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##
## selection.r - Selection schemes for EAs
##
## All selection methods should have the same signature:
##
## selection(values, n, ...)
##
##' Selection strategies for EMOA.
##'
##' The currently implemented strategies are nondominated sorting
##' followed by either hypervolume contribution or crowding distance
##' based ranking. Both of these implementations are currently
##' limited to selecting a single individual for replacement.
##'
##' @param values Matrix of function values.
##' @param n Number of individuals to select for replacement.
##' @param ... Optional parameters passed to
##' \code{\link{hypervolume_contribution}}.
##'
##' @title Selection strategies
##' @aliases nds_hv_selection nds_cd_selection
##'
##' @author Olaf Mersmann \email{olafm@@statistik.tu-dortmund.de}
##' @keywords optimize nonlinear
##' @export
nds_hv_selection <- function(values, n=1, ...) {
#stopifnot(n == 1)
sel <- which(is_maximally_dominated(values))
## Identify individual which gets replaced:
if (length(sel) == 1) {
sel
} else {
contrib <- if (length(sel) == ncol(values)) {
hypervolume_contribution(values, ...)
} else {
hypervolume_contribution(values[,sel], ...)
}
sel[which.min(contrib)]
}
}
##' @export
##' @rdname nds_hv_selection
nds_cd_selection <- function(values, n=1, ...) {
#stopifnot(n == 1)
N <- ncol(values)
k <- N - n
ranks <- nds_rank(values)
sel <- rep(FALSE, N)
cr <- 0
while(sum(sel) < k) {
cr <- cr + 1
sel[ranks == cr] <- TRUE
}
if (sum(sel) != k) {
nelim <- sum(sel) - k
dist <- crowding_distance(values[,ranks == cr])
cdr <- rank(dist, ties.method="random")
s <- which(ranks == cr)[cdr <= nelim]
sel[s] <- FALSE
}
which(sel == FALSE)
}
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