[R] generating unordered combinations

[Dan Halligan]

That's brilliant - thanks.

On 17 Sep 2009, at 23:36, William Dunlap wrote:

> There is a 1-1 correspondance between your n-sets
> consisting of m possible element types (0 through m-1
> in your example) and the number of n-subsets of a (n+m-1)-set.
> E.g., your example had m=3 and n=3 and subtracting
> 1:3 from each column of combn(3+3-1,3) gives your result:
>
>> t(combn(3+3-1, 3)-(1:3))
>      [,1] [,2] [,3]
> [1,]    0    0    0
> [2,]    0    0    1
> [3,]    0    0    2
> [4,]    0    1    1
> [5,]    0    1    2
> [6,]    0    2    2
> [7,]    1    1    1
> [8,]    1    1    2
> [9,]    1    2    2
> [10,]    2    2    2
>
> Bill Dunlap
> TIBCO Software Inc - Spotfire Division
> wdunlap tibco.com
>
>> -----Original Message-----
>> From: r-help-bounces at r-project.org
>> [mailto:r-help-bounces at r-project.org] On Behalf Of Dan Halligan
>> Sent: Thursday, September 17, 2009 1:31 PM
>> To: r-help at r-project.org
>> Subject: [R] generating unordered combinations
>>
>> Hi,
>>
>> I am trying to generate all unordered combinations of a set of
>> numbers / characters, and I can only find a (very) clumsy way of  
>> doing
>> this using expand.grid.  For example, all unordered combinations of
>> the numbers 0, 1, 2 are:
>> 0, 0, 0
>> 0, 0, 1
>> 0, 0, 2
>> 0, 1, 1
>> 0, 1, 2
>> 0, 2, 2
>> 1, 1, 1
>> 1, 1, 2
>> 1, 2, 2
>> 2, 2, 2
>>
>> (I have not included, for example, 1, 0, 0, since it is equivalent to
>> 0, 0, 1).
>>
>> I have found a way to generate this data.frame using expand.grid as
>> follows:
>>
>> g <- expand.grid(c(0,1,2), c(0,1,2), c(0,1,2))
>> for(i in 1:nrow(g)) {
>> 	g[i,] <- sort(as.character(g[i,]))
>> }
>> o <- order(g$Var1, g$Var2, g$Var3)
>> unique(g[o,]).
>>
>> This is obviously quite clumsy and hard to generalise to a greater
>> number of characters, so I'm keen to find any other solutions.  Can
>> anyone suggest a better (more general, quicker) method?
>>
>> Cheers
>>
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