[R] why sd() can be applied to character vector?

[R. Michael Weylandt]

On Sat, Oct 27, 2012 at 10:16 AM, Jinsong Zhao <jszhao at yeah.net> wrote:
> Hi there,
>
> In the following example, sd() can be applied to a character vector.
> However, mean() can not be run in a similar way. Why?
>
> I have read sd() man page, however, I don't find information about that
> behavior.
>
>> x <- as.character(1:10)
>> sd(x)
> [1] 3.02765
>> mean(x)
> [1] NA
> Warning message:
> In mean.default(x) : argument is not numeric or logical: returning NA

Practically, I see

$R_HOME/src/library/stats/src/cov.c:633 has

x = PROTECT(coerceVector(x, REALSXP));

which seems to be responsible for the behavior you note (eventually
calling $R_HOME/src/main/util.c:1463), but mean.default catches the
character case early and returns NA_real_.

None of that explains the reasoning however and it does seem a little anomalous

Cheers,
Michael