Hi,
R seems to have a feature that isn't used much, which I don't really
now how to call. But, the dimnames function, can in addition to giving
names to rows/columns/dim 3 rows/dim 4 rows... can also give labels to
the dimensions themselves.
Thus, I can do:
A = matrix(1:9,3,3)
dimnames(A) = list(from=c(), to=c() )
and now, printing a prints these dimension labels nicely:
> A
to
from [,1] [,2] [,3]
[1,] 1 4 7
[2,] 2 5 8
[3,] 3 6 9
I don't know exactly what these labels are called, but they can be
handy sometimes.
For example, they could be handy when using aperm. Instead of writing
aperm(A, c(2,1) )
It would be nice to be able to write:
aperm(A, c("from","to") )
This way, I don't have to remember what is the order of the dims of A.
It is easy to extend the functionality of aperm, so that the above
code works:
---
my.aperm=function( A, d , ...) {
n=names(dimnames(A))
if( is.null(n) ) {
return( aperm(A,d, ...))
}
dimnum = seq(along=n)
names(dimnum) = n
return( aperm(A, dimnum[d], ... ))
}
--
It seems some functions support these dim labels (print, for example),
and some don't (for example, matrix multiplication doesn't, aperm
doesn't, apply doesn't).
Another cool way to use these, would be labeled arguments in array
subsetting:
> a[to=1:2,from=2:3]
to
from [,1] [,2]
[1,] 4 7
[2,] 5 8
> a[from=1:2,to=2:3]
to
from [,1] [,2]
[1,] 4 7
[2,] 5 8
No, doesn't seem to work.
My questions are:
1. Do these have names, so that I can search the documentation for them?
2. How are they usually used?
thanks,
Michael
______________________________________________
R-help@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.
