# R/classPrior.R In BayesianTools: General-Purpose MCMC and SMC Samplers and Tools for Bayesian Statistics

#### Documented in createBetaPriorcreatePriorcreatePriorDensitycreateTruncatedNormalPriorcreateUniformPrior

```#' Creates a standardized prior class
#' @author Florian Hartig
#' @param density Prior density
#' @param sampler Sampling function for density (optional)
#' @param lower vector with lower bounds of parameters
#' @param upper vector with upper bounds of parameter
#' @param best vector with "best" parameter values
#' @details This is the general prior generator. It is highly recommended to not only implement the density, but also the sampler function. If this is not done, the user will have to provide explicit starting values for many of the MCMC samplers. Note the existing, more specialized prior function. If your prior can be created by those, they are preferred. Note also that priors can be created from an existing MCMC output from BT, or another MCMC sample, via \code{\link{createPriorDensity}}.
#' @note min and max truncate, but not re-normalize the prior density (so, if a pdf that integrated to one is truncated, the integral will in general be smaller than one). For MCMC sampling, this doesn't make a difference, but if absolute values of the prior density are a concern, one should provide a truncated density function for the prior.
#' @export
#' @example /inst/examples/createPrior.R
createPrior <- function(density = NULL, sampler = NULL, lower = NULL, upper = NULL, best = NULL){

# case density is a Bayesian Posterior
if(inherits(density,"bayesianOutput")) return(createPriorDensity(density, lower = lower, upper = upper, best = best))

if(! is.null(lower) & ! is.null(upper)) if(any(lower > upper)) stop("prior with lower values > upper")

if(is.null(best) & ! is.null(lower) & ! is.null(upper)) best = (upper + lower) / 2

# if no density is provided
if (is.null(density)){
density <- function(x){
return(0)
}
}

catchingPrior <- function(x){

# check if bounds are respected
if(!is.null(lower)){
if (any(x < lower)) return(-Inf)
}
if(!is.null(upper)){
if (any(x > upper)) return(-Inf)
}

# calculate prior density within try-catch statement
out <- tryCatch(
{
density(x)
},
error=function(cond) {
warning("Problem in the prior", cond)
return(-Inf)
}
)
# extra check
if (out == Inf) stop("Inf encountered in prior")

return(out)
}

parallelDensity<- function(x){
if (is.vector(x)) return(catchingPrior(x))
else if(is.matrix(x)) return(apply(x, 1, catchingPrior))
else stop("parameter must be vector or matrix")
}

# Check and parallelize the sampler
# if no sampler is passed, but lower and upper, generate uniform sampler
if (is.null(sampler) && !is.null(lower) && !is.null(upper)) {
sampler <- function(n = 1) {
runif(n, lower, upper)
}
}

if(!is.null(sampler)){
npar <- length(sampler())
parallelSampler <- function(n=NULL){
if(is.null(n)) out = sampler()
else{
if (npar == 1) out = matrix(replicate(n, sampler()))
else if (npar >1) out = t(replicate(n, sampler(), simplify = T))
else stop("sampler provided doesn't work")
}
return(out)
}
} else parallelSampler = function(n = NULL){
stop("Attept to call the sampling function of the prior, although this function has not been provided in the Bayesian setup. A likely cause of this error is that you use a function or sampling algorithm that tries to sample from the prior. Either change the settings of your function, or provide a sampling function in your BayesianSetup (see ?createBayesianSetup, and ?createPrior)")
}

checkPrior <- function(x = NULL, z = FALSE){
if(is.null(x)) x <- parallelSampler(1000)
if(is.function(x)) x <- x()
if(!is.matrix(x)) x <- parallelSampler(1000)
check <- parallelDensity(x)
if(any(is.infinite(check))) {
if(z) warning("Z matrix values outside prior range", call. = FALSE)
else warning("Start values outside prior range", call. = FALSE)
}
}

out<- list(density = parallelDensity, sampler = parallelSampler, lower = lower, upper = upper, best = best, originalDensity = density, checkStart = checkPrior)
class(out) <- "prior"
return(out)
}

#' Convenience function to create a simple uniform prior distribution
#' @author Florian Hartig
#' @param lower vector of lower prior range for all parameters
#' @param upper vector of upper prior range for all parameters
#' @param best vector with "best" values for all parameters
#' @note for details see \code{\link{createPrior}}
#' @example /inst/examples/createPrior.R
#' @export
createUniformPrior<- function(lower, upper, best = NULL){
len = length(lower)
density <- function(x){
if (length(x) != len) stop("parameter vector does not match prior")
else return(sum(dunif(x, min = lower, max = upper, log = T)))
}
sampler <- function() runif(len, lower, upper)

out <- createPrior(density = density, sampler = sampler, lower = lower, upper = upper, best = best)
return(out)
}

#' Convenience function to create a truncated normal prior
#' @author Florian Hartig
#' @param mean best estimate for each parameter
#' @param sd sdandard deviation
#' @param lower vector of lower prior range for all parameters
#' @param upper vector of upper prior range for all parameters
#' @note for details see \code{\link{createPrior}}
#' @export
#' @example /inst/examples/createPrior.R
createTruncatedNormalPrior<- function(mean, sd, lower, upper){
len = length(mean)
density <- function(x){
if (length(x) != len) stop("parameter vector does not match prior")
else return(sum(msm::dtnorm(x, mean = mean, sd = sd, lower = lower, upper = upper, log = T)))
}
sampler <- function(){
msm::rtnorm(n = length(mean), mean = mean, sd = sd, lower = lower, upper = upper)
}
out <- createPrior(density = density, sampler = sampler, lower = lower, upper = upper)
return(out)
}

#' Convenience function to create a beta prior
#' @author Florian Hartig
#' @param a shape1 of the beta distribution
#' @param b shape2 of the beta distribution
#' @param upper upper values for the parameters
#' @param lower lower values for the parameters
#' @note for details see \code{\link{createPrior}}
#' @details This creates a beta prior, assuming that lower / upper values for parameters are are fixed. The beta is the calculated relative to this lower / upper space.
#' @example /inst/examples/createPrior.R
#' @export
createBetaPrior<- function(a, b, lower=0, upper=1){
len = length(lower)
if (! any(upper > lower)) stop("wrong values in beta prior")
range = upper - lower
density <- function(x){
x = (x - lower) / range
if (length(x) != len) stop("parameter vector does not match prior")
else return(sum( dbeta(x, shape1 = a, shape2 = b, log=T) ))
}
sampler <- function(){
out = rbeta(n = len, shape1 = a, shape2 = b)
out = (out * range) + lower
return(out)
}
out <- createPrior(density = density, sampler = sampler, lower = lower, upper = upper)
return(out)
}

#' Fits a density function to a multivariate sample
#'
#' @author Florian Hartig
#' @export
#' @param sampler an object of class BayesianOutput or a matrix
#' @param method method to generate prior - default and currently only option is multivariate
#' @param eps numerical precision to avoid singularity
#' @param lower vector with lower bounds of parameter for the new prior, independent of the input sample
#' @param upper vector with upper bounds of parameter for the new prior, independent of the input sample
#' @param best vector with "best" values of parameter for the new prior, independent of the input sample
#' @param scaling optional scaling factor for the covariance. If scaling > 1 will create a prior wider than the posterior, < 1 a prior more narrow than the posterior. Scaling is linear to the posterior width, i.e. scaling = 2 will create a prior that with 2x the sd of the original posterior.
#' @param ... parameters to pass on to the getSample function
#'
#' @details This function fits a density estimator to a multivariate (typically a posterior) sample. The main purpose is to summarize a posterior sample as a pdf, in order to include it as a prior in a new analysis, for example when new data becomes available, or to calculate a fractional Bayes factor (see \code{\link{marginalLikelihood}}).
#'
#' The limitation of this function is that we currently only implement a multivariate normal density estimator, so you will have a loss of information if your posterior is not approximately multivariate normal, which is likely the case if you have weak data. Extending the function to include more flexible density estimators (e.g. gaussian processes) is on our todo list, but it's quite tricky to get this stable, so I'm not sure when we will have this working. In general, creating reliable empirical density estimates in high-dimensional parameter spaces is extremely tricky, regardless of the software you are using.
#'
#' For that reason, it is usually recommended to not update the posterior with this option, but rather:
#'
#' 1. If the full dataset is available, to make a single, or infrequent updates, recompute the entire model with the full / updated data
#'
#' 2. For frequent updates, consider using SMC instead of MCMC sampling. SMC sampling doesn't require an analytical summary of the posterior.
#'
#' @example /inst/examples/createPrior.R
createPriorDensity <- function(sampler, method = "multivariate", eps = 1e-10, lower = NULL, upper = NULL, best = NULL, scaling = 1, ...){

x = getSample(sampler, ...)

if(method == "multivariate"){
nPars = ncol(x)
covar = cov(x) * scaling^2
mean = apply(x, 2, mean)
if(is.null(lower)) lower = rep(-Inf, length = length(mean))
if(is.null(upper)) upper = rep(Inf, length = length(mean))

density = function(par){
dens = tmvtnorm::dtmvnorm(x = par, mean = mean, sigma = covar + eps, log = T, lower = lower, upper = upper)
return(dens)
}

sampler = function(n=1){
par <- tmvtnorm::rtmvnorm(n = n, mean = mean, sigma = covar + eps, lower = lower, upper = upper, algorithm = "rejection")
if (n == 1) par = as.vector(par)
return(par)
}

out <- createPrior(density = density, sampler = sampler, lower = lower, upper = upper, best = best)
return(out)
}
}

#' @author Maximilian Pichler

#' @export

print.prior <- function(x, ...){
cat('Prior: \n\n')

prior = x
info = c( "lower", "upper","best")
maxPar =  max(length(prior\$lower),length(prior\$lupper))
if(maxPar == 0) maxPar = ncol(prior\$sampler())
priorInfo = data.frame(matrix(NA, ncol = 3, nrow = maxPar))
colnames(priorInfo) = info
for(i in 1:3) if(!is.null(prior[[info[i]]])) priorInfo[,i] <- prior[[info[i]]]
rownames(priorInfo) <- sapply(1:maxPar, FUN = function(x) return(paste("par",x)))
print(priorInfo)

}
```

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BayesianTools documentation built on Feb. 16, 2023, 8:44 p.m.