#' @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`, `power` (to 2nd power, like `I(x^2)`), and
#' `inverse` (like `1/y`).
#'
#' @param x A regression model or a character string of the response value.
#' @param ... Currently not used.
#'
#' @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)
#'
#' # inverse, response provided as character string
#' find_transformation("1 / y")
#' @export
find_transformation <- function(x, ...) {
UseMethod("find_transformation")
}
#' @export
find_transformation.default <- function(x, ...) {
# validation check
if (is.null(x) || is.data.frame(x) || !is_model(x)) {
return(NULL)
}
# sanity check for multivariate models
if (is_multivariate(x)) {
result <- lapply(find_terms(x), function(i) {
find_transformation(i[["response"]])
})
unlist(result)
} else {
rv <- find_terms(x)[["response"]]
find_transformation(rv)
}
}
#' @export
find_transformation.character <- function(x, ...) {
transform_fun <- "identity"
# remove whitespaces
x <- gsub(" ", "", x, fixed = TRUE)
# log-transformation
if (any(grepl("log\\((.*)\\)", x))) {
# do we have log-log models?
if (grepl("log\\(log\\((.*)\\)\\)", x)) {
transform_fun <- "log-log"
} else {
# 1. try: log(x + number)
plus_minus <- .safe(
eval(parse(text = gsub("log\\(([^,\\+)]*)(.*)\\)", "\\2", x)))
)
# 2. try: log(number + x)
if (is.null(plus_minus)) {
plus_minus <- .safe(
eval(parse(text = gsub("log\\(([^,\\+)]*)(.*)\\)", "\\1", x)))
)
}
if (is.null(plus_minus) || is.function(plus_minus)) {
transform_fun <- "log"
} else {
transform_fun <- paste0("log(x+", plus_minus, ")")
}
}
} else if (any(grepl("log1p\\((.*)\\)", x))) {
# log1p-transformation
transform_fun <- "log1p"
} else if (any(grepl("expm1\\((.*)\\)", x))) {
# expm1-transformation
transform_fun <- "expm1"
} else if (any(grepl("log2\\((.*)\\)", x))) {
# log2/log10-transformation
transform_fun <- "log2"
} else if (any(grepl("log10\\((.*)\\)", x))) {
transform_fun <- "log10"
} else if (any(grepl("exp\\((.*)\\)", x))) {
# exp-transformation
transform_fun <- "exp"
} else if (any(grepl("sqrt\\((.*)\\)", x))) {
# sqrt-transformation
plus_minus <- eval(parse(text = gsub("sqrt\\(([^,\\+)]*)(.*)\\)", "\\2", x)))
if (is.null(plus_minus) || is.function(plus_minus)) {
transform_fun <- "sqrt"
} else {
transform_fun <- paste0("sqrt(x+", plus_minus, ")")
}
} else if (any(startsWith(x, "1/"))) {
# inverse-transformation
transform_fun <- "inverse"
} else if (any(grepl("(.*)(\\^|\\*\\*)\\s?-?(\\d+|[()])", x))) {
# power-transformation
transform_fun <- "power"
} else if (any(grepl("I\\((.*)\\)", x))) {
# (unknown) I-transformation
transform_fun <- NULL
}
transform_fun
}
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