#' UfsCov algorithm for unsupervised feature selection
#'
#' Applies the UfsCov algorithm based on the space filling concept,
#' by using a sequatial forward search (SFS).This function
#' offers a parellel computing.
#' @usage UfsCov_par(data, ncores=2)
#' @param data Data of class: \code{matrix} or \code{data.frame}.
#' @param ncores Number of cores to use (by default: \code{ncores=2}).
#' @return A list of two elements:
#' \itemize{
#' \item \code{CovD} a vector containing the coverage measure of
#' each step of the SFS.
#' \item \code{IdR} a vector containing the added variables during
#' the selection procedure.
#' }
#' @author Mohamed Laib \email{Mohamed.Laib@@unil.ch}
#' @note The algorithm does not deal with missing values and constant
#' features. Please make sure to remove them. Note that it is not recommanded to
#' use this function with small data, it takes more time than using the
#' standard \code{\link{UfsCov}} function.
#'
#' @details Since the algorithm is based on pairwise distances, and
#' according to the computing power of your machine, large number of
#' data points needs more memory.
#'
#'
#' @examples
#' N <- 800
#' dat<-SimData(N)
#' Results<- UfsCov_par(dat,ncores=2)
#'
#' cou<-colnames(dat)
#' nom<-cou[Results[[2]]]
#' par(mfrow=c(1,1), mar=c(5,5,2,2))
#' names(Results[[1]])<-cou[Results[[2]]]
#' plot(Results[[1]] ,pch=16,cex=1,col="blue", axes = FALSE,
#' xlab = "Added Features", ylab = "Coverage measure")
#' lines(Results[[1]] ,cex=2,col="blue")
#' grid(lwd=1.5,col="gray" )
#' box()
#' axis(2)
#' axis(1,1:length(nom),nom)
#' which.min(Results[[1]])
#'
#' \dontrun{
#'
#' N<-5000
#' dat<-SimData(N)
#'
#' ## Little comparison:
#' system.time(Uf<-UfsCov(dat))
#' system.time(Uf.p<-UfsCov_par(dat, ncores = 4))
#'
#' }
#' @references
#' M. Laib, M. Kanevski,
#' \href{https://www.elen.ucl.ac.be/Proceedings/esann/esannpdf/es2018-57.pdf}{A novel
#' filter algorithm for unsupervised feature selection based on a space filling measure}.
#' Proceedings of the 26rd European Symposium on Artificial Neural Networks, Computational
#' Intelligence and Machine Learning (ESANN), pp. 485-490, Bruges (Belgium), 2018.
#'
#' M. Laib and M. Kanevski, A new algorithm for redundancy minimisation in
#' geo-environmental data, 2019.
#' \href{https://www.sciencedirect.com/science/article/pii/S0098300418310975}{Computers &
#' Geosciences, 133 104328}.
#'
#'
#' @import Biobase doParallel parallel
#' @importFrom utils setTxtProgressBar txtProgressBar
#' @export
#'
UfsCov_par<-function(data, ncores=2){
so<-so1<-as.matrix(1:ncol(data))
sf<-0
ind<-c()
vlu<-c()
dat<- apply(data , MARGIN = 2,FUN = function(X) (X - min(X))/diff(range(X)))
cl <- makeCluster(ncores)
bprog <- txtProgressBar(min = 0, max = ncol(data))
clusterExport(cl, varlist = c("measr","dat","sf","so"), envir=environment())
for (i in 1:ncol(data)){
disA<-parRapply(cl, so, function(X) (measr(dat[,c(X,sf)])))
ind<-c(ind,so[which.min(disA)])
vlu<-c(vlu,min(disA, na.rm = TRUE))
sf<-ind
so<-as.matrix(so1[-sf])
setTxtProgressBar(bprog,i)}
stopCluster(cl)
if(!is.null(colnames(dat))){
sf <- colnames(data)[sf]
}
return(list(CoValue=vlu, VarName=sf))}
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