Re: [R] Generating groupings of ordered observations

From: Gavin Simpson <gavin.simpson_at_ucl.ac.uk>
Date: Sat, 21 Jun 2008 23:19:44 +0100

On Sat, 2008-06-21 at 17:56 -0400, Gabor Grothendieck wrote:
> On Sat, Jun 21, 2008 at 12:40 PM, Gavin Simpson <gavin.simpson@ucl.ac.uk> wrote:
> > Dear List,
> >
> > I have a problem I'm finding it difficult to make headway with.
> >
> > Say I have 6 ordered observations, and I want to find all combinations
> > of splitting these 6 ordered observations in g groups, where g = 1, ...,
> > 6. Groups can only be formed by adjacent observations, so observations 1
> > and 4 can't be in a group on their own, only if 1,2,3&4 are all in the
> > group.
> >
<snip />
> >
> > I'd like to be able to do this automagically, for any (reasonable,
> > small, say n = 10-20) number of observations, n, and for g = 1, ..., n
> > groups.
> >
> > I can't see the pattern here or a way forward. Can anyone suggest an
> > approach?
> >
>
> Peter Wolf has APL-style encode/decode functions on his web site that
> can readily be used for this. The output of the encode below are the binary
> digits expansions of the numbers 0:15, one per column, and the remainder
> transforms that matrix to the required one (but columns are in a different
> order than yours):

Thanks for this Gabor. Will take a look.

One additional complication that I failed to mention, is that I would like to state the number of groups. So in my example, instead of splitting the 6 observations into 1,2,...,6 groups, I would like to get only the sets that partition the 6 observations into say 4 groups.

For larger problems, where n is > 100 it is not possible to do the required calculations on the choose(n-1, 0:(n-1)) possible combinations. Instead we would evaluate the combinations of the n observations relating to a small number of groups, say g = 1:10 where n = 100 for example.

So Chuck and your solutions answer the question I asked, but I can't see how to modify them to set the number of groups to be less than the number of observations.

All the best,

G

>
> > source("http://www.wiwi.uni-bielefeld.de/~wolf/software/R-wtools/decodeencode/decodeencode.R")
> > n <- 6
> > n1 <- n-1
> > apply(rbind(1, encode(0:(2^n1-1), rep(2, n1))), 2, cumsum)
> [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10] [,11] [,12]
> [,13] [,14] [,15] [,16] [,17] [,18] [,19] [,20] [,21] [,22] [,23]
> [,24] [,25] [,26]
> [1,] 1 1 1 1 1 1 1 1 1 1 1 1
> 1 1 1 1 1 1 1 1 1 1 1 1
> 1 1
> [2,] 1 1 1 1 1 1 1 1 1 1 1 1
> 1 1 1 1 2 2 2 2 2 2 2 2
> 2 2
> [3,] 1 1 1 1 1 1 1 1 2 2 2 2
> 2 2 2 2 2 2 2 2 2 2 2 2
> 3 3
> [4,] 1 1 1 1 2 2 2 2 2 2 2 2
> 3 3 3 3 2 2 2 2 3 3 3 3
> 3 3
> [5,] 1 1 2 2 2 2 3 3 2 2 3 3
> 3 3 4 4 2 2 3 3 3 3 4 4
> 3 3
> [6,] 1 2 2 3 2 3 3 4 2 3 3 4
> 3 4 4 5 2 3 3 4 3 4 4 5
> 3 4
> [,27] [,28] [,29] [,30] [,31] [,32]
> [1,] 1 1 1 1 1 1
> [2,] 2 2 2 2 2 2
> [3,] 3 3 3 3 3 3
> [4,] 3 3 4 4 4 4
> [5,] 4 4 4 4 5 5
> [6,] 4 5 4 5 5 6

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 Dr. Gavin Simpson             [t] +44 (0)20 7679 0522
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Received on Sat 21 Jun 2008 - 22:22:30 GMT

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