[Rd] Differences between "$" and "[["

[Gabor Grothendieck]

Gabor Grothendieck <ggrothendieck <at> myway.com> writes:

: 
: Eric Lecoutre <lecoutre <at> stat.ucl.ac.be> writes:
: 
: : 
: : Hi,
: : 
: : If I define the following list:
: : 
: :  > (l<-list("text-align"="right"))
: : $"text-align"
: : [1] "right"
: : 
: : I know that I can't use l$text-align, as the parser will find a '-' 
: operation.
: : If I want (need) to use special names, as "text-align", I have to enclose 
: : it between "". So I can use:
: : 
: : l$"text-align" or l[["text-align"]]
: : 
: : If now I have the text "text-align" defined in a variable:
: : p<-"text-align"
: : 
: : I can use:
: :  > l[[p]]
: : [1] "right"
: : 
: : But I can't use l$p
: : 
: : where as it is said in the help page that 'x$name' is equivalent to 
: : 'x[["name"]]'.
: : 
: : Anyway I will use "[[" but I dont clearly understand this behavior.
: 
: [[ evaluates its right argument and $ does not.  The "..." notation is
: just to allow one to specify non-syntactic arguments.  One could
: alternately use l$`text-align` .  I think the "..." notation
: is a holdover from before `...` was implemented.
: 
: Its also possible to define your own class and have $ operate any
: way you like on it (although its probably best to stick with
: standard behavior and the following is not really recommended):
: 
: 	l<-list("text-align"="right", a=2)
: 	class(l) <- c("mylist", "list")
: 	"$.mylist" <- function(x, idx) { 
: 	     y <- x[[idx]]
: 	     if (is.null(y)) x[[eval.parent(parse(text=idx))]] else y
: 	}
: 	p <- "text-align"
: 	l$p # "right"
: 	l$"text-align" # same
: 	l$`text-align` # same
: 	a <- 99
: 	l$a # 2
: 	l$"a" # same
: 	l$`a` # same
: 	l[["a"]] # same
: 	l[[a]] # 99

The comment on the last one is wrong.  It should give an error since
there is no 99th component to l.