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中指定的參數（限定了輸入參數的范圍）
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函數只有一個參數arg，choices會被自動設置為arg，即choices = arg。此時，在外部函數調用時，每一個輸入的參數都會與choices(其實是在外部函數定義時的arg的一個默認參數（長度可以是1，也可以是大於1的向量）)
Matching is done using pmatch, so arg may be abbreviated. - 允許輸入前幾個字母
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)