modify {purrr}

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, ...)

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.

See Also

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

Other modify variants: map_depth(), modify_tree()

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)