match()

match(x, table, nomatch = NA_integer_, incomparables = FALSE)

Returns: integer · Updated March 13, 2026 · Base Functions

base matching vectors

match() returns the position of the first match between elements of x in table.

Syntax

match(x, table, nomatch = NA_integer_, incomparables = FALSE)

Parameters

Parameter	Type	Default	Description
`x`	vector	—	The values to look up
`table`	vector	—	The values to match against
`nomatch`	integer	`NA_integer_`	Value to return when no match is found
`incomparables`	vector	`FALSE`	Values in `x` that cannot be matched

Examples

Basic usage

x <- c("a", "b", "c", "d")
table <- c("b", "c", "e")

match(x, table)
# [1] NA  1  2 NA

Using nomatch parameter

match(c("apple", "cherry"), c("apple", "banana"), nomatch = 0)
# [1] 1 0

Finding unique or duplicate values

new_values <- c("apple", "banana", "cherry", "apple")
seen <- c("apple", "banana")

# Get indices of matches
idx <- match(new_values, seen)
idx
# [1]  1  2 NA  1

Practical data cleaning

df <- data.frame(
  category = c("A", "B", "C", "D", "A"),
  value = 1:5
)

allowed <- c("A", "B")
idx <- match(df$category, allowed)
idx
# [1]  1  2 NA NA  1

Common Patterns

Deduplication: Identify which values in one vector appear in another:

existing_ids <- c(101, 102, 103)
new_ids <- c(101, 104, 105, 103)

# Which new IDs already exist?
!is.na(match(existing_ids, new_ids))
# [1]  TRUE FALSE FALSE

# Get only the new IDs
setdiff(new_ids, existing_ids)
# [1] 104 105