help with population matrix

-----Original Message-----
From: sf62 at st-andrews.ac.uk
Sent: Mon, 18 Feb 2013 16:09:49 +0000
To: r-help at r-project.org
Subject: [R] help with population matrix

Hi guys, I am a biologist and an R newbie, and I'm learning how to create
a
simple population model.

So, I have a population matrix ("pop")of 30 age classes of female (1:4
are
non-breeders, 5:30 are breeders) which will be modelled for 100 years.

pop <- matrix(0,30,100)

I then populate this matrix with 3 young adult females.

pop[5, 1] <- 3

I then want to run this for 100 years, with stochasticity, to see how
this
population does over time.

for (y in 1:100)  {
    pop[1,t+1] <- rbinom(1,colSums(pop[5:30, t]), b/2)

(I haven't filled these in but you don't need them, they all have
different
survival probabilities to the sexually-mature adults.)

   pop[5, t+1] <- rbinom(1, pop[4, t], s2)
   pop[6, t+1] <- rbinom(1, pop[5, t], s2)

.....

   pop[30, t+1] <- rbinom(1, pop[29, t], s2)

}

So my question is: is there any way of populating this matrix without
having to explicitly write 30 lines of code? Because lines 5 - 30 are all
going to be the same, and yet even after 5 hours (literally) of web
searching and R manual reading I can't find a way to index the rows,
which
seems to be what's needed here.

Any insight is welcome here, including a different way of modelling this
population.
Many thanks, Sam.