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

Michael Friendly friendly at yorku.ca
Sat Nov 27 23:56:49 CET 2010

```  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?")
}
}

> 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