我正在寻找类似于“msm”包的东西,但是针对离散马尔可夫链。例如,如果我有一个定义为下面这样的转移矩阵:
Pi <- matrix(c(1/3,1/3,1/3,
0,2/3,1/6,
2/3,0,1/2))
针对状态A、B、C,如何根据该转移矩阵模拟马尔可夫链?
我正在寻找类似于“msm”包的东西,但是针对离散马尔可夫链。例如,如果我有一个定义为下面这样的转移矩阵:
Pi <- matrix(c(1/3,1/3,1/3,
0,2/3,1/6,
2/3,0,1/2))
针对状态A、B、C,如何根据该转移矩阵模拟马尔可夫链?
不久前,我编写了一组用于模拟和估计离散马尔可夫链概率矩阵的函数:http://www.feferraz.net/files/lista/DTMC.R。
以下是您所需的相关代码:
simula <- function(trans,N) {
transita <- function(char,trans) {
sample(colnames(trans),1,prob=trans[char,])
}
sim <- character(N)
sim[1] <- sample(colnames(trans),1)
for (i in 2:N) {
sim[i] <- transita(sim[i-1],trans)
}
sim
}
#example
#Obs: works for N >= 2 only. For higher order matrices just define an
#appropriate mattrans
mattrans <- matrix(c(0.97,0.03,0.01,0.99),ncol=2,byrow=TRUE)
colnames(mattrans) <- c('0','1')
row.names(mattrans) <- c('0','1')
instancia <- simula(mattrans,255) # simulates 255 steps in the process
啊,我正在为你写解释的时候,你已经找到了答案。这里是我想出来的一个简单示例:
run = function()
{
# The probability transition matrix
trans = matrix(c(1/3,1/3,1/3,
0,2/3,1/3,
2/3,0,1/3), ncol=3, byrow=TRUE);
# The state that we're starting in
state = ceiling(3 * runif(1, 0, 1));
cat("Starting state:", state, "\n");
# Make twenty steps through the markov chain
for (i in 1:20)
{
p = 0;
u = runif(1, 0, 1);
cat("> Dist:", paste(round(c(trans[state,]), 2)), "\n");
cat("> Prob:", u, "\n");
newState = state;
for (j in 1:ncol(trans))
{
p = p + trans[state, j];
if (p >= u)
{
newState = j;
break;
}
}
cat("*", state, "->", newState, "\n");
state = newState;
}
}
run();
R
的人的注意。希望这有所帮助! - iciosample(1:3, 1)
比使用ceiling(3 * runif(1, 0, 1))
更容易理解。对于最内层的for循环,你可以简单地使用newState <- sample(1:ncol(trans), 1, prob=trans[state,])
。这更清晰地展示了正在发生的事情。而且,你甚至不必再归一化行了。 - Ken Williams