[R] Help with first S3-class

[Duncan Murdoch]

On 02/01/2018 6:38 PM, Martin Møller Skarbiniks Pedersen wrote:
> Hi,
> 
>    I am trying to understand S3 classes. I have read several tutorials about
> the topics but I am still a bit confused. I guess it is because it is
> so different from
> Java OOP.

What you do below isn't S3.  S3 is a system where the classes are 
secondary to the generic functions.  Methods "belong" to generics, they 
don't belong to classes.  As far as I can see, you hardly make use of S3 
methods and generics at all.

For the style you're using, the R6 or R.oo packages might be more suitable.

> 
>    I have pasted my attempt at creating a bank-account class below and
> my problems are:
> 
> 1. What should be added some plot.default() calls the account$plot() method ?

No, you need to define a function called "plot.account" whose signature 
is compatible with plot, i.e. arguments (x, y, ...), possibly with extra 
parameters as well.  It should do the plotting.

> 2. What should the account$plot() be implemented to show some kind of plot ?

The plot.account function could call account$plot(); it can do whatever 
you want to plot the object.

> 3. How can one function inside "me"-list called another function. Eg.
> How can account$plot() call account$pastSaldo() ?

There's no particular support for having them see each other the way 
you've defined them.  A better way to define them would be as local 
functions within the account body; then they'd be able to see each other 
without any prefix.

Duncan Murdoch
> 
> Here are my code. Feel free to comment on any aspect of the code.
> ps. I hope I have manage to strip HTML.
> 
> Regards
> Martin
> 
> 
> 
> account <- function(owner = NULL) {
>      thisEnv <- environment()
> 
>      pastSaldo <- vector()
>      saldo <- 0
>      owner <- owner
> 
>      me <- list(
>      thisEnv = thisEnv,
> 
>           getEnv = function() { return(get("thisEnv", thisEnv)) },
> 
>           balance = function() { return(get("saldo", thisEnv)) },
> 
>          deposit = function(value) {
>              assign("pastSaldo", append(pastSaldo, saldo), thisEnv)
>              assign("saldo", saldo + value, thisEnv)
>          },
> 
>          withdraw = function(value) {
>             assign("pastSaldo", append(pastSaldo, saldo), thisEnv)
>             assign("saldo", saldo - value, thisEnv)
>          },
> 
>         pastSaldo = function() {
>            return(pastSaldo)
>        },
> 
>        plot = function() {
>           plot.new()
>           lines(list(y = pastSaldo, x = seq_along(pastSaldo)))
>         }
>      )
>      assign('this', me, envir = thisEnv)
> 
>      class(me) <- append(class(me), "account")
>      return(me)
> }
> 
> FirstAccount <- account("Martin")
> FirstAccount$deposit(100)
> FirstAccount$withdraw(50)
> FirstAccount$deposit(200)
> FirstAccount$balance()
> FirstAccount$pastSaldo()
> 
> FirstAccount$plot()
> 
> plot(FirstAccount)                            # fails
> plot.account(FirstAccount)               # fails
> 
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