purrr: map_if – R documentation

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map_if

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.

map_depth() allows to apply .f to a specific depth level of a nested vector.

Usage

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

map_at(.x, .at, .f, ...)

map_depth(.x, .depth, .f, ..., .ragged = 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, 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. 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 the mapped function.
`.else`	A function applied to elements of `.x` for which `.p` returns `FALSE`.
`.at`	A character vector of names, positive numeric vector of positions to include, or a negative numeric vector of positions to exlude. Only those elements corresponding to `.at` will be modified. If the `tidyselect` package is installed, you can use `vars()` and the `tidyselect` helpers to select elements.
`.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, ~ map(., fun))`
`.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`.

Examples

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

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

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

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

# 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))
str(x)
map_depth(x, 2, paste, collapse = "/")

# Equivalent to:
map(x, map, paste, collapse = "/")

purrr

Functional Programming Tools

v0.3.4

GPL-3 | file LICENSE

Authors

Lionel Henry [aut, cre], Hadley Wickham [aut], RStudio [cph, fnd]

Initial release