On Mon, Feb 15, 2010 at 5:07 PM, blue sky <bluesky...@gmail.com> wrote:
>> x=3
>> `class<-`(x,'something')#this command prints
> [1] 3
> attr(,"class")
> [1] "something"
>> x=3
>> class(x)='something'#this command doesn't print anything
>
> The first of the above two commands print the content of 'x' but the
> second doesn't, although both of them set the argument 'x'. I'm
> wondering which is method is called in the latter one.
>
The same thing is called. It's not a question of the method doing the
printing - the printing is done by the R interpreter when it finishes
evaluating your line.
Normally evaluations (eg sqrt(2)) print out but if you wrap them in
'invisible' they dont - try "invisible(sqrt(2))".
All assignments have their invisibility set when run interactively:
> x=1:10
> dim(x)=c(2,5)
> x
see how nothing is printed, either at the 'x=1:10' or the
'dim(x)=...'? Calling the assignment function directly returns a value
without the invisibility cloak, which is what you want when typing
'sqrt(2)':
> x=1:10
> `dim<-`(x,c(2,5))
[,1] [,2] [,3] [,4] [,5]
[1,] 1 3 5 7 9
[2,] 2 4 6 8 10
I suspect this behaviour is specified somewhere in the R parser. But
be honest, why does it matter? You would quickly get irritated by R
telling you what you just did every time you typed an assignment:
> x=1:10
[1] 1 2 3 4 5 6 7 8 9 10
Barry
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