R help - Oct 2012 - why sd() can be applied to character vector?

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

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

On 27/10/2012 10:16, Jinsong Zhao 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.
Well, it is there:

        x: a numeric vector or an R object which is coercible to one by
           ?as.vector?.

 > as.vector(x, 'numeric')
  [1]  1  2  3  4  5  6  7  8  9 10

And it is different from mean() because that is generic and allows many 
other forms of input, whereas for sd() one knows what to coerce to.
>
>  > 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
>
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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