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#' Distance between pairs of fatty acid signatures
#'
#' The utility function \code{dist_sum_pairwise} computes the total distance
#' between all possible pairs of fatty acid signatures.
#'
#' @param sig_data A numeric matrix of fatty acid signatures in column-major
#' format.
#' @param dist_meas An integer indicator of the distance measure to compute.
#' Default value 1.
#' @param gamma The power parameter of the chi-square distance measure. Default
#' value 1.
#'
#' @return The summed distance between all possible pairs of signatures.
#'
#' @section Details:
#' \code{dist_sum_pairwise} is an internal utility function. The signature
#' data in \code{sig_data} are presumed to be ready for analysis, which is best
#' accomplished by a call to the function \code{prep_sig}. Consequently, to
#' increase execution speed during simulations, no numeric error checking of the
#' signatures is performed. Please refer to documentation for the function
#' \code{\link{prep_sig}} for information regarding signature preparation.
#'
#' Please refer to documentation for the function
#' \code{\link{dist_between_2_sigs}} for additional information regarding the
#' arguments \code{dist_meas} and \code{gamma}.
#'
#' Utility functions called by \code{dist_sigs_2_mean}:
#' \itemize{
#' \item \code{\link{dist_between_2_sigs}}
#' }
#'
################################################################################
dist_sum_pairwise <- function(sig_data, dist_meas = 1, gamma = 1){
# Return a distance of 0 if a single signature is passed.
if(is.vector(sig_data)){
return(0)
}
# Consider all possible pairs of signatures.
n_sig <- ncol(sig_data)
dist <- 0
for(li1 in 1:(n_sig-1)){
for(li2 in (li1+1):n_sig){
# Compute the distance between these two signatures and accumulate.
dist <- dist + dist_between_2_sigs(sig_data[,li1], sig_data[,li2],
dist_meas, gamma)
}
}
# Return.
return(dist)
}
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