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
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