R语言关于可逆跳跃mcmc的代码
时间: 2023-07-10 09:03:17 浏览: 91
可逆跳跃MCMC(reversible jump MCMC)是一种用于贝叶斯模型选择的方法,它可以在不同的参数空间中进行参数估计。下面是一个简单的R语言实现:
```R
# 定义一个例子模型,假设有两个参数,可以是正态分布或指数分布
model <- function(x, type) {
if (type == "normal") {
mu <- x[1]
sd <- x[2]
return(list(mu = mu, sd = sd, type = type, loglik = sum(dnorm(data, mu, sd, log = TRUE))))
} else if (type == "exponential") {
rate <- x[1]
return(list(rate = rate, type = type, loglik = sum(dexp(data, rate, log = TRUE))))
}
}
# 初始化参数
init_mu <- rnorm(1)
init_sd <- abs(rnorm(1))
init_rate <- rexp(1)
# 假设数据是服从正态分布的
data <- rnorm(100)
# 定义先验概率分布
prior_normal <- function(mu, sd) {
dnorm(mu, 0, 10, log = TRUE) + dgamma(sd, 1, 1, log = TRUE)
}
prior_exponential <- function(rate) {
dgamma(rate, 1, 1, log = TRUE)
}
# 定义转移概率分布
trans_normal_to_exponential <- function(x) {
rate <- rexp(1)
return(c(rate, "exponential"))
}
trans_exponential_to_normal <- function(x) {
mu <- rnorm(1)
sd <- abs(rnorm(1))
return(c(mu, sd, "normal"))
}
# 进行可逆跳跃MCMC
n_iter <- 10000
result <- matrix(0, n_iter, 4)
result[1,] <- c(init_mu, init_sd, init_rate, "normal")
for (i in 2:n_iter) {
current <- result[i-1,]
if (current[4] == "normal") {
# 在正态分布和指数分布之间跳跃
proposal <- trans_normal_to_exponential(current)
prior_ratio <- prior_exponential(proposal[1]) - prior_normal(current[1], current[2])
likelihood_ratio <- model(proposal, proposal[2])$loglik - model(current[1:2], current[4])$loglik
acceptance_prob <- exp(prior_ratio + likelihood_ratio)
if (runif(1) < acceptance_prob) {
result[i,] <- c(proposal[1], NA, proposal[2], proposal[3])
} else {
result[i,] <- current
}
} else if (current[4] == "exponential") {
# 在指数分布和正态分布之间跳跃
proposal <- trans_exponential_to_normal(current)
prior_ratio <- prior_normal(proposal[1], proposal[2]) - prior_exponential(current[3])
likelihood_ratio <- model(proposal[1:2], proposal[3])$loglik - model(current[1], current[3], current[4])$loglik
acceptance_prob <- exp(prior_ratio + likelihood_ratio)
if (runif(1) < acceptance_prob) {
result[i,] <- c(proposal[1], proposal[2], NA, proposal[3])
} else {
result[i,] <- current
}
}
}
# 输出结果
library(ggplot2)
library(dplyr)
result_df <- as.data.frame(result)
result_df$loglik <- sapply(result_df[,1:3], function(x) model(x[!is.na(x)], result_df$type[!is.na(result_df$type)])$loglik)
result_df %>%
filter(!is.na(sd)) %>%
ggplot(aes(x = mu, y = sd, color = loglik)) +
geom_point() +
scale_color_gradient(low = "red", high = "blue")
```
上面的代码中,我们定义了一个简单的模型,假设有两个参数,可以是正态分布或指数分布,然后使用可逆跳跃MCMC进行参数估计。在这个例子中,我们首先随机初始化参数,然后在正态分布和指数分布之间跳跃,直到采样到足够多的样本为止。最后,我们将结果绘制成散点图,其中颜色表示对数似然值。
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