[R] Generating permutations that always include one specific element

[Stephan Kolassa]

Hi Ray,

First possibility: just select those combinations that contain "AL":
combos.with.AL <- possible.combos[rowSums(possible.combos=="AL")>0,]

Second possibility: create all 3-combos *without* "AL":
bands.without.AL <- c("B", "DB", "DG", "G", "K", "LB", "LG", "MG", "O", 
"P", "PI", "PK", "PU", "R", "V", "W", "Y")
possible.combos.without.AL <- permutations(17, 3, bands.without.AL)

Then insert "AL" in the first, second, third and fourth position of this 
matrix:
foo <- cbind("AL",possible.combos.without.AL)
combos.with.AL <- 
rbind(foo,foo[,c(2,1,3,4)],foo[,c(2,3,1,4)],foo[,c(2,3,4,1)])

The first one is easier to understand but requires you to first build 
the big object possible combos, most of which you discard. May be a 
problem in larger instances.

HTH,
Stephan


Raymond Danner schrieb:
> Dear R community,
> 
> I am trying to create a matrix of permutations of a vector:
> bands <- c("AL", "B", "DB", "DG", "G", "K", "LB", "LG", "MG", "O", "P",
> "PI", "PK", "PU", "R", "V", "W", "Y")
> 
> Each permutation must be 4 characters long. permutations() from the gtools
> package does this easy enough:
> possible.combos <- permutations(18, 4, bands)
> 
> However, ³AL² must be one of the elements in each permutation.
> Any ideas?
> 
> Thanks in advance,
> Ray 
> 
> 
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> 
> 
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