[R] question on mean, sum

[Jim Lemon]

Hi mokuram,

As others have noted, you will profit from a bit more knowledge about
"extraction":

sum(mtcars)
[1] 13942.2

This works because you have "extracted" the first column of the
"mtcars" data frame _as a data frame_

mtcars[1]
                    mpg
Mazda RX4           21.0
Mazda RX4 Wag       21.0
...
Volvo 142E          21.4

is.data.frame(mtcars[1])
[1] TRUE

mean(mtcars)
[1] NA
Warning message:
In mean.default(mtcars) :
argument is not numeric or logical: returning NA

Here you are trying to take the mean of the entire data frame. While
the mean function will "coerce" a single column to a vector, it won't
do so for an entire data frame. However, if you coerce the data frame
to a matrix:

mean(as.matrix(mtcars))
[1] 39.60853

That's the good news. The bad news is that the result is meaningless.
The same thing goes for:

sd(as.matrix(mtcars))
[1] 84.20792

It is almost always a good idea to read the error messages carefully
and try to understand them.

Jim


On Mon, Nov 14, 2016 at 2:01 PM,  <mokuram at sina.com> wrote:
>  Hi,
> I am working on functions such as sum(), mean() ...
>> sum(mtcars)[1] 13942.2> mean(mtcars)[1] NAWarning message:In mean.default(mtcars) : NA> sd(mtcars)Error in is.data.frame(x) : ()'double'
> why got different reply?Is this a BUG for the current version of R?my version info:version.string R version 3.3.1 (2016-06-21)
>
> Thank you very much for the help.
> mokuram
>
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