R: Problems with updating list objects using a while-loop - r

Question

I am currently trying to simulate a (random) market in R using a while-loop that runs while the market is open: while the time is less than 600 minutes.
On this market only one of four events may happen at any time: birth of a supply, birth of a demand, death of a supply or death of a demand.
These are all drawn from exponential distributions using the rexp()-command with their own intensity. Their amounts and respective prices are each drawn from their own normal distribution (only values greater than 0), and the time is then updated depending on which of the events is drawn first.
Then I would like to update these intensities (using cox-regression), and for this to happen I need to store previous information about each of the events, preferably in a list, so that I can for example draw samples from the living supplies and remove them, to imitate a purchase. I basically want to keep track of what is "alive" on the market at a given time. Here is some of my code:
TIME <- 0
count <- 1
...
my.stores <- c()
while(TIME < 600){
time.supply.birth <- rexp(1, intensity1)
time.supply.death <- rexp(1, intensity2)
time.demand.birth <- rexp(1, intensity3)
time.demand.death <- rexp(1, intensity4)
case1 <- time.supply.birth == min(time.supply.birth, time.demand.birth, time.supply.death, time.demand.death)
case2 <- time.supply.death == min(time.supply.birth, time.demand.birth, time.supply.death, time.demand.death)
case3 <- time.demand.birth == min(time.supply.birth, time.demand.birth, time.supply.death, time.demand.death)
case4 <- time.demand.death == min(time.supply.birth, time.demand.birth, time.supply.death, time.demand.death)
TIME <- TIME + time.supply.birth*case1 + time.supply.death*case2 + time.demand.birth*case3 + time.demand.death*case4
if(case1 == T){
amount.supply.birth <- rnorm() # with values
price.supply.birth <- rnorm()
count.supply.birth.event <- count.supply.birth.event + 1
my.stores[[count]]$amount.supply.birth <- c(my.stores[[count-1]]$amount.supply.birth, amount.supply.birth)
my.stores[[count]]$price.supply.birth <- c(my.stores[[count-1]]$price.supply.birth, price.supply.birth)
} else if(case2 ==T) {
# Death supply event: here a sample from the living supplies should be drawn
} else if(case3 == T){
# Similar to case 1
} else if(case4 == T){
# similar to case 2
} else{
}
count <- count + 1
}
My problem is that I cannot even store any information in the list, since the while-loop breaks immediately after one iteration, which results in the length of the list my.stores to be only 1 - I bet it is something about my indexing in the list, but I'm not sure how to get around it. I get the following warning:
Error in my.stores[[count - 1]] :
attempt to select less than one element in get1index <real>
and when I print the list I get the following:
> my.stores[[1]]
$amount.demand.birth
[1] 6.044815
Say I draw a demand.birth with an amount and a price, and then the next iteration I similarly draw a supply.birth, I would have liked something like:
> my.stores[[1]]
$amount.demand.birth
[1] 6.044815
$amount.supply.birth
[1] 0
$price.demand.birth
[1] 50.78
$price.supply.birth
[1] 0
> my.stores[[2]]
$amount.demand.birth
[1] 6.044815
[2] 6.044815
$amount.supply.birth
[1] 0
[2] 7.1312
$price.demand.birth
[1] 50.78
[2] 50.78
$price.supply.birth
[1] 0
[2] 95.00
Anyone who can help me with this or who has other suggestions?
Sorry about the long post.
Cheers!

Answer 1

Since my.stores[[0]] is not valid, did you try :
if (count==1) {
my.stores[[count]]$amount.supply.birth <- amount.supply.birth
my.stores[[count]]$price.supply.birth <- price.supply.birth
}
else {
my.stores[[count]]$amount.supply.birth <- c(my.stores[[count-1]]$amount.supply.birth, amount.supply.birth)
my.stores[[count]]$price.supply.birth <- c(my.stores[[count-1]]$price.supply.birth, price.supply.birth)
}