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
Boeretang 200, 2400 MOL, Belgium
Tel. + 32 14 33 58 22
Skype: laurent.franckx
laurent.fran...@vito.be
Visit our website: www.vito.be/english and http://www.vito.be/transport
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