[R] adding class attributes to strings: works in a loop, but not directly

[Franckx Laurent]

Dear Nello
Thanks for the clarification.
It was of course an elementary mistake to assume that the for loop would modify the original AONtptmodelist.
I also understand now that the issues was that I cannot attribute a class to a literal.
Your proposed alternative seems to work now, including the method dispatch.
Regards
Laurent


-----Original Message-----
From: Blaser Nello [mailto:nblaser at ispm.unibe.ch]
Sent: woensdag 29 mei 2013 15:56
To: Franckx Laurent; r-help at r-project.org
Subject: RE: [R] adding class attributes to strings: works in a loop, but not directly

There are two separate issues that seem to be unclear.
Concerning the class assignment: You cannot assign a class to a character string. You can assign a class to an object that contains a character string:

> a <- "b"
# ok
> class(a) <- "AONmode"
# not ok
> class("b") <- "AONmode"
Error in class("b") <- "AONmode" :
  target of assignment expands to non-language object

Concerning your for-loop: for (tptmode in AONtptmodelist) creates a new object tptmode and changes this. This will in no way alter your object AONtptmodelist. If you want to have the elements of AONtptmodelist have a different class, you could for instance create a new object newAONtptmodelist as follows:

newAONtptmodelist <- lapply(AONtptmodelist, function(x){
  class(x) <- "AONmode"
  return(x)
})

Best,
Nello

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Franckx Laurent
Sent: Mittwoch, 29. Mai 2013 13:49
To: 'r-help at r-project.org'
Subject: [R] adding class attributes to strings: works in a loop, but not directly

Dear all

I try to assign class attributes to strings. Depending on the approach I use, it works (including method dispatch) or fails.

Let me clarify with an example.

Let:

> AONtptmodelist <- c("FOOT","BICY")


When I assign "class" attributes directly to these strings, it fails:

> class("BICY") <- "AONmode"
Error in class("BICY") <- "AONmode" :
  target of assignment expands to non-language object

However, when I assign the attributes in a loop, it does work:

> for(tptmode in AONtptmodelist) {
+ class(tptmode) <- "AONmode"
+ cat("The value of is.object(tptmode) for " , tptmode , "is: " ,
+ is.object(tptmode) , ".\n") cat("The class of " , tptmode , "is: " ,
+ class(tptmode) , ".\n") }
The value of is.object(tptmode) for  FOOT is:  TRUE .
The class of  FOOT is:  AONmode .
The value of is.object(tptmode) for  BICY is:  TRUE .
The class of  BICY is:  AONmode .

Moreover, within this loop, method dispatch works correctly.

However, when I start a new loop over the same vector, I get:

> for(tptmode in AONtptmodelist) {
+ cat("The value of is.object(tptmode) for " , tptmode , "is: " ,
+ is.object(tptmode) , ".\n") cat("The class of " , tptmode , "is: " ,
+ class(tptmode) , ".\n") }
The value of is.object(tptmode) for  FOOT is:  FALSE .
The class of  FOOT is:  character .
The value of is.object(tptmode) for  BICY is:  FALSE .
The class of  BICY is:  character .


I find this troublesome for two reasons. First, I do not understand the difference between the two approaches. Why can I assign a class attribute to a string when it is called in a loop, but not directly? And why is the class attribution not permanent? Second, the problem was concealed until now precisely because I assigned the attributes in a link. However, I would like to centralise my class definition in one single place in my code, thus outside the loop where the methods are called.



Laurent Franckx, PhD
VITO NV
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Tel. + 32 14 33 58 22
Skype: laurent.franckx
laurent.franckx at vito.be
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