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

[Jinsong Zhao]

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

Regards,
Jinsong