[R] A random insertion position in a vector

[Douglas Bates]

A student asked me about simulation he is doing.  This simulation
presented an interesting R programming puzzle that we did solve but I
am not happy with the solution.  I feel there should be a better way
of doing this.

Briefly, he is simulating births and deaths in a linear array of cells.
Births and deaths are required to alternate.

For a birth, one of the cells is selected at random to be the parent.
The offspring cell is generated according to some rules based on the
type of the parent then inserted into the array either to the left or
to the right of the parent.  The left/right choice is random.

The part that took us the longest time was deciding how to insert the
offspring cell into the array in a way that doesn't break down at the
boundaries.

I ended up using code based on an argument 'pos', which is the number of
elements from the original array that will be to the left of the new
element.  Another way of thinking of pos is as one less than the index
of the new element in the result.

insert <-
    function(orig = 1:10, new = 0, pos = sample(0:length(orig), 1))
{
    ## Insert a new element at a random position in a vector.
    ## The default values of the arguments were for testing and do not
    ## reflect the process descibed above.
    if (pos < 1) return(c(new, orig))
    left <- 1:pos
    c(orig[left], new, orig[-left])
}

iterate <-
    function(current)
{
    ## Update the array of cells with a birth followed by a death
    parent <- sample(seq(along = current), 1)  # index of the parent
    pos <- parent - sample(c(0,1), 1)          
    newcell <- ## generate the new cell based on the type of the parent
    newvec <- insert(current, newcell, pos)
    newvec[-sample(seq(along = newvec), 1)]
}

Can anyone come up with a cleaner way of doing the insertion or
perhaps the complete birth/death step?
-- 
Douglas Bates                            bates at stat.wisc.edu
Statistics Department                    608/262-2598
University of Wisconsin - Madison        http://www.stat.wisc.edu/~bates/
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