Programming state switches in R - r

Question

I am trying to write a program that sets a state from A to state B and vice versa.
rnumbers <- data.frame(replicate(5,runif(2000, 0, 1)))
I am imagining this data frame of random numbers in a uniform distribution, except it has 10000 rows instead of 20 rows.
Setting the probability of going to state A and state B :
dt <- c(.02)
A <- dt*1
B <- dt*.5
Making a function that goes through data frame rnumbers and putting in a 0 if the number is less than B and a 1 if the number is less than A.
step_generator <- function(x){
step <- ifelse ( x < B, 0, ifelse(x < A, 1, NA))
return(step)
}
state <- apply(rnumbers, 2, step_generator)
This essentially gives me what I want - a data frame with columns that contain 0, 1, or NA depending on the value of the random number in rnumbers. However, I am missing a couple of things--
1) I would like to keep track of how long each state lasts. What I mean by that, is if you imagine each row as a change in time as above (dt <- c(.02)). I want to be able to plot "state vs. time". In order to address this, this is what I tried :
state1 <- transform(state, time = rep(dt))
state2 <- transform(state1, cumtime = cumsum(time))
This gets me close to what I want, cumtime goes from .02 to .4. However, I want the clock to start at 0 in the 1st row and add .02 to every subsequent row.
2) I need to know how long each state lasts for. Essentially, I want to be able to go through each column, and ask for how much time (cumsum) does each state last. This would then give me a distribution of times for state A and state B. I want this stored in another data frame.
I think this makes sense, if anything is unclear please let me know and I will clarify.
Thanks for any and all help!

Answer 1

The range between "number is less than .02*1 and greater than .02*.5" is very narrow so if you are setting this simulation up, most of the first row will most probably be zero. You cannot really hope to get success with ifelse when the conditions have any look-back features. That function doesn't allow "back-indexing".
rstate <- rnumbers # copy the structure
rstate[] <- NA # preserve structure with NA's
# Init:
rstate[1, ] <- rnumbers[1, ] < .02 & rnumbers[1, ] > 0.01
step_generator <- function(col, rnum){
for (i in 2:length(col) ){
if( rnum[i] < B) { col[i] <- 0 }
else { if (rnum[i] < A) {col[i] <- 1 }
else {col[i] <- col[i-1] } }
}
return(col)
}
# Run for each column index:
for(cl in 1:5){ rstate[ , cl] <-
step_generator(rstate[,cl], rnumbers[,cl]) }