[R] S4; Setter function is not chaning slot value as expected

[Martin Morgan]

On 11/10/2013 03:54 AM, daniel schnaider wrote:
> Thanks Martin. It worked well.
> Two new questions related to the same subject.
>
> 1) Why create this semantic of a final argument name specifically names value?

I do not know. It is a requirement of replacement methods in R in general, not 
just S4 methods. See section 3.4.4 of RShowDoc("R-lang").

> 2) Regarding performance. When CustomerID(ac) <- "54321" runs, does it only
> change the slot from whatever it was to 54321, or it really create another
> object and change all the value of all slots, keeping technically all the other
> values equal and changing 54321?

Copying is tricky in R. It behaves as though a copy has been made of the entire 
object. Whether a copy is actually made, or just marked as necessary on 
subsequent modification, requires deep consideration of the code. This is the 
way R works, not just the way S4 classes work.

If instead of a single account you modelled 'Accounts', i.e., all accounts, then 
updating 1000 account id's would only make one copy, whereas if you model each 
account separately this would require 1000 copies.

Martin

>
> thanks..
>
>
> On Sat, Nov 9, 2013 at 4:20 PM, Martin Morgan <mtmorgan at fhcrc.org
> <mailto:mtmorgan at fhcrc.org>> wrote:
>
>     On 11/09/2013 06:31 AM, daniel schnaider wrote:
>
>         It is my first time programming with S4 and I can't get the setter fuction
>         to actually change the value of the slot created by the constructor.
>
>         I guess it has to do with local copy, global copy, etc. of the variable -
>         but, I could't find anything relevant in documentation.
>
>         Tried to copy examples from the internet, but they had the same problem.
>
>         # The code
>               setClass ("Account" ,
>                          representation (
>                          customer_id = "character",
>                          transactions = "matrix")
>               )
>
>
>               Account <- function(id, t) {
>                       new("Account", customer_id = id, transactions = t)
>                       }
>
>
>               setGeneric ("CustomerID<-", function(obj,
>         id){standardGeneric("__CustomerID<-")})
>
>
>     Replacement methods (in R in general) require that the final argument (the
>     replacement value) be named 'value', so
>
>          setGeneric("CustomerID<-",
>              function(x, ..., value) standardGeneric("CustomerID"))
>
>          setReplaceMethod("CustomerID", c("Account", "character"),
>              function(x, ...., value)
>          {
>              x at customer_id <- value
>              x
>          })
>
>     use this as
>
>         CustomerID(ac) <- "54321"
>
>
>
>               setReplaceMethod("CustomerID", "Account", function(obj, id){
>                   obj at customer_id <- id
>                   obj
>                   })
>
>                   ac <- Account("12345", matrix(c(1,2,3,4,5,6), ncol=2))
>                   ac
>                   CustomerID <- "54321"
>                   ac
>
>         #Output
>                   > ac
>                   An object of class "Account"
>                   Slot "customer_id":
>                   [1] "12345"
>
>                   Slot "transactions":
>                        [,1] [,2]
>                   [1,]    1    4
>                   [2,]    2    5
>                   [3,]    3    6
>
>         # CustomerID is value has changed to 54321, but as you can see it does't
>                   > CustomerID <- "54321"
>
>
>                   > ac
>                   An object of class "Account"
>                   Slot "customer_id":
>                   [1] "12345"
>
>                   Slot "transactions":
>                        [,1] [,2]
>                   [1,]    1    4
>                   [2,]    2    5
>                   [3,]    3    6
>
>
>         Help!
>
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>
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> Daniel Schnaider
>
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