# R/inv.reparam.pi.R In mmds: Mixture Model Distance Sampling (mmds)

#### Documented in inv.reparam.pi

```#' Reparametrize mixture proportions
#'
#' Re-parameterise the mixture proportions so that when there is more than a
#' 2-point mixture, the proportions sum to 1.
#'
#' @param mix.prop A set of mixture proportions.
#' @param lastpar Is the last parameter provided, i.e. does sum(mix.prop)=1?
#' @return a vector of parameters
#'
#' @export
#'
#' @section Notes:
#' See Miller and Thomas for information on exactly how these are calculated.
#' Thanks go to David Borchers for proposing the trick.
#'
#' @references
#' Miller, D.L. and L. Thomas (in prep.). Mixture model distance sampling detection functions.
#'
#' @seealso reparam.pi
#'
#' @author David L. Miller
#' @examples
#' library(mmds)
#' reparam.pi(inv.reparam.pi(0.3))
#' reparam.pi(inv.reparam.pi(c(0.3,0.4,0.1),lastpar=TRUE))
inv.reparam.pi<-function(mix.prop,lastpar=FALSE){
# re-parameterise the pis so that when there is more
# than a 2 point mixture
# INVERSE VERSION!
# pis->thetas

# functions to use for the transforms
F<-function(x) pgamma(x,3,2)
Finv<-function(x) qgamma(x,3,2)
#F<-function(x){exp(x)/(1+exp(x))}
#Finv<-function(x){log(x/(1-x))}

thetas<-c()

if(lastpar){
looper<-length(mix.prop)
}else{
looper<-(length(mix.prop)-1)
}

for(i in 1:looper){
if(i==1){
theta<-log(Finv(mix.prop[i]))
}else{
theta<-log(
Finv(mix.prop[i]+F(sum(exp(thetas))))-
sum(exp(thetas)))
}
thetas<-c(thetas,theta)
}

# return the thetas
return(thetas)
}
```

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mmds documentation built on May 2, 2019, 8:55 a.m.