[Rd] Differences between "$" and "[["
p.dalgaard at biostat.ku.dk
Mon Nov 29 16:19:24 CET 2004
Eric Lecoutre <lecoutre at stat.ucl.ac.be> writes:
> If I define the following list:
> > (l<-list("text-align"="right"))
>  "right"
> I know that I can't use l$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$"text-align" or l[["text-align"]]
> If now I have the text "text-align" defined in a variable:
> I can use:
> > l[[p]]
>  "right"
> But I can't use l$p
> where as it is said in the help page that 'x$name' is equivalent to
> Anyway I will use "[[" but I dont clearly understand this behavior.
It also says
The operators '$' and '$<-' do not evaluate their second argument.
It is translated to a string and that string is used to locate
the correct component of the first argument.
so l$p looks for l[["p"]] since the p is treated as a symbol. The fact
that there is an object called p and that it contains a special name
is immaterial (the opposite would be truly scary...).
O__ ---- Peter Dalgaard Blegdamsvej 3
c/ /'_ --- Dept. of Biostatistics 2200 Cph. N
(*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk) FAX: (+45) 35327907
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