Data manipulation in columns (with apply?)

x <- "NUM sim N

a <- read.table(textConnection(x), header=T)
lambda <- by(a, a$NUM, function(x) x$N[-1] / x$N[-length(x$N)])
lambda

# sum of lambdas
sapply(lambda, sum)

# mean
sapply(lambda, mean)

# sd
sapply(lambda, sd)

R Users,
I have written a small simulation model in R which outputs a datafile consisting of ending population sizes for each simulation run (year).  The data (see short data example below) is labeled by NUM (simulation run), sim (year) and N (yearly count).   After searching the help files and coming up empty (probably because I used the wrong terms) I am appealing for some help for working with the output dataset.

What I want to do is for each of the i simulation runs (NUM) I want to

1) take N(t+1)/N(t)=lambda(t) for each year (where in the below example t=1,...,10--total years of the simulation)
2) Sum lambda(t) and divide by t (e.g., output both the mean/se of lambda for each simulation run)
3) Take the mean of the mean(lambda's) (and associated stddev, min, max) over all NUM

I think I have to write a function for use within an apply statement, but I am not quite there yet on the learning curve so most of my recent attempts in R have been useful learning experiences of what not to do...

Any suggestions/direction is greatly appreciated.

Bret Collier
TX A&M

NUM sim N
1 1  466
1 2  450
1 3  473
1 4  531
1 5  515
1 6  502
1 7  471
1 8  460
1 9  458
1 10 434
2 1  289
2 2  356
2 3 387
2 4 440
2 5 457
2 6 466
2 7 467
2 8 449
2 9 387
2 10 394
3 1 367
3 2 400
3 3 476
3 4 508
3 5 478
3 6 501
3 7 513
3 8 505
3 9 492
3 10 465

platform       i386-pc-mingw32
arch           i386
os             mingw32
system         i386, mingw32
status
major          2
minor          3.0
year           2006
month          04
day            24
svn rev        37909
language       R
version.string Version 2.3.0 (2006-04-24) (yeah, I need to update)