[R] return vector of element names for vector, matrix or array

[Joshua Wiley]

Hi Dr. Friendly,

On Sat, Nov 27, 2010 at 2:56 PM, Michael Friendly <friendly at yorku.ca> wrote:
>  Just as as.vector() takes a vector, matrix or array and returns a vector in
> row-major order,
> I'd like to write a function to take such an object and return the dimension
> names,
> pasted with some separator, as a similar vector.
>
> Here is something ugly cobbled together to demonstrate what I want: a
> function to work
> for any number of dimensions.
>
> vecnames <- function(x, sep=':') {
>    if (is.vector(x) || length(dim(x))==1) return(names(x))
>    else if (length(dim(x)==2)) {
>      snames <- dimnames(x)
>        return (as.vector(outer(snames[[1]], snames[[2]], paste, sep=sep)))
>        }
>    else {
>      stop("How to handle 3+ dimensions?")
>        }
>    }

Interesting use of outer, but I think you're overthinking it ;)

expand.grid(dimnames(x3))

If you want it as a vector separated by colons...

apply(expand.grid(dimnames(x3)), 1, paste, collapse = ":")

Cheers,

Josh

>
>> x1 <- c(a=1, b=2, c=3)
>> vecnames(x1)
> [1] "a" "b" "c"
>>
>> x2 <- matrix(rpois(2 * 3, 7), ncol = 3, nrow = 2)
>> dimnames(x2) <- list(R=letters[1:2], C=LETTERS[1:3])
>> vecnames(x2)
> [1] "a:A" "b:A" "a:B" "b:B" "a:C" "b:C"
>>
>> x3 <- array(rpois(2 * 3 * 2, 7), dim=c(2,3,2))
>> dimnames(x3)=list(R=letters[1:2], C=LETTERS[1:3], L=tail(letters,2))
>>
>> # wanted for this case:
>> as.vector(outer(vecnames(x2), dimnames(x3)[[3]], paste, sep=':'))
>  [1] "a:A:y" "b:A:y" "a:B:y" "b:B:y" "a:C:y" "b:C:y" "a:A:z" "b:A:z" "a:B:z"
> "b:B:z" "a:C:z" "b:C:z"
>>
>
> --
> Michael Friendly     Email: friendly AT yorku DOT ca
> Professor, Psychology Dept.
> York University      Voice: 416 736-5115 x66249 Fax: 416 736-5814
> 4700 Keele Street    Web:   http://www.datavis.ca
> Toronto, ONT  M3J 1P3 CANADA
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.


-- 
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://www.joshuawiley.com/