if-statement - 在WinBugs/JAGS中基于if-else条件选择不同的发行版

Question

我正在尝试编写一个Winbugs/Jags模型来为多 Cereal 主题模型建模(正是本文-> http://www.ryanmcd.com/papers/mg_lda.pdf)

在这里，我想根据特定值选择其他分布。
对于例如:我想做类似的事情

`if ( X[i] > 0.5 )
{
Z[i] ~ dcat(theta-gl[D[i], 1:K-gl])
W[i] ~ dcat(phi-gl[z[i], 1:V])
}
else 
{
Z[i] ~ dcat(theta-loc[D[i], 1:K-loc])
W[i] ~ dcat(phi-loc[z[i], 1:V])
}
`

这可以在Winbugs/JAGS中完成吗？

Answer 1

Winbugs/JAGS不是一种过程语言，因此您不能使用这样的构造。使用step函数。引用手册:

step(e) ...... 1 if e >= 0; 0 otherwise

因此，您需要一个技巧来更改条件:

X[i] > 0.5   <=>     
X[i] - 0.5 > 0  <=> 
!(X[i] - 0.5 <= 0) <=>
!(-(X[i] - 0.5) >= 0) <=>
!(step(-(X[i] - 0.5)) == 1) <=>
step(-(X[i] - 0.5)) == 0

然后将其用于索引技巧:

# then branch
Z_branch[i, 1] ~ dcat(theta-gl[D[i], 1:K-gl])
W_branch[i, 1] ~ dcat(phi-gl[z[i], 1:V])

# else branch
Z_branch[i, 2] ~ dcat(theta-loc[D[i], 1:K-loc])
W_branch[i, 2] ~ dcat(phi-loc[z[i], 1:V])

# decision here
if_branch[i] <- 1 + step(-(X[i] - 0.5)) # 1 for "then" branch, 2 for "else" branch
Z[i] ~ Z_branch[i, if_branch[i]]
W[i] ~ W_branch[i, if_branch[i]]