On Thu, 11 Dec 2008, Reuben Cummings wrote:
Hi,
This has been asked before but not sufficiently answered from what I
could find. How do you create combinations
with repetitions (multisets) in R?
If I have
set <- array(1:3)
Why wrap 1:3 in array() ??
And I want to choose all combinations of picking 2 numbers, I want to
get a print out like
[,1] [,2]
[1,] 1 1
[2,] 1 2
[3,] 1 3
[4,] 2 2
[5,] 2 3
[6,] 3 3
For small problems (n < 100, say) :
which( lower.tri( diag( n ), diag=TRUE), arr.ind=TRUE )[,2:1]
For larger problems, something like :
foo <- function(n) {
brks <- cumsum( n:1 )
k <- 1:choose( n+1, 2 )
j <- findInterval( k, brks+1 ) + 1
i <- k - ( brks-brks[1] )[ j ]
cbind( j, i ) }
If the number in 'set' are not 1:n, you can do a lookup using the results
from above.
HTH,
Chuck
subsets(set, 2, allow.repeat=T) should work, but I can't get the
multic package to install, t(combn(set,2)) was suggested but it
doesn't produce repetitions; expand.grid(rep(list(1:3), 2)) was also
suggested, but it produces permuations, not combinations.
Additionally, I would like to iterate through each resultant set for
large n (similar to the description for getNextSet {pcalg}). Any
suggestions?
Reuben Cummings
______________________________________________
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.
Charles C. Berry (858) 534-2098
Dept of Family/Preventive Medicine
E mailto:cbe...@tajo.ucsd.edu UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/ La Jolla, San Diego 92093-0901
______________________________________________
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.
