[英]Manual simulation of Markov Chain in R (2)
Consider the Markov chain with state space S = {1, 2} , transition matrix考虑状态空间S = {1, 2}的马尔可夫链,转移矩阵
and initial distribution α = (1/2, 1/2) .和初始分布α = (1/2, 1/2) 。
- Simulate 5 steps of the Markov chain (that is, simulate X 0 , X 1 , . . . , X 5 ).模拟马尔可夫链的 5 个步骤(即模拟X 0 , X 1 , . . , X 5 )。 Repeat the simulation 100 times.重复模拟 100 次。
My solution:我的解决方案:
states <- c(1, 2)
alpha <- c(1, 1)/2
mat <- matrix(c(1/2, 1/2, 0, 1), nrow = 2, ncol = 2)
nextX <- function(X, pMat)
{
probVec <- vector()
if(X == states[1])
{
probVec <- pMat[1,]
}
if(X==states[2])
{
probVec <- pMat[2,]
}
return(sample(states, 1, replace=TRUE, prob=probVec))
}
steps <- function(alpha1, mat1, n1)
{
X0 <- sample(states, 1, replace=TRUE, prob=alpha1)
if(n1 <=0)
{
return (X0)
}
else
{
vec <- vector(mode="numeric", length=n1)
for (i in 1:n1)
{
X <- nextX(X0, mat1)
vec[i] <- X
}
return (vec)
}
}
# steps(alpha1=alpha, mat1=mat, n1=5)
simulate <- function(alpha1, mat1, n1)
{
for (i in 1:n1)
{
vec <- steps(alpha1, mat1, 5)
print(vec)
}
}
simulate(alpha, mat, 100)
Output输出
> simulate(alpha, mat, 100)
[1] 1 2 2 2 2
[1] 2 1 2 2 2
[1] 1 1 1 1 1
[1] 1 1 1 1 1
[1] 2 2 2 2 2
[1] 2 1 1 2 2
[1] 1 1 1 1 1
[1] 2 2 1 2 2
[1] 1 1 1 1 1
[1] 2 2 1 1 2
[1] 2 2 1 2 2
[1] 1 1 1 1 1
[1] 1 1 1 1 1
[1] 1 1 2 2 2
[1] 1 1 1 1 1
[1] 2 1 2 2 2
[1] 1 1 1 1 1
[1] 1 1 1 1 1
[1] 2 1 2 1 1
[1] 1 1 1 1 1
[1] 1 1 1 1 1
[1] 1 1 1 1 1
[1] 1 1 1 1 1
[1] 1 2 1 2 2
[1] 2 2 2 1 2
[1] 1 1 1 1 1
[1] 1 1 1 1 1
[1] 1 1 1 1 1
[1] 2 1 2 2 1
[1] 1 2 2 2 2
[1] 1 1 2 2 2
[1] 1 2 2 1 2
[1] 1 1 1 1 1
[1] 2 2 1 2 2
[1] 2 2 2 1 1
[1] 1 1 1 1 1
[1] 1 1 1 1 1
[1] 1 1 1 1 1
[1] 1 1 1 2 2
[1] 1 2 1 1 2
[1] 2 2 1 1 1
[1] 1 1 1 1 1
[1] 2 2 2 2 1
[1] 1 1 1 1 1
[1] 1 1 1 1 1
[1] 1 2 2 2 2
[1] 2 1 1 2 2
[1] 1 1 1 1 1
[1] 1 2 1 2 1
[1] 1 1 1 1 1
[1] 1 1 1 1 1
[1] 2 1 2 1 2
[1] 1 1 1 1 1
[1] 1 1 1 1 1
[1] 1 1 1 1 1
[1] 1 1 1 1 1
[1] 2 1 2 1 2
[1] 1 1 1 1 1
[1] 1 1 1 1 1
[1] 2 2 1 1 2
[1] 1 1 1 1 1
[1] 1 1 1 1 1
[1] 2 2 1 1 2
[1] 1 1 1 1 1
[1] 1 2 1 1 2
[1] 1 1 1 1 1
[1] 2 1 1 2 1
[1] 1 1 1 1 1
[1] 2 1 2 2 2
[1] 1 1 1 1 1
[1] 1 1 1 1 1
[1] 1 2 2 2 2
[1] 1 1 1 1 1
[1] 2 2 2 1 2
[1] 2 2 2 1 1
[1] 1 1 2 2 2
[1] 1 1 1 1 1
[1] 2 2 2 1 2
[1] 1 1 1 1 1
[1] 2 2 1 2 2
[1] 2 2 1 2 1
[1] 1 1 1 1 1
[1] 2 2 2 2 2
[1] 1 1 1 1 1
[1] 1 2 1 2 2
[1] 1 1 1 1 1
[1] 2 1 1 2 1
[1] 2 2 2 2 1
[1] 2 2 2 2 2
[1] 1 1 1 1 1
[1] 2 2 2 1 1
[1] 2 2 2 2 2
[1] 1 1 1 1 1
[1] 1 1 1 1 1
[1] 2 1 2 2 1
[1] 2 2 1 1 1
[1] 1 1 1 1 1
[1] 2 2 1 2 2
[1] 2 1 2 2 2
[1] 1 1 1 1 1
As you can see, I am getting same output in each iteration.如您所见,我在每次迭代中都得到相同的输出。
How can I fix my code?我该如何修复我的代码?
There are two issues:有两个问题:
If you check the matrix you have input, it is the transpose of what you wanted:如果您检查输入的矩阵,它就是您想要的转置:
> mat
[,1] [,2]
[1,] 0.5 0
[2,] 0.5 1
So, change that.所以,改变它。
In the step
function, the returned state is not used to initiate the subsequent state.在step
函数中,返回的状态不用于启动后续状态。 Instead, X0
just keeps getting passed in repeatedly:相反, X0
只是不断地重复传入:
for (i in 1:n1)
{
X <- nextX(X0, mat1)
vec[i] <- X
}
Honestly, you don't need X0
at all.老实说,您根本不需要X0
。 Just change all the X0
s in the step
function to X
and it should work.只需将step
函数中的所有X0
更改为X
就可以了。
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