[R] How might one write this better?

Ajay Shah ajayshah at mayin.org
Sun Oct 3 13:35:42 CEST 2004

I am trying to simulate the trajectory of the pension assets of one
person. In C-like syntax, it looks like this:

daily.wage.growth = 1.001                 # deterministic
contribution.rate = 0.08                  # deterministic 8%
Wage = 10                                 # initial
Asset = 0                                 # initial
for (10,000 days) {
    Asset += contribution.rate * Wage           # accreting contributions
    Wage *= daily.wage.growth * Wage            # wage growth
    Asset *= draw from a normal distribution    # Asset returns
cat("Terminal asset = ", Asset, "\n")

How can one do this well in R? What I tried so far is to notice that
the wage trajectory is deterministic, it does not change from one run
to the next, and it can be done in one line. The asset returns
trajectory can be obtained using a single call to rnorm(). Both these
can be done nicely using R functions (if you're curious, I can give
you my code). Using these, I efficiently get a vector of contributions
c[] and a vector of returns r[]. But that still leaves the loop:

  Asset <- 0
  for (t in 1:T) {
    Asset <- c[t] + r[t]*Asset

How might one do this better?

I find that using this code, it takes roughly 0.3 seconds per
computation of Asset (on my dinky 500 MHz Celeron). I need to do
50,000 of these every now and then, and it's a pain to have to wait 3
hours. It'll be great if there is some neat R way to rewrite the
little loop above.

Ajay Shah                                                   Consultant
ajayshah at mayin.org                      Department of Economic Affairs
http://www.mayin.org/ajayshah           Ministry of Finance, New Delhi

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