map2.Rd

% Generated by roxygen2: do not edit by hand
% Please edit documentation in R/map2-pmap.R
\name{map2}
\alias{map2}
\alias{map2_lgl}
\alias{map2_int}
\alias{map2_dbl}
\alias{map2_chr}
\alias{map2_raw}
\alias{map2_dfr}
\alias{map2_dfc}
\alias{map2_df}
\alias{walk2}
\alias{pmap}
\alias{pmap_lgl}
\alias{pmap_int}
\alias{pmap_dbl}
\alias{pmap_chr}
\alias{pmap_raw}
\alias{pmap_dfr}
\alias{pmap_dfc}
\alias{pmap_df}
\alias{pwalk}
\title{Map over multiple inputs simultaneously.}
\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_raw(.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_raw(.l, .f, ...)

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

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

pwalk(.l, .f, ...)
}
\arguments{
\item{.x, .y}{Vectors of the same length. A vector of length 1 will
be recycled.}

\item{.f}{A function, formula, or vector (not necessarily atomic).

If a \strong{function}, it is used as is.

If a \strong{formula}, e.g. \code{~ .x + 2}, it is converted to a function. There
are three ways to refer to the arguments:
\itemize{
\item For a single argument function, use \code{.}
\item For a two argument function, use \code{.x} and \code{.y}
\item For more arguments, use \code{..1}, \code{..2}, \code{..3} etc
}

This syntax allows you to create very compact anonymous functions.

If \strong{character vector}, \strong{numeric vector}, or \strong{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 \code{\link[=get-attr]{get-attr()}}
to extract named attributes. If a component is not present, the value of
\code{.default} will be returned.}

\item{...}{Additional arguments passed on to \code{.f}.}

\item{.id}{Either a string or \code{NULL}. If a string, the output will contain
a variable with that name, storing either the name (if \code{.x} is named) or
the index (if \code{.x} is unnamed) of the input. If \code{NULL}, the default, no
variable will be created.

Only applies to \code{_dfr} variant.}

\item{.l}{A list of vectors, such as a data frame. The length of \code{.l}
determines the number of arguments that \code{.f} will be called with. List
names will be used if present.}
}
\value{
An atomic vector, list, or data frame, depending on the suffix.
Atomic vectors and lists will be named if \code{.x} or the first
element of \code{.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.
}
\description{
These functions are variants of \code{\link[=map]{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. They
share the same notion of "parallel" as \code{\link[base:pmax]{base::pmax()}} and \code{\link[base:pmin]{base::pmin()}}.
\code{map2()} and \code{walk2()} are specialised for the two argument case; \code{pmap()}
and \code{pwalk()} allow you to provide any number of arguments in a list. Note
that a data frame is a very important special case, in which case \code{pmap()}
and \code{pwalk()} apply the function \code{.f} to each row.
}
\details{
Note that arguments to be vectorised over come before \code{.f},
and arguments that are supplied to every call come after \code{.f}.
}
\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, `+`)

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))

# 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)

# 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
\dontrun{
# this won't work
pmap(df, plus)
}
# 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)
}
\seealso{
Other map variants: \code{\link{imap}},
  \code{\link{invoke}}, \code{\link{lmap}},
  \code{\link{map}}, \code{\link{modify}}
}