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#' @title Return function of transformed response variables
#' @name get_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, and returns the related function that was used for
#' transformation.
#'
#' @param x A regression model.
#'
#' @return
#'
#' A list of two functions: `$transformation`, the function that was used to
#' transform the response variable; `$inverse`, the inverse-function of
#' `$transformation` (can be used for "back-transformation"). If no
#' transformation was applied, both list-elements `$transformation` and
#' `$inverse` just return `function(x) x`. If transformation is unknown,
#' `NULL` is returned.
#'
#' @examples
#' # identity, no transformation
#' model <- lm(Sepal.Length ~ Species, data = iris)
#' get_transformation(model)
#'
#' # log-transformation
#' model <- lm(log(Sepal.Length) ~ Species, data = iris)
#' get_transformation(model)
#'
#' # log-function
#' get_transformation(model)$transformation(0.3)
#' log(0.3)
#'
#' # inverse function is exp()
#' get_transformation(model)$inverse(0.3)
#' exp(0.3)
#' @export
get_transformation <- function(x) {
transform_fun <- find_transformation(x)
# unknown
if (is.null(transform_fun)) {
return(NULL)
}
if (transform_fun == "identity") {
out <- list(transformation = function(x) x, inverse = function(x) x)
} else if (transform_fun == "log") {
out <- list(transformation = log, inverse = exp)
} else if (transform_fun %in% c("log1p", "log(x+1)")) {
out <- list(transformation = log1p, inverse = expm1)
} else if (transform_fun == "log10") {
out <- list(transformation = log10, inverse = function(x) 10^x)
} else if (transform_fun == "log2") {
out <- list(transformation = log2, inverse = function(x) 2^x)
} else if (transform_fun == "exp") {
out <- list(transformation = exp, inverse = log)
} else if (transform_fun == "sqrt") {
out <- list(transformation = sqrt, inverse = function(x) x^2)
} else if (transform_fun == "power") {
## TODO: detect power - can we turn this into a function?
# power <- .safe(gsub("\\(|\\)", "", gsub("(.*)(\\^|\\*\\*)\\s*(\\d+|[()])", "\\3", find_terms(x)[["response"]])))
# if (is.null(power)) {
# power <- 2
# }
out <- list(transformation = function(x) x^2, inverse = sqrt)
} else if (transform_fun == "expm1") {
out <- list(transformation = expm1, inverse = log1p)
} else if (transform_fun == "log-log") {
out <- list(
transformation = function(x) log(log(x)),
inverse = function(x) exp(exp(x))
)
}
out
}
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