checkFileExists = function(filenames) {
if(!is.character(filenames)) {
stop("filenames must be a character vector.")
}
nonExistFiles = filenames[!file.exists(filenames)]
if(length(nonExistFiles) > 0) {
stop(sprintf("Non-existent file(s): \n%s", paste(nonExistFiles, collapse="\n")))
}
invisible(NULL)
}
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