What does the “slot doesn't exist” error message mean? - r

Question

I'm trying to write an object and access to his parameters. I've got two files, menus.R, where I define the object, and main.R, where I use the object and try to access to a slot (parameter).
The code of both files are next:
menus.R
menu <- setClass("menu", slots=list(competition="numeric", stats="numeric"))
setMethod("show", "menu", function(object){
while (TRUE){
#Clean console
cat("\014")
cat("COMPARATIVA ENTRE EQUIPOS DE LA MISMA COMPETICION\n")
cat("-------------------------------------------------\n\n")
cat("1. Comparativa entre clubes de Liga DIA\n")
cat("2. Comparativa entre clubes de Liga Femenina 2 - Grupo 'A'\n")
cat("3. Comparativa entre clubes de Liga Femenina 2 - Grupo 'B'\n")
cat("0. Salir\n\n")
option <- readline("Selecciona opción: ")
option <- suppressWarnings(as.numeric(option))
if (!is.na(option)){
if (option == 1){
object#competition <- 14
}
if (option == 2){
object#competition <- 22
}
if (option == 3){
object#competition <- 23
}
readline("Espera ...")
if (option == 0)
break
}else{
readline("No es un número. Pulsa una tecla para introducir otra opción.")
}
}
})
main.R
menu(competition=0, stats=0)
print(menu#competition)
getClass(class(menu))
When I call menu(competition=0, stats=0) I can see what the method show gives me to me. This is correct. In show method I assign a value to competition. When I exit from show method the next instruction is print(menu#competition) and here is where I've got this error:
Error in print(menu#competition) : there is no a slot with name
"competition" for this object class "classGeneratorFunction"
Then with getClass(class(menu)) I've got this:
What am I doing wrong? How can I get access to competition or stats?

Answer 1

You are confusing the object constructor with the object itself.
menu(competition = 0, stats=0) generates you a new object of class menu, but you fail to save it somewhere, so it prints on the screen. Therefore your first, correct output.
But then, you want to manipulate the object. But you didn't save it! Instead, you try to manipulate the "object factory", menu(). The Type of the "object factory" is classGeneratorFunction, that's what you see.
This should work:
myMenuObject <- menu(competition=0, stats=0)
print(myMenuObject)
print(myMenuObject#competition)
getClass(class(myMenuObject))