[R] random walk w/ reflecting boundary: avoid control construct?

[Charles C. Berry]

On Wed, 24 Oct 2007, Johannes Hüsing wrote:

> Dear expeRts,
> recently I asked for a nice way to re-program a problem
> without using control constructs such as "for" or
> "sapply(1:length(x), ...". Is there a way to program
> a random walk with a reflecting boundary without resorting
> to such constructs? A working solution is
>
> ranwalk <- function(length, bound) {
>    k <- cumsum(sample(c(-1, 1), length, replace=TRUE))
>    while( any(abs(k) > bound) ) {
>        ri <- min(which(abs(k) > bound))
>        k[ri:length] <- k[ri:length] - 2 * sign(k[ri])
>    }
>    k
> }
>
> but it uses "while" and has the same expression in the
> "while" statement and the following line. Is there
> a sensible way to reprogram it using the "whole object"
> approach?

Does this satisfy your 'sensible' sensibility?

ranwalk2 <-
  function(length., bound) {
       k <- cumsum(sample(c(-1, 1), length., replace=TRUE))
       lk <- rep( c( 0:bound, (bound-1):(-bound), (1-bound):(-1)),
 			length=max(abs(k))+1)
       sign(k)*lk[ 1 + abs(k) ]
}

HTH,

  Chuck

>
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Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	            UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901