Argument Verification Using Partial Matching

Description

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

Usage

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

Arguments

Details

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.

Value

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.

Warning

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

See Also

pmatch, match.fun, match.call.

Examples

require(stats)
## Extends the example for 'switch'
center <- function(x, type = c("mean", "median", "trimmed")) {
  type <- match.arg(type)
  switch(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"