R/ifMissing.R

In Orcs: Omnidirectional R Code Snippets

#' Take Measures in Case of Nonexisting Target Files
#' 
#' @description 
#' If a target file already exists, it is simply being imported into R. However,
#' if the specified target file does not exist, it is first created by a 
#' user-defined function and subsequently returned, thus rendering explicit 
#' calls to [file.exists()] unnecessary.
#' 
#' @param ofl Target file name as `character`.
#' @param fun0 If 'ofl' exists, `function` to be applied to it. Defaults to 
#'   [terra::rast()]). 
#' @param fun1 If 'ofl' does not exist, `function` used to create it. Defaults 
#'   to [terra::writeRaster()]).
#' @param arg1 Argument in 'fun1' (as `character`) that corresponds to 'ofl', 
#'   e.g. 'filename' in [terra::writeRaster()] or 'file' in 
#'   [utils::write.table()]. If missing (default), the target file name passed 
#'   to 'fun1' needs to be explicitly included via '...'.
#' @param ... Additional arguments passed to 'fun0' and 'fun1'.
#' 
#' @return 
#' If 'ofl' has already existed, the contents of 'ofl' derived from 'fun0'; else
#' the output resultant from 'fun1'.
#' 
#' @seealso [file.exists()], [do.call()].
#' 
#' @author Florian Detsch
#' 
#' @examples
#' # simply import existing file
#' logo <- system.file("ex/logo.tif", package = "terra")
#' s <- ifMissing(logo) 
#' 
#' # create nonexisting file and import it afterwards
#' logo2 <- file.path(tempdir(), "rlogo.tif")
#' s2 <- ifMissing(logo2, arg1 = "filename", x = s, datatype = "INT1U")
#' 
#' # this also works with text files and more sophisticated custom functions
#' fun = function(x, file = "", ...) {
#'   write.csv(x, file, ...)
#'   read.csv(file)
#' }
#' 
#' data(iris)
#' ofl <- file.path(tempdir(), "iris.csv")
#' iris2 <- ifMissing(ofl, fun1 = fun, x = iris, file = ofl, quote = FALSE, row.names = FALSE)
#' 
#' @export
ifMissing = function(
    ofl
    , fun0 = terra::rast
    , fun1 = terra::writeRaster
    , arg1
    , ...
) {
  
  if (file.exists(ofl)) {
    do.call(fun0, args = list(ofl))
    
  } else {
    dots = list(...)
    
    if (!missing(arg1)) {
      ofl1 = list(ofl) 
      names(ofl1) = arg1
      dots = append(dots, ofl1)
    }
    
    do.call(fun1, args = dots)
  }
}