#' Remove features with low variation

#'

#'

#' \code{remove.low.cv} that removes variables with low variation.

#' From a matrix/data.frame (samples in rows, features in columns), it computes the coefficient of variation for every features (columns) 

#' and return a filtered data.frame with features for which the coefficient of variation is above a given threshold.

#'

#' @param X a matrix/data.frame

#' @param cutoff a numeric value

#'

#' @return

#' a data.frame/matrix

#' 

#' @examples 

#' mat <- matrix(sample(1:3, size = 200, replace = TRUE), ncol=20)

#' remove.low.cv(mat, 0.4)

#' 

#' @export

remove.low.cv <- function(X, cutoff = 0.5){

    stopifnot(is(X, "data.frame") | is(X, "matrix"))

    stopifnot(is.vector(cutoff) & is.numeric(cutoff) & (length(cutoff) == 1))

    # var.coef

    cv <- unlist(lapply(as.data.frame(X), 

                        function(x) abs(sd(x)/mean(x))))

    return(X[,cv > cutoff])

}