与match.arg（）匹配的数字参数

Question

In match.arg() , why can the choices argument be numeric but the arg argument value cannot? Is the function not supposed to be used to match numeric arguments?

The documentation file for match.arg shows the following, but makes no mention of numeric values

choices a character vector of candidate values

Ok, but it's not in the following example.

f <- function(year){
    match.arg(year, choices = 1995:2005)
}

f(2000)
# Error in match.arg(year, 1995:2005) : 
#  'arg' must be NULL or a character vector

g <- function(year){
   match.arg(as.character(year), choices = 1995:2005)
}

g(2000)
# [1] 2000

So a character string just matched an numeric value...interesting.

My purpose for using this function is to determine if the user-entered year is a member of a large vector of years, which is not necessarily a sequence (some are missing).

Also note that year = NULL won't work for me here, because that would mean the default year would be 1871 in my actual data, which I don't want.

Answer 1

Snippet from match.arg

else if (!is.character(arg)) 
        stop("'arg' must be NULL or a character vector")

with no attempt at coercion to a character string.

From ?match.arg

Matching is done using pmatch , so arg may be abbreviated.

From the ?pmatch Arguments section

x
the values to be matched: converted to a character vector by as.character. Long vectors are supported.

Therefore, with match.arg , a user cannot use a numeric arg but can use a numeric choices vector. It has been my experience that 5 is not the same as "5" .