R语言 函数参数的局部匹配验证(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.

• 用arg匹配choices中指定的参数（限定了输入参数的范围）

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.

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.)

• 如果match.arg函数只有一个参数arg，choices会被自动设置为arg，即choices = arg。此时，在外部函数调用时，每一个输入的参数都会与choices(其实是在外部函数定义时的arg的一个默认参数（长度可以是1，也可以是大于1的向量）)
  Matching is done using pmatch, so arg may be abbreviated.
• 允许输入前几个字母

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.

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)