match.arg {base}R Documentation

Argument Verification Using Partial Matching


match.arg matches a character arg against a table of candidate values as specified by choices.


match.arg(arg, choices, several.ok = FALSE)



a character vector (of length one unless several.ok is TRUE) or NULL which means to take choices[1].


a character vector of candidate values, often missing, see ‘Details’.


logical specifying if arg should be allowed to have more than one element.


In the one-argument form match.arg(arg), the choices are obtained from a default setting for the formal argument arg of the function from which match.arg was called. (Since default argument matching will set arg to choices, this is allowed as an exception to the ‘length one unless several.ok is TRUE’ rule, and returns the first element.)

Matching is done using pmatch, so arg may be abbreviated and the empty string ("") never matches, not even itself, see pmatch.


The unabbreviated version of the exact or unique partial match if there is one; otherwise, an error is signalled if several.ok is false, as per default. When several.ok is true and (at least) one element of arg has a match, all unabbreviated versions of matches are returned.


The error messages given are liable to change and did so in R 4.2.0. Do not test them in packages.

See Also



## Extends the example for 'switch'
center <- function(x, type = c("mean", "median", "trimmed")) {
  type <- match.arg(type)
         mean = mean(x),
         median = median(x),
         trimmed = mean(x, trim = .1))
x <- rcauchy(10)
center(x, "t")       # Works
center(x, "med")     # Works
try(center(x, "m"))  # Error
stopifnot(identical(center(x),       center(x, "mean")),
          identical(center(x, NULL), center(x, "mean")) )

## Allowing more than one 'arg' and hence more than one match:
match.arg(c("gauss", "rect", "ep"),
          c("gaussian", "epanechnikov", "rectangular", "triangular"),
          several.ok = TRUE)
match.arg(c("a", ""),  c("", NA, "bb", "abc"), several.ok=TRUE) # |-->  "abc"

[Package base version 4.2.1 Index]