On Mon, 21 Jun 2010, Duplisea, Daniel wrote:
Hello. Given this group assignment vector x:
x= rep(letters[1:3],c(9,21,9))
I would like to know the number of all possible unique permutations of x
and also obtain the matrix of them.
Do you mean what would
ncol( unique( replicate( Inf, sample(x) ) , MARGIN=2 ) )
be if you had a computer capable of carrying out the calculation?
Well, the answer would be choose(39,9)*choose(30,9) > 3e+15.
So on earthly computers you have no chance of enumerating the matrix.
Thus, each vector should still
contain 9*a, 21*b and 9*c, but case assigment must be different for one
case (default at least 2) for each permuation.
??
Unless you meant what I suggested above, define 'case'. Better still, give
an example.
a, b and c can be
interspersed in permuted vectors, i.e. order does not matter. If this
unique permutation matrix is very large then I would like to choose a
subset of it. I can easily permute the vector with sample(x) but if I
repeat this n>1 times there is no guarantee of uniqueness.
Do you really need uniqueness?
If this is for some statistical procedure, a small probability of
duplicates will likely not matter. And you can always toss out the
duplicates if you ever encounter them.
For a small
number of samples, I could look for unique vectors and keep only those
but this seems awkward and I suspect there are more elegant ways to do
it.
Is there an r function which will allow me to calculate both how many
unique permutations of x exist and also obtain a matrix of them and/or a
subset of that matrix?
See ?choose for this problem, and ?unique for smaller problems.
HTH,
Chuck
Thanks very much,
Daniel
______________________
Daniel Duplisea
Pêches et Océans / Fisheries and Oceans Canada
Institut Maurice-Lamontagne
Mont-Joli, QC
G5H 3Z4
Canada
tel: 1 418 775 0881
fax: 1 418 775 0740
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Charles C. Berry (858) 534-2098
Dept of Family/Preventive Medicine
E mailto:cbe...@tajo.ucsd.edu UC San Diego
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