Fixed size permutations

Nick Fankhauser wrote:

I'm using functions to return a matrix of all permutations of a
specified size from a larger list for predictor selection.
For each predictor size I use a seperate function like this:

bag2 <- function(n) {
    rl <- c()
    for (i1 in seq(n)) {
        for (i2 in seq(n)) {
            if (length(unique(c(i1,i2)))==1) {next}
            rl <- cbind(rl,matrix(c(i1,i2)))
        }
    }
    rl
}

bag3 <- function(n) {
    rl <- c()
    for (i1 in seq(n)) {
        for (i2 in seq(n)) {
            for (i3 in seq(n)) {
                if (length(unique(c(i1,i2,i3)))==1) {next}
                rl <- cbind(rl,matrix(c(i1,i2,i3)))
            }
        }
    }
    rl
}

But I think it should be somehow possible in R to use one general
function for all sizes. Can someone help?
I don't exactly know how this kind of permutation is called, maybe this
would help to find a solution.

Nick

Looks to me like you're just doing expand.grid, but without
the 'all variables equal' cases. If that's correct and if
n > 1, then this might do it:

bag <- function(n,K){
  L <- vector(mode='list', length=K)
  L <- lapply(L, function(x) {x <- seq_len(n)})
  d <- expand.grid(L)
  keeprow <- apply(d, 1, function(x) {var(x)!=0})
    ## I'm blanking out on a better way to test for equal values

  td <- t(as.matrix(d[keeprow,]))
  dimnames(td) <- c(NULL, NULL)
  rtd <- td[nrow(td):1,]
  rtd
}
all.equal(bag(5,2), bag2(5))
#[1] TRUE

all.equal(bag(9,3), bag3(9))
#[1] TRUE

 -Peter Ehlers