R help - Mar 2009 - code to find all distinct subsets of size r from a set of size n

I'm doing a permutation test and need to efficiently generate all
distinct subsets of size r from a set of size n.  P 138 of MASS (4th
ed) notes that  "The code to generate this efficiently is in the
scripts".  I was unable to find this code on quick inspection of the
\library\MASS\scripts file for Chapter 5 and 'subsets' is not a
function in MASS.

I did find this problem is discussed in RNews, Programmer's Niche
1(1):27 - 30 and RNews, 1(2):29-31.  Is there function in 'base'  R or
other package to do this?  Thanks.  --Dale

For permutations a couple of options are 'permutations' in package
gtools, and 'urnsamples' in package prob

hth,
Kingsford Jones

On Fri, Mar 13, 2009 at 6:35 AM, Dale Steele <dale.w.steele at gmail.com>
wrote:
> I'm doing a permutation test and need to efficiently generate all
> distinct subsets of size r from a set of size n. ?P 138 of MASS (4th
> ed) notes that ?"The code to generate this efficiently is in the
> scripts". ?I was unable to find this code on quick inspection of the
> \library\MASS\scripts file for Chapter 5 and 'subsets' is not a
> function in MASS.
>
> I did find this problem is discussed in RNews, Programmer's Niche
> 1(1):27 - 30 and RNews, 1(2):29-31. ?Is there function in 'base' ?R
or
> other package to do this? ?Thanks. ?--Dale
>
> ______________________________________________
> 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.
>

If your subsets are to be taken from the rows of a dataframe, df, and  
the size is r, then something like this could satisfy:

apply(combn(nrow(df), r), 2, function(x) df[x, ])

(Although these are not really permutations as I understand that term.)

-- 
David Winsemius

On Mar 13, 2009, at 8:35 AM, Dale Steele wrote:
> I'm doing a permutation test and need to efficiently generate all
> distinct subsets of size r from a set of size n.  P 138 of MASS (4th
> ed) notes that  "The code to generate this efficiently is in the
> scripts".  I was unable to find this code on quick inspection of the
> \library\MASS\scripts file for Chapter 5 and 'subsets' is not a
> function in MASS.
>
> I did find this problem is discussed in RNews, Programmer's Niche
> 1(1):27 - 30 and RNews, 1(2):29-31.  Is there function in 'base'  R
or
> other package to do this?  Thanks.  --Dale
>
> ______________________________________________
> 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.
David Winsemius, MD
Heritage Laboratories
West Hartford, CT