R: Map over multiple inputs simultaneously.

map2 {purrr} | R Documentation

## Map over multiple inputs simultaneously.

### Description

These functions are variants of `map()` iterate over multiple arguments in parallel. `map2()` and `walk2()` are specialised for the two argument case; `pmap()` and `pwalk()` allow you to provide any number of arguments in a list.

### Usage

    map2(.x, .y, .f, ...)

    map2_lgl(.x, .y, .f, ...)

    map2_int(.x, .y, .f, ...)

    map2_dbl(.x, .y, .f, ...)

    map2_chr(.x, .y, .f, ...)

    map2_dfr(.x, .y, .f, ..., .id = NULL)

    map2_dfc(.x, .y, .f, ...)

    walk2(.x, .y, .f, ...)

    pmap(.l, .f, ...)

    pmap_lgl(.l, .f, ...)

    pmap_int(.l, .f, ...)

    pmap_dbl(.l, .f, ...)

    pmap_chr(.l, .f, ...)

    pmap_dfr(.l, .f, ..., .id = NULL)

    pmap_dfc(.l, .f, ...)

    pwalk(.l, .f, ...)

### Arguments

`.x, .y` | Vectors of the same length. A vector of length 1 will be recycled.
`.f` | A function, formula, or atomic vector. 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. Within a list, wrap strings in `get-attr()` to extract named attributes. If a component is not present, the value of `.default` will be returned.
`...` | Additional arguments passed on to `.f`.
`.id` | If not `NULL` a variable with this name will be created giving either the name or the index of the data frame.
`.l` | A list of lists. The length of `.l` determines the number of arguments that `.f` will be called with. List names will be used if present.

### Details

Note that arguments to be vectorised over come before the `.f`, and arguments that are supplied to every call come after `.f`.

### Value

An atomic vector, list, or data frame, depending on the suffix. Atomic vectors and lists will be named if `.x` or the first element of `.l` is named.

If all input is length 0, the output will be length 0. If any input is length 1, it will be recycled to the length of the longest.

### See Also

Other map variants: `imap`, `invoke`, `lmap`, `map`, `modify`

### Examples

    x <- list(1, 10, 100)
    y <- list(1, 2, 3)
    z <- list(5, 50, 500)

    map2(x, y, ~ .x + .y)
    # Or just
    map2(x, y, `+`)

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

    pmap(list(x, y, z), sum)

    # Matching arguments by position
    pmap(list(x, y, z), function(a, b ,c) a / (b + c))

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

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

    ## Use `...` to absorb unused components of input list .l
    df <- data.frame(
      x = 1:3 + 0.1,
      y = 3:1 - 0.1,
      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)

[Package purrr version 0.2.4 Index]