vectorization & modifying globals in functions

I have the following code:

--8<---------------cut here---------------start------------->8---
d <- rep(10,10)
for (i in 1:100) {
  a <- sample.int(length(d), size = 2)
  if (d[a[1]] >= 1) {
    d[a[1]] <- d[a[1]] - 1
    d[a[2]] <- d[a[2]] + 1
  }
}
--8<---------------cut here---------------end--------------->8---

it does what I want, i.e., modified vector d 100 times.

Now, if I want to repeat this 1e6 times instead of 1e2 times, I want to
vectorize it for speed, so I do this:

--8<---------------cut here---------------start------------->8---
update <- function (i) {
  a <- sample.int(n.agents, size = 2)
  if (d[a[1]] >= delta) {
    d[a[1]] <- d[a[1]] - 1
    d[a[2]] <- d[a[2]] + 1
  }
  entropy(d, unit="log2")
}
system.time(entropy.history <- sapply(1:1e6,update))
--8<---------------cut here---------------end--------------->8---

however, the global d is not modified, apparently update modifies the
local copy.

so,
1. is there a way for a function to modify a global variable?
2. how would you vectorize this loop?

thanks!

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Sam Steingold (http://sds.podval.org/) on Ubuntu 12.04 (precise) X 11.0.11103000
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