base (version 3.3)

match.arg: Argument Verification Using Partial Matching

Description

match.arg matches arg against a table of candidate values as specified by choices, where NULL means to take the first one.

Usage

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

Arguments

arg

a character vector (of length one unless several.ok is TRUE) or NULL.

choices

a character vector of candidate values

several.ok

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

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 more than one element of arg has a match, all unabbreviated versions of matches are returned.

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.

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 match:
match.arg(c("gauss", "rect", "ep"),
          c("gaussian", "epanechnikov", "rectangular", "triangular"),
          several.ok = TRUE)