mortalrabbits

lifespan <- 18
months <- 85
pop <- vector(mode="numeric", length=months)  # this will hold the population each month as we build it up
pop[1:2] <- 1                                 # rabbits take a month to mature so we go ahead and fill in month 1 and 2
for(i in 3:months){                           # this loop will just build our population month by month
  if(i-(lifespan+1) < 0){                     # if we are early on none have died yet this if statement takes care of that
    pop[i] <- pop[i-1]+pop[i-2]-0       
  }
  if(i-(lifespan+1) == 0){                    # this one takes care of the first rabbit dying
    pop[i] <- pop[i-1]+pop[i-2]-1
  }
  
  if(i-(lifespan+1) > 0){                     # this handles all subsequent months
    pop[i] <- pop[i-1]+pop[i-2]-pop[i-(lifespan+1)]
  }
}
pop[months]