#' Encode a given factor variable using difference encoding
#'
#' @description Transforms the original design matrix using a difference encoding.
#'
#' @param X The data.frame/data.table to transform.
#' @param fact The factor variable to encode by - either a positive integer specifying the
#' column number, or the name of the column.
#' @param keep_factor Whether to keep the original factor column(defaults to **FALSE**).
#' @param encoding_only Whether to return the full transformed dataset or only the new
#' columns. Defaults to FALSE and returns the full dataset.
#'
#' @return A new data.table X which contains the new columns and optionally the old factor.
#'
#' @importFrom data.table data.table
#' @importFrom data.table setkeyv
#' @importFrom data.table .SD
#' @importFrom data.table ':='
#' @export
#'
#' @examples
#'
#' design_mat <- cbind( data.frame( matrix(rnorm(5*100),ncol = 5) ),
#' sample( sample(letters, 10), 100, replace = TRUE)
#' )
#' colnames(design_mat)[6] <- "factor_var"
#'
#' encode_difference(X = design_mat, fact = "factor_var", keep_factor = FALSE)
#'
encode_difference <- function(X, fact, keep_factor = FALSE, encoding_only = FALSE){
if(is.numeric(fact)){
fact <- colnames(X)[fact]
}
X <- data.table::data.table(X)
length_diff <- length(levels(as.factor(unlist(X[, .SD, .SDcols = fact]))))
diff <- matrix(-1 / (col(matrix(0,length_diff, length_diff)) + 1),
length_diff, length_diff)
diff[lower.tri(diff)] <- 0
diff <- diff[, 1:(length_diff - 1)]
diff[row(diff) == (col(diff) + 1)] <- - apply(diff, 2, sum)
diff <- data.table::data.table(diff)
colnames(diff) <- paste( fact,"_",
c((1:(ncol(diff)))),
"_diff", sep = "")
factor_var <- levels(as.factor(unlist(X[, .SD, .SDcols = fact])))
diff <- cbind( factor_var, diff )
colnames(diff)[1] <- fact
diff <- data.table::data.table(diff)
if(encoding_only == TRUE){
if(keep_factor == FALSE){
return(diff[,-1])
}
else{
return(diff)
}
}
X <- X[diff, on = fact]
if(keep_factor == FALSE){
X[,(fact) := NULL]
}
return(X)
}
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