variable data in loop in R - r

Question

I have the code below where I have a loop above which is fed with a single value X:
n = 25
X = 1
p = 0.2 #probability
P = matrix( c(p, 1-p, 0, 0, 0, 0,
p, 0, 1-p, 0, 0, 0,
p, 0, 0, 1-p, 0, 0,
0, p, 0, 0, 1-p, 0,
0, 0, p, 0, 0, 1-p,
0, 0, 0, p, 0, 1-p),
ncol=6, nrow=6, byrow = TRUE) #transition matrix
for(i in 1:n){
Y = runif(1) #uniform sample
k = P[X[i], ] #calculate k values
k = cumsum(k)
if(Y <= k[1]){ #update the chain
X[i+1] = 1}
else if(Y <= k[2]){
X[i+1] = 2}
else if (Y <= k[3]){
X[i+1] = 3}
else if (Y<=k[4]){
X[i+1] = 4}
else if (Y<=k[5]){
X[i+1] = 5}
else {X[i+1]=6}
}
plot(1:n, X[1:i], type = 's')
I'm wondering that can I set my X be multi data like X = c(1,3,4), such that I can run all three values for X in only one line of code without having to resimulate by changing the value for X? The three graphs should be produced as a result.

Answer 1

First of all, you will want to put your code inside a function, if you aim to re-use that code. Secondly, in the code you posted, you plot the graph in the graphic device, which will be lost after you plot something else, so you might wanna save it as a PDF or PNG or something like that:
xtoplot <- function(X, n = 25, p = 0.2, transit = P){
for(i in 1:n){
Y <- runif(1) # uniform sample
k <- P[X[i], ] # calculate k values
k <- cumsum(k)
if(Y <= k[1]){ # update the chain
X[i+1] <- 1}
else if(Y <= k[2]){
X[i+1] <- 2}
else if(Y <= k[3]){
X[i+1] <- 3}
else if(Y <= k[4]){
X[i+1] <- 4}
else if(Y <= k[5]){
X[i+1] <- 5}
else{
X[i+1] <- 6}
}
pltname <- paste0("plot_", X, ".pdf") # The document name of the plot
pdf(pltname) # Tell R to prepare to export something to PDF
plot(1:n, X[1:i], type = 's') # The plot to be exported
dev.off()
}
Now that the function is in place, you can use a member of the "apply()-family" to run the function for multiple input values for X like this:
lapply(c(1,3,4), xtoplot)
In your working directory, you will find three PDFs called plot_1.pdf, plot_3.pdf and plot_4.pdf with the graphs you were looking for.