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#' @title Find possible transformation of response variables
#' @name find_transformation
#'
#' @description This functions checks whether any transformation, such as log-
#' or exp-transforming, was applied to the response variable (dependent
#' variable) in a regression formula. Currently, following patterns are
#' detected: `log`, `log1p`, `log2`, `log10`, `exp`, `expm1`, `sqrt`,
#' `log(x+<number>)`, `log-log` and `power` (to 2nd power, like `I(x^2)`).
#'
#' @param x A regression model.
#' @return A string, with the name of the function of the applied transformation.
#' Returns `"identity"` for no transformation, and e.g. `"log(x+3)"` when
#' a specific values was added to the response variables before
#' log-transforming. For unknown transformations, returns `NULL`.
#'
#' @examples
#' # identity, no transformation
#' model <- lm(Sepal.Length ~ Species, data = iris)
#' find_transformation(model)
#'
#' # log-transformation
#' model <- lm(log(Sepal.Length) ~ Species, data = iris)
#' find_transformation(model)
#'
#' # log+2
#' model <- lm(log(Sepal.Length + 2) ~ Species, data = iris)
#' find_transformation(model)
#' @export
find_transformation <- function(x) {
# validation check
if (is.null(x) || is.data.frame(x) || !is_model(x)) {
return(NULL)
}
rv <- find_terms(x)[["response"]]
transform_fun <- "identity"
# log-transformation
if (any(grepl("log\\((.*)\\)", rv))) {
# do we have log-log models?
if (grepl("log\\(log\\((.*)\\)\\)", rv)) {
transform_fun <- "log-log"
} else {
# 1. try: log(x + number)
plus_minus <- .safe(
eval(parse(text = gsub("log\\(([^,\\+)]*)(.*)\\)", "\\2", rv)))
)
# 2. try: log(number + x)
if (is.null(plus_minus)) {
plus_minus <- .safe(
eval(parse(text = gsub("log\\(([^,\\+)]*)(.*)\\)", "\\1", rv)))
)
}
if (is.null(plus_minus)) {
transform_fun <- "log"
} else {
transform_fun <- paste0("log(x+", plus_minus, ")")
}
}
}
# log1p-transformation
if (any(grepl("log1p\\((.*)\\)", rv))) {
transform_fun <- "log1p"
}
# expm1-transformation
if (any(grepl("expm1\\((.*)\\)", rv))) {
transform_fun <- "expm1"
}
# log2/log10-transformation
if (any(grepl("log2\\((.*)\\)", rv))) {
transform_fun <- "log2"
}
if (any(grepl("log10\\((.*)\\)", rv))) {
transform_fun <- "log10"
}
# exp-transformation
if (any(grepl("exp\\((.*)\\)", rv))) {
transform_fun <- "exp"
}
# sqrt-transformation
if (any(grepl("sqrt\\((.*)\\)", rv))) {
plus_minus <- eval(parse(text = gsub("sqrt\\(([^,\\+)]*)(.*)\\)", "\\2", rv)))
if (is.null(plus_minus)) {
transform_fun <- "sqrt"
} else {
transform_fun <- paste0("sqrt(x+", plus_minus, ")")
}
}
# (unknown) I-transformation
if (any(grepl("I\\((.*)\\)", rv))) {
transform_fun <- NULL
}
# power-transformation
if (any(grepl("(.*)(\\^|\\*\\*)\\s?-?(\\d+|[()])", rv))) {
transform_fun <- "power"
}
transform_fun
}
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