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
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David Winsemius, MD
Heritage Laboratories
West Hartford, CT
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