# [R] help about R basic

Sarah Goslee sarah.goslee at gmail.com
Sun Sep 25 14:21:54 CEST 2011

```Hi,

On Sun, Sep 25, 2011 at 5:03 AM, 阮铮 <rz1991 at foxmail.com> wrote:
> This is my first time to ask for help in the R mailing list, so sorry for my misbehavior.
>
>
> The question is actually an example of the apply function embedded in R. Code is here:
>
>> x <- cbind(x1 = 3, x2 = c(4:1, 2:5)) > dimnames(x)[[1]] <- letters[1:8] > x   x1 x2 a  3  4 b  3  3 c  3  2 d  3  1 e  3  2 f  3  3 g  3  4 h  3  5 > cave <- function(x, c1, c2) c(mean(x[c1]), mean(x[c2])) > apply(x,1, cave,  c1="x1", c2=c("x1","x2"))        a b   c d   e f   g h [1,] 3.0 3 3.0 3 3.0 3 3.0 3 [2,] 3.5 3 2.5 2 2.5 3 3.5 4 > x["x1"] [1] NA > mean(x["x1"]) [1] NA > mean(x[c("x1", "x2")]) [1] NA
> What confused me is how do apply work to deal with x[c1] and x[c2] (the output seems unchanged) ? Why can't I call them out the apply function ?

This is rather mangled, but I tried to untangle it. Sending only plain-text
email to the list would help.

For referencing columns by name rather than position, you need to have a
data frame rather than a matrix, so try this:
> class(x)
[1] "matrix"
>
> class(x)
[1] "matrix"
> x <- data.frame(x)
> x["x1"]
x1
a  3
b  3
c  3
d  3
e  3
f  3
g  3
h  3
> mean(x["x1"])
x1
3

apply() is doing something more complex internally that results in cave()
being able to reference by name, but that doesn't help outside of
apply()

Also, note that you need to assign the results of apply to something:
x2 <- apply(x,1, cave,  c1="x1", c2=c("x1","x2"))

Sarah
--
Sarah Goslee
http://www.functionaldiversity.org

```