Map

Updated May 19, 2026· Base Functions

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Map() is a base R function that applies a function to multiple arguments in parallel. It is equivalent to mapply() with simplification disabled, meaning it always returns a list regardless of the result shape.

Relationship to mapply

Map() is a thin wrapper around mapply:

Map(f, x, y)
# identical to
mapply(f, x, y, SIMPLIFY = FALSE)

The difference is that Map() never attempts to simplify the result. mapply() tries to collapse the list into a matrix or vector when all results are scalar — Map() skips that step.

Basic Usage

# Apply a function to two vectors element-wise
Map(\(x, y) x + y, 1:3, 10:12)
# [[1]]
# [1] 11
# [[2]]
# [1] 13
# [[3]]
# [1] 15

# Compare to mapply (which simplifies)
mapply(\(x, y) x + y, 1:3, 10:12)
# [1] 11 13 15

Parameter Details

Parameter	Type	Default	Description
`FUN`	function	Required	Function to apply
`...`	vectors/lists	Required	Arguments to iterate over
`MoreArgs`	list	`NULL`	Additional fixed arguments to pass to FUN
`USE.names`	logical	`TRUE`	If TRUE and any argument has names, the result inherits those names
`SIMPLIFY`	logical	`FALSE`	Always FALSE for Map (enforced)

Working With Names

# Names are preserved when USE.names = TRUE (default)
Map(\(name, val) paste0(name, ": ", val),
    c(a = "apple", b = "banana"),
    1:2)
# $a
# [1] "apple: 1"
# $b
# [1] "banana: 2"

Passing Extra Arguments

Use MoreArgs to pass additional fixed arguments:

f <- function(x, y, z) x + y * z

Map(f, 1:3, 10:12, MoreArgs = list(z = 2))
# [[1]]
# [1] 21
# [[2]]
# [1] 25
# [[3]]
# [1] 29

Multiple List Inputs

# Apply function across multiple lists
x <- list(a = 1, b = 2)
y <- list(a = 10, b = 20)
z <- list(a = 100, b = 200)

Map(\(a, b, c) a + b + c, x, y, z)
# $a
# [1] 111
# $b
# [1] 222

When to Use Map vs mapply

Use Map() when you want predictable list output. Use mapply() when you want the result simplified to a vector or matrix — but be aware that simplification can fail in unexpected ways if the return types vary.

# Safe: always list, no surprises
result <- Map(function(x) if (x > 1) "big" else "small", 1:3)
str(result)
# List of 3

# mapply simplifies when possible
result <- mapply(function(x) if (x > 1) "big" else "small", 1:3)
str(result)
#  chr [1:3] "small" "big" "big"