, fit a
# model to each piece, summarise and extract R^2
mtcars |>
  split(mtcars$cyl) |>
  map(\(df) lm(mpg ~ wt, data = df)) |>
  map(summary) |>
  map_dbl("r.squared")

Map/modify elements at given depth

Description

map_depth() calls map(.y, .f) on all .y at the specified .depth in .x. modify_depth() calls modify(.y, .f) on .y at the specified .depth in .x.

Usage

map_depth(.x, .depth, .f, ..., .ragged = .depth < 0, .is_node = NULL)

modify_depth(.x, .depth, .f, ..., .ragged = .depth < 0, .is_node = NULL)
map_depth(.x, .depth, .f, ..., .ragged = .depth < 0, .is_node = NULL)

modify_depth(.x, .depth, .f, ..., .ragged = .depth < 0, .is_node = NULL)

Arguments

`.x`	A list or atomic vector.
`.depth`	Level of `.x` to map on. Use a negative value to count up from the lowest level of the list. `map_depth(x, 0, fun)` is equivalent to `fun(x)`. `map_depth(x, 1, fun)` is equivalent to `x <- map(x, fun)` `map_depth(x, 2, fun)` is equivalent to `⁠x <- map(x, \(y) map(y, fun))⁠`
`.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 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.
`.ragged`	If `TRUE`, will apply to leaves, even if they're not at depth `.depth`. If `FALSE`, will throw an error if there are no elements at depth `.depth`.
`.is_node`	A predicate function that determines whether an element is a node (by returning `TRUE`) or a leaf (by returning `FALSE`). The default value, `NULL`, treats simple lists as nodes and everything else (including richer objects like data frames and linear models) as leaves, using `vctrs::obj_is_list()`. To recurse into all objects built on lists use `is.list()`.

Examples

# map_depth() -------------------------------------------------
# Use `map_depth()` to recursively traverse nested vectors and map
# a function at a certain depth:
x <- list(a = list(foo = 1:2, bar = 3:4), b = list(baz = 5:6))
x |> str()
x |> map_depth(2, \(y) paste(y, collapse = "/")) |> str()

# Equivalent to:
x |> map(\(y) map(y, \(z) paste(z, collapse = "/"))) |> str()

# When ragged is TRUE, `.f()` will also be passed leaves at depth < `.depth`
x <- list(1, list(1, list(1, list(1, 1))))
x |> str()
x |> map_depth(4, \(x) length(unlist(x)), .ragged = TRUE) |> str()
x |> map_depth(3, \(x) length(unlist(x)), .ragged = TRUE) |> str()
x |> map_depth(2, \(x) length(unlist(x)), .ragged = TRUE) |> str()
x |> map_depth(1, \(x) length(unlist(x)), .ragged = TRUE) |> str()
x |> map_depth(0, \(x) length(unlist(x)), .ragged = TRUE) |> str()

# modify_depth() -------------------------------------------------
l1 <- list(
  obj1 = list(
    prop1 = list(param1 = 1:2, param2 = 3:4),
    prop2 = list(param1 = 5:6, param2 = 7:8)
  ),
  obj2 = list(
    prop1 = list(param1 = 9:10, param2 = 11:12),
    prop2 = list(param1 = 12:14, param2 = 15:17)
  )
)

# In the above list, "obj" is level 1, "prop" is level 2 and "param"
# is level 3. To apply sum() on all params, we map it at depth 3:
l1 |> modify_depth(3, sum) |> str()

# modify() lets us pluck the elements prop1/param2 in obj1 and obj2:
l1 |> modify(c("prop1", "param2")) |> str()

# But what if we want to pluck all param2 elements? Then we need to
# act at a lower level:
l1 |> modify_depth(2, "param2") |> str()

# modify_depth() can be with other purrr functions to make them operate at
# a lower level. Here we ask pmap() to map paste() simultaneously over all
# elements of the objects at the second level. paste() is effectively
# mapped at level 3.
l1 |> modify_depth(2, \(x) pmap(x, paste, sep = " / ")) |> str()
# map_depth() -------------------------------------------------
# Use `map_depth()` to recursively traverse nested vectors and map
# a function at a certain depth:
x <- list(a = list(foo = 1:2, bar = 3:4), b = list(baz = 5:6))
x |> str()
x |> map_depth(2, \(y) paste(y, collapse = "/")) |> str()

# Equivalent to:
x |> map(\(y) map(y, \(z) paste(z, collapse = "/"))) |> str()

# When ragged is TRUE, `.f()` will also be passed leaves at depth < `.depth`
x <- list(1, list(1, list(1, list(1, 1))))
x |> str()
x |> map_depth(4, \(x) length(unlist(x)), .ragged = TRUE) |> str()
x |> map_depth(3, \(x) length(unlist(x)), .ragged = TRUE) |> str()
x |> map_depth(2, \(x) length(unlist(x)), .ragged = TRUE) |> str()
x |> map_depth(1, \(x) length(unlist(x)), .ragged = TRUE) |> str()
x |> map_depth(0, \(x) length(unlist(x)), .ragged = TRUE) |> str()

# modify_depth() -------------------------------------------------
l1 <- list(
  obj1 = list(
    prop1 = list(param1 = 1:2, param2 = 3:4),
    prop2 = list(param1 = 5:6, param2 = 7:8)
  ),
  obj2 = list(
    prop1 = list(param1 = 9:10, param2 = 11:12),
    prop2 = list(param1 = 12:14, param2 = 15:17)
  )
)

# In the above list, "obj" is level 1, "prop" is level 2 and "param"
# is level 3. To apply sum() on all params, we map it at depth 3:
l1 |> modify_depth(3, sum) |> str()

# modify() lets us pluck the elements prop1/param2 in obj1 and obj2:
l1 |> modify(c("prop1", "param2")) |> str()

# But what if we want to pluck all param2 elements? Then we need to
# act at a lower level:
l1 |> modify_depth(2, "param2") |> str()

# modify_depth() can be with other purrr functions to make them operate at
# a lower level. Here we ask pmap() to map paste() simultaneously over all
# elements of the objects at the second level. paste() is effectively
# mapped at level 3.
l1 |> modify_depth(2, \(x) pmap(x, paste, sep = " / ")) |> str()

Apply a function to each element of a vector conditionally

Description

The functions map_if() and map_at() take .x as input, apply the function .f to some of the elements of .x, and return a list of the same length as the input.

map_if() takes a predicate function .p as input to determine which elements of .x are transformed with .f.
map_at() takes a vector of names or positions .at to specify which elements of .x are transformed with .f.

Usage

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

map_at(.x, .at, .f, ..., .progress = FALSE)
map_if(.x, .p, .f, ..., .else = NULL)

map_at(.x, .at, .f, ..., .progress = FALSE)

Arguments

`.x`	A list or atomic vector.
`.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.
`.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 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.
`.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.
`.progress`	Whether to show a progress bar. Use `TRUE` to turn on a basic progress bar, use a string to give it a name, or see progress_bars for more details.

Examples

# Use a predicate function to decide whether to map a function:
iris |> map_if(is.factor, as.character) |> str()

# Specify an alternative with the `.else` argument:
iris |> map_if(is.factor, as.character, .else = as.integer) |> str()

# Use numeric vector of positions select elements to change:
iris |> map_at(c(4, 5), is.numeric) |> str()

# Use vector of names to specify which elements to change:
iris |> map_at("Species", toupper) |> str()
# Use a predicate function to decide whether to map a function:
iris |> map_if(is.factor, as.character) |> str()

# Specify an alternative with the `.else` argument:
iris |> map_if(is.factor, as.character, .else = as.integer) |> str()

# Use numeric vector of positions select elements to change:
iris |> map_at(c(4, 5), is.numeric) |> str()

# Use vector of names to specify which elements to change:
iris |> map_at("Species", toupper) |> str()

Map over two inputs

Description

These functions are variants of map() that iterate over two arguments at a time.

Usage

map2(.x, .y, .f, ..., .progress = FALSE)

map2_lgl(.x, .y, .f, ..., .progress = FALSE)

map2_int(.x, .y, .f, ..., .progress = FALSE)

map2_dbl(.x, .y, .f, ..., .progress = FALSE)

map2_chr(.x, .y, .f, ..., .progress = FALSE)

map2_vec(.x, .y, .f, ..., .ptype = NULL, .progress = FALSE)

walk2(.x, .y, .f, ..., .progress = FALSE)
map2(.x, .y, .f, ..., .progress = FALSE)

map2_lgl(.x, .y, .f, ..., .progress = FALSE)

map2_int(.x, .y, .f, ..., .progress = FALSE)

map2_dbl(.x, .y, .f, ..., .progress = FALSE)

map2_chr(.x, .y, .f, ..., .progress = FALSE)

map2_vec(.x, .y, .f, ..., .ptype = NULL, .progress = FALSE)

walk2(.x, .y, .f, ..., .progress = FALSE)

Arguments

`.x`, `.y`	A pair of vectors, usually the same length. If not, a vector of length 1 will be recycled to the length of the other.
`.f`	A function, specified in one of the following ways: A named function. An anonymous function, e.g. `⁠\(x, y) x + y⁠` or `function(x, y) x + y`. A formula, e.g. `~ .x + .y`. You must use `.x` to refer to the current element of `x` and `.y` to refer to the current element of `y`. Only recommended if you require backward compatibility with older versions of R.
`...`	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.
`.progress`	Whether to show a progress bar. Use `TRUE` to turn on a basic progress bar, use a string to give it a name, or see progress_bars for more details.
`.ptype`	If `NULL`, the default, the output type is the common type of the elements of the result. Otherwise, supply a "prototype" giving the desired type of output.

Value

The output length is determined by the length of the input. The output names are determined by the input names. The output type is determined by the suffix:

No suffix: a list; .f() can return anything.
⁠_lgl()⁠, ⁠_int()⁠, ⁠_dbl()⁠, ⁠_chr()⁠ return a logical, integer, double, or character vector respectively; .f() must return a compatible atomic vector of length 1.
⁠_vec()⁠ return an atomic or S3 vector, the same type that .f returns. .f can return pretty much any type of vector, as long as its length 1.
walk() returns the input .x (invisibly). This makes it easy to use in a pipe. The return value of .f() is ignored.

Any errors thrown by .f will be wrapped in an error with class purrr_error_indexed.

Examples

x <- list(1, 1, 1)
y <- list(10, 20, 30)

map2(x, y, \(x, y) x + y)
# Or just
map2(x, y, `+`)

# Split into pieces, fit model to each piece, then predict
by_cyl <- mtcars |> split(mtcars$cyl)
mods <- by_cyl |> map(\(df) lm(mpg ~ wt, data = df))
map2(mods, by_cyl, predict)
x <- list(1, 1, 1)
y <- list(10, 20, 30)

map2(x, y, \(x, y) x + y)
# Or just
map2(x, y, `+`)

# Split into pieces, fit model to each piece, then predict
by_cyl <- mtcars |> split(mtcars$cyl)
mods <- by_cyl |> map(\(df) lm(mpg ~ wt, data = df))
map2(mods, by_cyl, predict)

Modify elements selectively

Description

Unlike map() and its variants which always return a fixed object type (list for map(), integer vector for map_int(), etc), the modify() family always returns the same type as the input object.

modify() is a shortcut for ⁠x[[i]] <- f(x[[i]]); return(x)⁠.
modify_if() only modifies the elements of x that satisfy a predicate and leaves the others unchanged. modify_at() only modifies elements given by names or positions.
modify2() modifies the elements of .x but also passes the elements of .y to .f, just like map2(). imodify() passes the names or the indices to .f like imap() does.
modify_in() modifies a single element in a pluck() location.

Usage

modify(.x, .f, ...)

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

modify_at(.x, .at, .f, ...)

modify2(.x, .y, .f, ...)

imodify(.x, .f, ...)
modify(.x, .f, ...)

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

modify_at(.x, .at, .f, ...)

modify2(.x, .y, .f, ...)

imodify(.x, .f, ...)

Arguments

`.x`	A vector.
`.f`	A function specified in the same way as the corresponding map function.
`...`	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.
`.y`	A vector, usually the same length as `.x`.

Details

Since the transformation can alter the structure of the input; it's your responsibility to ensure that the transformation produces a valid output. For example, if you're modifying a data frame, .f must preserve the length of the input.

Value

An object the same class as .x

Genericity

modify() and variants are generic over classes that implement length(), [[ and ⁠[[<-⁠ methods. If the default implementation is not compatible for your class, you can override them with your own methods.

If you implement your own modify() method, make sure it satisfies the following invariants:

modify(x, identity) === x
modify(x, compose(f, g)) === modify(x, g) |> modify(f)

These invariants are known as the functor laws in computer science.

Examples

# Convert factors to characters
iris |>
  modify_if(is.factor, as.character) |>
  str()

# Specify which columns to map with a numeric vector of positions:
mtcars |> modify_at(c(1, 4, 5), as.character) |> str()

# Or with a vector of names:
mtcars |> modify_at(c("cyl", "am"), as.character) |> str()

list(x = sample(c(TRUE, FALSE), 100, replace = TRUE), y = 1:100) |>
  list_transpose(simplify = FALSE) |>
  modify_if("x", \(l) list(x = l$x, y = l$y * 100)) |>
  list_transpose()

# Use modify2() to map over two vectors and preserve the type of
# the first one:
x <- c(foo = 1L, bar = 2L)
y <- c(TRUE, FALSE)
modify2(x, y, \(x, cond) if (cond) x else 0L)

# Use a predicate function to decide whether to map a function:
modify_if(iris, is.factor, as.character)

# Specify an alternative with the `.else` argument:
modify_if(iris, is.factor, as.character, .else = as.integer)
# Convert factors to characters
iris |>
  modify_if(is.factor, as.character) |>
  str()

# Specify which columns to map with a numeric vector of positions:
mtcars |> modify_at(c(1, 4, 5), as.character) |> str()

# Or with a vector of names:
mtcars |> modify_at(c("cyl", "am"), as.character) |> str()

list(x = sample(c(TRUE, FALSE), 100, replace = TRUE), y = 1:100) |>
  list_transpose(simplify = FALSE) |>
  modify_if("x", \(l) list(x = l$x, y = l$y * 100)) |>
  list_transpose()

# Use modify2() to map over two vectors and preserve the type of
# the first one:
x <- c(foo = 1L, bar = 2L)
y <- c(TRUE, FALSE)
modify2(x, y, \(x, cond) if (cond) x else 0L)

# Use a predicate function to decide whether to map a function:
modify_if(iris, is.factor, as.character)

# Specify an alternative with the `.else` argument:
modify_if(iris, is.factor, as.character, .else = as.integer)

Modify a pluck location

Description

assign_in() takes a data structure and a pluck location, assigns a value there, and returns the modified data structure.
modify_in() applies a function to a pluck location, assigns the result back to that location with assign_in(), and returns the modified data structure.

Usage

modify_in(.x, .where, .f, ...)

assign_in(x, where, value)
modify_in(.x, .where, .f, ...)

assign_in(x, where, value)

Arguments

`.x`, `x`	A vector or environment
`.where`, `where`	A pluck location, as a numeric vector of positions, a character vector of names, or a list combining both. The location must exist in the data structure.
`.f`	A function to apply at the pluck location given by `.where`.
`...`	Arguments passed to `.f`.
`value`	A value to replace in `.x` at the pluck location. Use `zap()` to instead remove the element.

Examples

# Recall that pluck() returns a component of a data structure that
# might be arbitrarily deep
x <- list(list(bar = 1, foo = 2))
pluck(x, 1, "foo")

# Use assign_in() to modify the pluck location:
str(assign_in(x, list(1, "foo"), 100))
# Or zap to remove it
str(assign_in(x, list(1, "foo"), zap()))

# Like pluck(), this works even when the element (or its parents) don't exist
pluck(x, 1, "baz")
str(assign_in(x, list(2, "baz"), 100))

# modify_in() applies a function to that location and update the
# element in place:
modify_in(x, list(1, "foo"), \(x) x * 200)

# Additional arguments are passed to the function in the ordinary way:
modify_in(x, list(1, "foo"), `+`, 100)
# Recall that pluck() returns a component of a data structure that
# might be arbitrarily deep
x <- list(list(bar = 1, foo = 2))
pluck(x, 1, "foo")

# Use assign_in() to modify the pluck location:
str(assign_in(x, list(1, "foo"), 100))
# Or zap to remove it
str(assign_in(x, list(1, "foo"), zap()))

# Like pluck(), this works even when the element (or its parents) don't exist
pluck(x, 1, "baz")
str(assign_in(x, list(2, "baz"), 100))

# modify_in() applies a function to that location and update the
# element in place:
modify_in(x, list(1, "foo"), \(x) x * 200)

# Additional arguments are passed to the function in the ordinary way:
modify_in(x, list(1, "foo"), `+`, 100)

Recursively modify a list

Description

modify_tree() allows you to recursively modify a list, supplying functions that either modify each leaf or each node (or both).

Usage

modify_tree(
  x,
  ...,
  leaf = identity,
  is_node = NULL,
  pre = identity,
  post = identity
)
modify_tree(
  x,
  ...,
  leaf = identity,
  is_node = NULL,
  pre = identity,
  post = identity
)

Arguments

`x`	A list.
`...`	Reserved for future use. Must be empty
`leaf`	A function applied to each leaf.
`is_node`	A predicate function that determines whether an element is a node (by returning `TRUE`) or a leaf (by returning `FALSE`). The default value, `NULL`, treats simple lists as nodes and everything else (including richer objects like data frames and linear models) as leaves, using `vctrs::obj_is_list()`. To recurse into all objects built on lists use `is.list()`.
`pre`, `post`	Functions applied to each node. `pre` is applied on the way "down", i.e. before the leaves are transformed with `leaf`, while `post` is applied on the way "up", i.e. after the leaves are transformed.

Examples

x <- list(list(a = 2:1, c = list(b1 = 2), b = list(c2 = 3, c1 = 4)))
x |> str()

# Transform each leaf
x |> modify_tree(leaf = \(x) x + 100) |>  str()

# Recursively sort the nodes
sort_named <- function(x) {
  nms <- names(x)
  if (!is.null(nms)) {
    x[order(nms)]
  } else {
    x
   }
}
x |> modify_tree(post = sort_named) |> str()
x <- list(list(a = 2:1, c = list(b1 = 2), b = list(c2 = 3, c1 = 4)))
x |> str()

# Transform each leaf
x |> modify_tree(leaf = \(x) x + 100) |>  str()

# Recursively sort the nodes
sort_named <- function(x) {
  nms <- names(x)
  if (!is.null(nms)) {
    x[order(nms)]
  } else {
    x
   }
}
x |> modify_tree(post = sort_named) |> str()

Negate a predicate function so it selects what it previously rejected

Description

Negating a function changes TRUE to FALSE and FALSE to TRUE.

Usage

negate(.p)
negate(.p)

Arguments

.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). You must use .x to refer to the first argument). Only recommended if you require backward compatibility with older versions of R.

Value

A new predicate function.

Adverbs

This function is called an adverb because it modifies the effect of a function (a verb). If you'd like to include a function created an adverb in a package, be sure to read faq-adverbs-export.

Examples

x <- list(x = 1:10, y = rbernoulli(10), z = letters)
x |> keep(is.numeric) |> names()
x |> keep(negate(is.numeric)) |> names()
# Same as
x |> discard(is.numeric)
x <- list(x = 1:10, y = rbernoulli(10), z = letters)
x |> keep(is.numeric) |> names()
x |> keep(negate(is.numeric)) |> names()
# Same as
x |> discard(is.numeric)

Partially apply a function, filling in some arguments

Description

Partial function application allows you to modify a function by pre-filling some of the arguments. It is particularly useful in conjunction with functionals and other function operators.

Usage

partial(
  .f,
  ...,
  .env = deprecated(),
  .lazy = deprecated(),
  .first = deprecated()
)
partial(
  .f,
  ...,
  .env = deprecated(),
  .lazy = deprecated(),
  .first = deprecated()
)

Arguments

`.f`	a function. For the output source to read well, this should be a named function.
`...`	named arguments to `.f` that should be partially applied. Pass an empty `⁠... = ⁠` argument to specify the position of future arguments relative to partialised ones. See `rlang::call_modify()` to learn more about this syntax. These dots support quasiquotation. If you unquote a value, it is evaluated only once at function creation time. Otherwise, it is evaluated each time the function is called.
`.env`	The environments are now captured via quosures.
`.lazy`	Please unquote the arguments that should be evaluated once at function creation time with `⁠!!⁠`.
`.first`	Please pass an empty argument `⁠... = ⁠` to specify the position of future arguments.

Details

partial() creates a function that takes ... arguments. Unlike compose() and other function operators like negate(), it doesn't reuse the function signature of .f. This is because partial() explicitly supports NSE functions that use substitute() on their arguments. The only way to support those is to forward arguments through dots.

Other unsupported patterns:

It is not possible to call partial() repeatedly on the same argument to pre-fill it with a different expression.
It is not possible to refer to other arguments in pre-filled argument.

Value

A function that takes the same arguments as .f, but returns a different value, as described above.

Adverbs

This function is called an adverb because it modifies the effect of a function (a verb). If you'd like to include a function created an adverb in a package, be sure to read faq-adverbs-export.

Examples

# Partial is designed to replace the use of anonymous functions for
# filling in function arguments. Instead of:
compact1 <- function(x) discard(x, is.null)

# we can write:
compact2 <- partial(discard, .p = is.null)

# partial() works fine with functions that do non-standard
# evaluation
my_long_variable <- 1:10
plot2 <- partial(plot, my_long_variable)
plot2()
plot2(runif(10), type = "l")

# Note that you currently can't partialise arguments multiple times:
my_mean <- partial(mean, na.rm = TRUE)
my_mean <- partial(my_mean, na.rm = FALSE)
try(my_mean(1:10))


# The evaluation of arguments normally occurs "lazily". Concretely,
# this means that arguments are repeatedly evaluated across invocations:
f <- partial(runif, n = rpois(1, 5))
f
f()
f()

# You can unquote an argument to fix it to a particular value.
# Unquoted arguments are evaluated only once when the function is created:
f <- partial(runif, n = !!rpois(1, 5))
f
f()
f()


# By default, partialised arguments are passed before new ones:
my_list <- partial(list, 1, 2)
my_list("foo")

# Control the position of these arguments by passing an empty
# `... = ` argument:
my_list <- partial(list, 1, ... = , 2)
my_list("foo")
# Partial is designed to replace the use of anonymous functions for
# filling in function arguments. Instead of:
compact1 <- function(x) discard(x, is.null)

# we can write:
compact2 <- partial(discard, .p = is.null)

# partial() works fine with functions that do non-standard
# evaluation
my_long_variable <- 1:10
plot2 <- partial(plot, my_long_variable)
plot2()
plot2(runif(10), type = "l")

# Note that you currently can't partialise arguments multiple times:
my_mean <- partial(mean, na.rm = TRUE)
my_mean <- partial(my_mean, na.rm = FALSE)
try(my_mean(1:10))


# The evaluation of arguments normally occurs "lazily". Concretely,
# this means that arguments are repeatedly evaluated across invocations:
f <- partial(runif, n = rpois(1, 5))
f
f()
f()

# You can unquote an argument to fix it to a particular value.
# Unquoted arguments are evaluated only once when the function is created:
f <- partial(runif, n = !!rpois(1, 5))
f
f()
f()


# By default, partialised arguments are passed before new ones:
my_list <- partial(list, 1, 2)
my_list("foo")

# Control the position of these arguments by passing an empty
# `... = ` argument:
my_list <- partial(list, 1, ... = , 2)
my_list("foo")

Safely get or set an element deep within a nested data structure

Description

pluck() implements a generalised form of [[ that allow you to index deeply and flexibly into data structures. It always succeeds, returning .default if the index you are trying to access does not exist or is NULL.

⁠pluck<-()⁠ is the assignment equivalent, allowing you to modify an object deep within a nested data structure.

pluck_exists() tells you whether or not an object exists using the same rules as pluck (i.e. a NULL element is equivalent to an absent element).

Usage

pluck(.x, ..., .default = NULL)

pluck(.x, ...) <- value

pluck_exists(.x, ...)
pluck(.x, ..., .default = NULL)

pluck(.x, ...) <- value

pluck_exists(.x, ...)

Arguments

`.x`, `x`	A vector or environment
`...`	A list of accessors for indexing into the object. Can be an positive integer, a negative integer (to index from the right), a string (to index into names), or an accessor function (except for the assignment variants which only support names and positions). If the object being indexed is an S4 object, accessing it by name will return the corresponding slot. Dynamic dots are supported. In particular, if your accessors are stored in a list, you can splice that in with `⁠!!!⁠`.
`.default`	Value to use if target is `NULL` or absent.
`value`	A value to replace in `.x` at the pluck location. Use `zap()` to instead remove the element.

Details

You can pluck or chuck with standard accessors like integer positions and string names, and also accepts arbitrary accessor functions, i.e. functions that take an object and return some internal piece.

This is often more readable than a mix of operators and accessors because it reads linearly and is free of syntactic cruft. Compare: accessor(x[[1]])$foo to pluck(x, 1, accessor, "foo").
These accessors never partial-match. This is unlike $ which will select the disp object if you write mtcars$di.

Examples

# Let's create a list of data structures:
obj1 <- list("a", list(1, elt = "foo"))
obj2 <- list("b", list(2, elt = "bar"))
x <- list(obj1, obj2)

# pluck() provides a way of retrieving objects from such data
# structures using a combination of numeric positions, vector or
# list names, and accessor functions.

# Numeric positions index into the list by position, just like `[[`:
pluck(x, 1)
# same as x[[1]]

# Index from the back
pluck(x, -1)
# same as x[[2]]

pluck(x, 1, 2)
# same as x[[1]][[2]]

# Supply names to index into named vectors:
pluck(x, 1, 2, "elt")
# same as x[[1]][[2]][["elt"]]

# By default, pluck() consistently returns `NULL` when an element
# does not exist:
pluck(x, 10)
try(x[[10]])

# You can also supply a default value for non-existing elements:
pluck(x, 10, .default = NA)

# The map() functions use pluck() by default to retrieve multiple
# values from a list:
map_chr(x, 1)
map_int(x, c(2, 1))

# pluck() also supports accessor functions:
my_element <- function(x) x[[2]]$elt
pluck(x, 1, my_element)
pluck(x, 2, my_element)

# Even for this simple data structure, this is more readable than
# the alternative form because it requires you to read both from
# right-to-left and from left-to-right in different parts of the
# expression:
my_element(x[[1]])

# If you have a list of accessors, you can splice those in with `!!!`:
idx <- list(1, my_element)
pluck(x, !!!idx)
# Let's create a list of data structures:
obj1 <- list("a", list(1, elt = "foo"))
obj2 <- list("b", list(2, elt = "bar"))
x <- list(obj1, obj2)

# pluck() provides a way of retrieving objects from such data
# structures using a combination of numeric positions, vector or
# list names, and accessor functions.

# Numeric positions index into the list by position, just like `[[`:
pluck(x, 1)
# same as x[[1]]

# Index from the back
pluck(x, -1)
# same as x[[2]]

pluck(x, 1, 2)
# same as x[[1]][[2]]

# Supply names to index into named vectors:
pluck(x, 1, 2, "elt")
# same as x[[1]][[2]][["elt"]]

# By default, pluck() consistently returns `NULL` when an element
# does not exist:
pluck(x, 10)
try(x[[10]])

# You can also supply a default value for non-existing elements:
pluck(x, 10, .default = NA)

# The map() functions use pluck() by default to retrieve multiple
# values from a list:
map_chr(x, 1)
map_int(x, c(2, 1))

# pluck() also supports accessor functions:
my_element <- function(x) x[[2]]$elt
pluck(x, 1, my_element)
pluck(x, 2, my_element)

# Even for this simple data structure, this is more readable than
# the alternative form because it requires you to read both from
# right-to-left and from left-to-right in different parts of the
# expression:
my_element(x[[1]])

# If you have a list of accessors, you can splice those in with `!!!`:
idx <- list(1, my_element)
pluck(x, !!!idx)

Compute the depth of a vector

Description

The depth of a vector is how many levels that you can index/pluck into it. pluck_depth() was previously called vec_depth().

Usage

pluck_depth(x, is_node = NULL)
pluck_depth(x, is_node = NULL)

Arguments

`x`	A vector
`is_node`	Optionally override the default criteria for determine an element can be recursed within. The default matches the behaviour of `pluck()` which can recurse into lists and expressions.

Value

An integer.

Examples

x <- list(
  list(),
  list(list()),
  list(list(list(1)))
)
pluck_depth(x)
x |> map_int(pluck_depth)
x <- list(
  list(),
  list(list()),
  list(list(list(1)))
)
pluck_depth(x)
x |> map_int(pluck_depth)

Map over multiple input simultaneously (in "parallel")

Description

These functions are variants of map() that iterate over multiple arguments simultaneously. They are parallel in the sense that each input is processed in parallel with the others, not in the sense of multicore computing, i.e. they share the same notion of "parallel" as base::pmax() and base::pmin().

Usage

pmap(.l, .f, ..., .progress = FALSE)

pmap_lgl(.l, .f, ..., .progress = FALSE)

pmap_int(.l, .f, ..., .progress = FALSE)

pmap_dbl(.l, .f, ..., .progress = FALSE)

pmap_chr(.l, .f, ..., .progress = FALSE)

pmap_vec(.l, .f, ..., .ptype = NULL, .progress = FALSE)

pwalk(.l, .f, ..., .progress = FALSE)
pmap(.l, .f, ..., .progress = FALSE)

pmap_lgl(.l, .f, ..., .progress = FALSE)

pmap_int(.l, .f, ..., .progress = FALSE)

pmap_dbl(.l, .f, ..., .progress = FALSE)

pmap_chr(.l, .f, ..., .progress = FALSE)

pmap_vec(.l, .f, ..., .ptype = NULL, .progress = FALSE)

pwalk(.l, .f, ..., .progress = FALSE)

Arguments

`.l`	A list of vectors. The length of `.l` determines the number of arguments that `.f` will be called with. Arguments will be supply by position if unnamed, and by name if named. Vectors of length 1 will be recycled to any length; all other elements must be have the same length. A data frame is an important special case of `.l`. It will cause `.f` to be called once for each row.
`.f`	A function, specified in one of the following ways: A named function. An anonymous function, e.g. `⁠\(x, y, z) x + y / z⁠` or `function(x, y, z) x + y / z` A formula, e.g. `~ ..1 + ..2 / ..3`. This syntax is not recommended as you can only refer to arguments by position.
`...`	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.
`.progress`	Whether to show a progress bar. Use `TRUE` to turn on a basic progress bar, use a string to give it a name, or see progress_bars for more details.
`.ptype`	If `NULL`, the default, the output type is the common type of the elements of the result. Otherwise, supply a "prototype" giving the desired type of output.

Value

The output length is determined by the maximum length of all elements of .l. The output names are determined by the names of the first element of .l. The output type is determined by the suffix:

No suffix: a list; .f() can return anything.
⁠_lgl()⁠, ⁠_int()⁠, ⁠_dbl()⁠, ⁠_chr()⁠ return a logical, integer, double, or character vector respectively; .f() must return a compatible atomic vector of length 1.
⁠_vec()⁠ return an atomic or S3 vector, the same type that .f returns. .f can return pretty much any type of vector, as long as it is length 1.
pwalk() returns the input .l (invisibly). This makes it easy to use in a pipe. The return value of .f() is ignored.

Any errors thrown by .f will be wrapped in an error with class purrr_error_indexed.

Examples

x <- list(1, 1, 1)
y <- list(10, 20, 30)
z <- list(100, 200, 300)
pmap(list(x, y, z), sum)

# Matching arguments by position
pmap(list(x, y, z), function(first, second, third) (first + third) * second)

# Matching arguments by name
l <- list(a = x, b = y, c = z)
pmap(l, function(c, b, a) (a + c) * b)

# Vectorizing a function over multiple arguments
df <- data.frame(
  x = c("apple", "banana", "cherry"),
  pattern = c("p", "n", "h"),
  replacement = c("P", "N", "H"),
  stringsAsFactors = FALSE
  )
pmap(df, gsub)
pmap_chr(df, gsub)

# Use `...` to absorb unused components of input list .l
df <- data.frame(
  x = 1:3,
  y = 10:12,
  z = letters[1:3]
)
plus <- function(x, y) x + y
## Not run: 
# this won't work
pmap(df, plus)

## End(Not run)
# but this will
plus2 <- function(x, y, ...) x + y
pmap_dbl(df, plus2)

# The "p" for "parallel" in pmap() is the same as in base::pmin()
# and base::pmax()
df <- data.frame(
  x = c(1, 2, 5),
  y = c(5, 4, 8)
)
# all produce the same result
pmin(df$x, df$y)
map2_dbl(df$x, df$y, min)
pmap_dbl(df, min)
x <- list(1, 1, 1)
y <- list(10, 20, 30)
z <- list(100, 200, 300)
pmap(list(x, y, z), sum)

# Matching arguments by position
pmap(list(x, y, z), function(first, second, third) (first + third) * second)

# Matching arguments by name
l <- list(a = x, b = y, c = z)
pmap(l, function(c, b, a) (a + c) * b)

# Vectorizing a function over multiple arguments
df <- data.frame(
  x = c("apple", "banana", "cherry"),
  pattern = c("p", "n", "h"),
  replacement = c("P", "N", "H"),
  stringsAsFactors = FALSE
  )
pmap(df, gsub)
pmap_chr(df, gsub)

# Use `...` to absorb unused components of input list .l
df <- data.frame(
  x = 1:3,
  y = 10:12,
  z = letters[1:3]
)
plus <- function(x, y) x + y
## Not run: 
# this won't work
pmap(df, plus)

## End(Not run)
# but this will
plus2 <- function(x, y, ...) x + y
pmap_dbl(df, plus2)

# The "p" for "parallel" in pmap() is the same as in base::pmin()
# and base::pmax()
df <- data.frame(
  x = c(1, 2, 5),
  y = c(5, 4, 8)
)
# all produce the same result
pmin(df$x, df$y)
map2_dbl(df$x, df$y, min)
pmap_dbl(df, min)

Wrap a function to return a value instead of an error

Description

Create a modified version of .f that return a default value (otherwise) whenever an error occurs.

Usage

possibly(.f, otherwise = NULL, quiet = TRUE)
possibly(.f, otherwise = NULL, quiet = TRUE)

Arguments

.f

A function to modify, 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. Only recommended if you require backward compatibility with older versions of R.

otherwise

Default value to use when an error occurs.

quiet

Hide errors (TRUE, the default), or display them as they occur?

Value

A function that takes the same arguments as .f, but returns a different value, as described above.

Adverbs

This function is called an adverb because it modifies the effect of a function (a verb). If you'd like to include a function created an adverb in a package, be sure to read faq-adverbs-export.

Examples

# To replace errors with a default value, use possibly().
list("a", 10, 100) |>
  map_dbl(possibly(log, NA_real_))

# The default, NULL, will be discarded with `list_c()`
list("a", 10, 100) |>
  map(possibly(log)) |>
  list_c()
# To replace errors with a default value, use possibly().
list("a", 10, 100) |>
  map_dbl(possibly(log, NA_real_))

# The default, NULL, will be discarded with `list_c()`
list("a", 10, 100) |>
  map(possibly(log)) |>
  list_c()

Progress bars in purrr

Description

purrr's map functions have a .progress argument that you can use to create a progress bar. .progress can be:

FALSE, the default: does not create a progress bar.
TRUE: creates a basic unnamed progress bar.
A string: creates a basic progress bar with the given name.
A named list of progress bar parameters, as described below.

It's good practice to name your progress bars, to make it clear what calculation or process they belong to. We recommend keeping the names under 20 characters, so the whole progress bar fits comfortably even on on narrower displays.

Progress bar parameters

clear: whether to remove the progress bar from the screen after termination. Defaults to TRUE.
format: format string. This overrides the default format string of the progress bar type. It must be given for the custom type. Format strings may contain R expressions to evaluate in braces. They support cli pluralization, and styling and they can contain special progress variables.
format_done: format string for successful termination. By default the same as format.
format_failed: format string for unsuccessful termination. By default the same as format.
name: progress bar name. This is by default the empty string and it is displayed at the beginning of the progress bar.
show_after: numeric scalar. Only show the progress bar after this number of seconds. It overrides the cli.progress_show_after global option.
type: progress bar type. Currently supported types are:
- iterator: the default, a for loop or a mapping function,
- tasks: a (typically small) number of tasks,
- download: download of one file,
- custom: custom type, format must not be NULL for this type. The default display is different for each progress bar type.

Further documentation

purrr's progress bars are powered by cli, so see Introduction to progress bars in cli and Advanced cli progress bars for more details.

Wrap a function to capture side-effects

Description

Create a modified version of .f that captures side-effects along with the return value of the function and returns a list containing the result, output, messages and warnings.

Usage

quietly(.f)
quietly(.f)

Arguments

`.f`	A function to modify, 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`. Only recommended if you require backward compatibility with older versions of R.

Value

A function that takes the same arguments as .f, but returns a different value, as described above.

Adverbs

This function is called an adverb because it modifies the effect of a function (a verb). If you'd like to include a function created an adverb in a package, be sure to read faq-adverbs-export.

Examples

f <- function() {
  print("Hi!")
  message("Hello")
  warning("How are ya?")
  "Gidday"
}
f()

f_quiet <- quietly(f)
str(f_quiet())
f <- function() {
  print("Hi!")
  message("Hello")
  warning("How are ya?")
  "Gidday"
}
f()

f_quiet <- quietly(f)
str(f_quiet())

Create delaying rate settings

Description

These helpers create rate settings that you can pass to insistently() and slowly(). You can also use them in your own functions with rate_sleep().

Usage

rate_delay(pause = 1, max_times = Inf)

rate_backoff(
  pause_base = 1,
  pause_cap = 60,
  pause_min = 1,
  max_times = 3,
  jitter = TRUE
)

is_rate(x)
rate_delay(pause = 1, max_times = Inf)

rate_backoff(
  pause_base = 1,
  pause_cap = 60,
  pause_min = 1,
  max_times = 3,
  jitter = TRUE
)

is_rate(x)

Arguments

`pause`	Delay between attempts in seconds.
`max_times`	Maximum number of requests to attempt.
`pause_base`, `pause_cap`	`rate_backoff()` uses an exponential back-off so that each request waits `pause_base * 2^i` seconds, up to a maximum of `pause_cap` seconds.
`pause_min`	Minimum time to wait in the backoff; generally only necessary if you need pauses less than one second (which may not be kind to the server, use with caution!).
`jitter`	Whether to introduce a random jitter in the waiting time.
`x`	An object to test.

Examples

# A delay rate waits the same amount of time:
rate <- rate_delay(0.02)
for (i in 1:3) rate_sleep(rate, quiet = FALSE)

# A backoff rate waits exponentially longer each time, with random
# jitter by default:
rate <- rate_backoff(pause_base = 0.2, pause_min = 0.005)
for (i in 1:3) rate_sleep(rate, quiet = FALSE)
# A delay rate waits the same amount of time:
rate <- rate_delay(0.02)
for (i in 1:3) rate_sleep(rate, quiet = FALSE)

# A backoff rate waits exponentially longer each time, with random
# jitter by default:
rate <- rate_backoff(pause_base = 0.2, pause_min = 0.005)
for (i in 1:3) rate_sleep(rate, quiet = FALSE)

Reduce a list to a single value by iteratively applying a binary function

Description

reduce() is an operation that combines the elements of a vector into a single value. The combination is driven by .f, a binary function that takes two values and returns a single value: reducing f over 1:3 computes the value f(f(1, 2), 3).

Usage

reduce(.x, .f, ..., .init, .dir = c("forward", "backward"))

reduce2(.x, .y, .f, ..., .init)
reduce(.x, .f, ..., .init, .dir = c("forward", "backward"))

reduce2(.x, .y, .f, ..., .init)

Arguments

`.x`	A list or atomic vector.
`.f`	For `reduce()`, a 2-argument function. The function will be passed the accumulated value as the first argument and the "next" value as the second argument. For `reduce2()`, a 3-argument function. The function will be passed the accumulated value as the first argument, the next value of `.x` as the second argument, and the next value of `.y` as the third argument. The reduction terminates early if `.f` returns a value wrapped in a `done()`.
`...`	Additional arguments passed on to the reduce function. We now generally recommend against using `...` to pass additional (constant) arguments to `.f`. Instead use a shorthand anonymous function: # Instead of x \|> reduce(f, 1, 2, collapse = ",") # do: x \|> reduce(\(x, y) f(x, y, 1, 2, collapse = ",")) This makes it easier to understand which arguments belong to which function and will tend to yield better error messages.
`.init`	If supplied, will be used as the first value to start the accumulation, rather than using `.x[[1]]`. This is useful if you want to ensure that `reduce` returns a correct value when `.x` is empty. If missing, and `.x` is empty, will throw an error.
`.dir`	The direction of reduction as a string, one of `"forward"` (the default) or `"backward"`. See the section about direction below.
`.y`	For `reduce2()` an additional argument that is passed to `.f`. If `init` is not set, `.y` should be 1 element shorter than `.x`.

Direction

Examples

# Reducing `+` computes the sum of a vector while reducing `*`
# computes the product:
1:3 |> reduce(`+`)
1:10 |> reduce(`*`)

# By ignoring the input vector (nxt), you can turn output of one step into
# the input for the next. This code takes 10 steps of a random walk:
reduce(1:10, \(acc, nxt) acc + rnorm(1), .init = 0)

# When the operation is associative, the direction of reduction
# does not matter:
reduce(1:4, `+`)
reduce(1:4, `+`, .dir = "backward")

# However with non-associative operations, the reduced value will
# be different as a function of the direction. For instance,
# `list()` will create left-leaning lists when reducing from the
# right, and right-leaning lists otherwise:
str(reduce(1:4, list))
str(reduce(1:4, list, .dir = "backward"))

# reduce2() takes a ternary function and a second vector that is
# one element smaller than the first vector:
paste2 <- function(x, y, sep = ".") paste(x, y, sep = sep)
letters[1:4] |> reduce(paste2)
letters[1:4] |> reduce2(c("-", ".", "-"), paste2)

x <- list(c(0, 1), c(2, 3), c(4, 5))
y <- list(c(6, 7), c(8, 9))
reduce2(x, y, paste)


# You can shortcircuit a reduction and terminate it early by
# returning a value wrapped in a done(). In the following example
# we return early if the result-so-far, which is passed on the LHS,
# meets a condition:
paste3 <- function(out, input, sep = ".") {
  if (nchar(out) > 4) {
    return(done(out))
  }
  paste(out, input, sep = sep)
}
letters |> reduce(paste3)

# Here the early return branch checks the incoming inputs passed on
# the RHS:
paste4 <- function(out, input, sep = ".") {
  if (input == "j") {
    return(done(out))
  }
  paste(out, input, sep = sep)
}
letters |> reduce(paste4)
# Reducing `+` computes the sum of a vector while reducing `*`
# computes the product:
1:3 |> reduce(`+`)
1:10 |> reduce(`*`)

# By ignoring the input vector (nxt), you can turn output of one step into
# the input for the next. This code takes 10 steps of a random walk:
reduce(1:10, \(acc, nxt) acc + rnorm(1), .init = 0)

# When the operation is associative, the direction of reduction
# does not matter:
reduce(1:4, `+`)
reduce(1:4, `+`, .dir = "backward")

# However with non-associative operations, the reduced value will
# be different as a function of the direction. For instance,
# `list()` will create left-leaning lists when reducing from the
# right, and right-leaning lists otherwise:
str(reduce(1:4, list))
str(reduce(1:4, list, .dir = "backward"))

# reduce2() takes a ternary function and a second vector that is
# one element smaller than the first vector:
paste2 <- function(x, y, sep = ".") paste(x, y, sep = sep)
letters[1:4] |> reduce(paste2)
letters[1:4] |> reduce2(c("-", ".", "-"), paste2)

x <- list(c(0, 1), c(2, 3), c(4, 5))
y <- list(c(6, 7), c(8, 9))
reduce2(x, y, paste)


# You can shortcircuit a reduction and terminate it early by
# returning a value wrapped in a done(). In the following example
# we return early if the result-so-far, which is passed on the LHS,
# meets a condition:
paste3 <- function(out, input, sep = ".") {
  if (nchar(out) > 4) {
    return(done(out))
  }
  paste(out, input, sep = sep)
}
letters |> reduce(paste3)

# Here the early return branch checks the incoming inputs passed on
# the RHS:
paste4 <- function(out, input, sep = ".") {
  if (input == "j") {
    return(done(out))
  }
  paste(out, input, sep = sep)
}
letters |> reduce(paste4)

Wrap a function to capture errors

Description

Creates a modified version of .f that always succeeds. It returns a list with components result and error. If the function succeeds, result contains the returned value and error is NULL. If an error occurred, error is an error object and result is either NULL or otherwise.

Usage

safely(.f, otherwise = NULL, quiet = TRUE)
safely(.f, otherwise = NULL, quiet = TRUE)

Arguments

.f

A function to modify, 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. Only recommended if you require backward compatibility with older versions of R.

otherwise

Default value to use when an error occurs.

quiet

Hide errors (TRUE, the default), or display them as they occur?

Value

A function that takes the same arguments as .f, but returns a different value, as described above.

Adverbs

This function is called an adverb because it modifies the effect of a function (a verb). If you'd like to include a function created an adverb in a package, be sure to read faq-adverbs-export.

Examples

safe_log <- safely(log)
safe_log(10)
safe_log("a")

list("a", 10, 100) |>
  map(safe_log) |>
  transpose()

# This is a bit easier to work with if you supply a default value
# of the same type and use the simplify argument to transpose():
safe_log <- safely(log, otherwise = NA_real_)
list("a", 10, 100) |>
  map(safe_log) |>
  transpose() |>
  simplify_all()
safe_log <- safely(log)
safe_log(10)
safe_log("a")

list("a", 10, 100) |>
  map(safe_log) |>
  transpose()

# This is a bit easier to work with if you supply a default value
# of the same type and use the simplify argument to transpose():
safe_log <- safely(log, otherwise = NA_real_)
list("a", 10, 100) |>
  map(safe_log) |>
  transpose() |>
  simplify_all()

Wrap a function to wait between executions

Description

slowly() takes a function and modifies it to wait a given amount of time between each call.

Usage

slowly(f, rate = rate_delay(), quiet = TRUE)
slowly(f, rate = rate_delay(), quiet = TRUE)

Arguments

f

A function to modify, 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. Only recommended if you require backward compatibility with older versions of R.

rate

A rate object. Defaults to a constant delay.

quiet

Hide errors (TRUE, the default), or display them as they occur?

Value

A function that takes the same arguments as .f, but returns a different value, as described above.

Adverbs

This function is called an adverb because it modifies the effect of a function (a verb). If you'd like to include a function created an adverb in a package, be sure to read faq-adverbs-export.

Examples

# For these example, we first create a custom rate
# with a low waiting time between attempts:
rate <- rate_delay(0.1)

# slowly() causes a function to sleep for a given time between calls:
slow_runif <- slowly(\(x) runif(1), rate = rate, quiet = FALSE)
out <- map(1:5, slow_runif)
# For these example, we first create a custom rate
# with a low waiting time between attempts:
rate <- rate_delay(0.1)

# slowly() causes a function to sleep for a given time between calls:
slow_runif <- slowly(\(x) runif(1), rate = rate, quiet = FALSE)
out <- map(1:5, slow_runif)

Package 'purrr'

Help Index

Accumulate intermediate results of a vector reduction

Description

Usage

Arguments

Value

Life cycle

Direction

See Also

Examples

Coerce array to list

Description

Usage

Arguments

Details

Examples

Convert an object into a mapper function

Description

Usage

Arguments

Examples

Create an attribute getter function

Description

Usage

Arguments

See Also

Examples

Wrap a function so it will automatically browse() on error

Description

Usage

Arguments

Value

Adverbs

See Also

Examples

Get an element deep within a nested data structure, failing if it doesn't exist

Description

Usage

Arguments

See Also

Examples

Compose multiple functions together to create a new function

Description

Usage

Arguments

Value

Adverbs

See Also

Examples

Find the value or position of the first match

Description

Usage

Arguments

Value

See Also

Examples

Do every, some, or none of the elements of a list satisfy a predicate?

Description

Usage

Arguments

Value

Examples

Does a list contain an object?

Description

Usage

Arguments

Examples

Find head/tail that all satisfies a predicate.

Description

Usage

Arguments

Value

Examples

Apply a function to each element of a vector, and its index

Description

Usage

Arguments

Value

See Also

Wrap a function so it will automatically `browse()` on error