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Dear Larisa,
Dear All,
I have a seemingly easy question. I need to create a
matrix consisting of all possible permutations of the following
vector:
0.5 1.0 1.5 2.0
3.0 4.0 5.0 6.0 8.0 10.0 12.0 16.0
I know that "sample" function with (replace=F) will
generate a permutation, however, I need all possible permutations.
[WNV] You do realise that there are 12! = 479 001 600
permutations in all. That's going to make a matrix with 12! x 12 = 5
748 019 200 entries in it, so I hope you did get that extra few Gb's of
RAM...
If
you have a more realistic case you can do this with a very simple
function. For example:
> permutations
<- function(x)
if (length(x) <= 1) as.matrix(x) else
{
M <- NULL
for(i in
1:length(x))
M <- rbind(M,
cbind(x[i], Recall(x[-i])))
M
}
[WNV] As a
check:
> permutations(1)
[,1]
[1,] 1
>
permutations(1:2)
[,1]
[,2]
[1,] 1 2
[2,]
2 1
> permutations(1:3)
[,1] [,2] [,3]
[1,] 1
2 3
[2,] 1
3 2
[3,] 2
1 3
[4,] 2
3 1
[5,] 3
1 2
[6,] 3
2 1
[WNV] Writing code that
will generate all possible permutations sequentially is more difficult,
of course, but not impossible.
Bill
Venables.
Thank you for your help.
Larisa
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