Great help, & thanks to both of you. I actually think I get it.
I think I was locked into eval(expression ... ) as the solution. I did
search the archives for this question, but it must not have clicked with
me. The Thomas Lumley R-help (February 2005) was on the money. I was
missing the power & flexibility of the [[ operation. It is certainly more
direct.
I do believe this pattern will satisfy my original problem.
> x = list(y=list(y1="hello",y2="world"),z=list(z1="foo",z2="bar"))
> ids = list( zIds=c("z1","z2"),yIds=c("y1","y2"))
> str="y1"
> x$y[[str]]
[1] "hello"
btw: I set my prompt to $, so the first post, the $ at the beginning of
the line was the prompt. apologies for the confusion.
cheers.
Prof Brian Ripley <[EMAIL PROTECTED]>
06/29/2006 04:06 AM
To
Joerg van den Hoff <[EMAIL PROTECTED]>
cc
[EMAIL PROTECTED], [email protected]
Subject
Re: [R] Help needed understanding eval,quote,expression
On Thu, 29 Jun 2006, Joerg van den Hoff wrote:
> Prof Brian Ripley wrote:
>> You are missing eval(parse(text=)). E.g.
>>
>>> x <- list(y=list(y1="hello",y2="world"),z=list(z1="foo",z2="bar"))
>> (what do you mean by the $ at the start of these lines?)
>>> eval(parse(text="x$y$y1"))
>> [1] "hello"
>>
>> However, bear in mind
>>
>>> fortune("parse")
>>
>> If the answer is parse() you should usually rethink the question.
>> -- Thomas Lumley
>> R-help (February 2005)
>>
>> In your indicated example you could probably use substitute() as
>> effectively.
>>
>>
>> On Wed, 28 Jun 2006, [EMAIL PROTECTED] wrote:
>>
>>> I am trying to build up a quoted or character expression representing
a
>>> component in a list in order to reference it indirectly.
>>> For instance, I have a list that has data I want to pull, and another
list
>>> that has character vectors and/or lists of characters containing the
names
>>> of the components in the first list.
>>>
>>> It seems that the way to do this is as evaluating expressions, but I
seem
>>> to be missing something. The concept should be similar to the snippet
>>> below:
>>>
>>>
>>> For instance:
>>>
>>> $x = list(y=list(y1="hello",y2="world"),z=list(z1="foo",z2="bar"))
>>> $y = quote(x$y$y1)
>>> $eval(y)
>>> [1] "hello"
>>>
>>>
>>> but, I'm trying to accomplish this by building up y as a character and
>>> then evaluating it, and having no success.
>>>
>>> $y1=paste("x$y$","y1",sep="")
>>> $y1
>>> [1] "x$y$y1"
>>>
>>>
>>> How can I evaluate y1 as I did with y previously? or can I?
>>>
>>>
>>> Much Thanks !
>>>
>>>
>
> if I understand you correctly you can achieve your goal much easier than
with
> eval, parse, substitute and the like:
>
> x <- list(y=list(y1="hello",y2="world"),z=list(z1="foo",z2="bar"))
>
> s1 <- 'y'
> s2 <- 'y1'
>
> x[[s1]][[s2]]
>
> i.e. using `[[' instead of `$' for list component extraction allows to
use
> characters for indexing (in other words: x$y == x[['y']])
But what he actually asked for was
>>> I am trying to build up a quoted or character expression representing
a
>>> component in a list in order to reference it indirectly.
You just typed in x[[s1]][[s2]], not 'built [it] up'. Suppose the
specification had been
r <- "x"
s <- c("y", "y1")
and s was of variable length? Then you need to construct a call similar
to x[["y"]][["y1"]] from r and s.
[There was another reason for sticking with $ rather than using [[: the
latter makes unnecessary copies in released versions of R.]
--
Brian D. Ripley, [EMAIL PROTECTED]
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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