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

From: Gabor Grothendieck <ggrothendieck_at_myway.com>
Date: Tue 30 Nov 2004 - 02:48:16 EST

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.



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https://stat.ethz.ch/mailman/listinfo/r-devel Received on Tue Nov 30 02:56:36 2004

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