R help - Jun 2006 - Help needed understanding eval,quote,expression

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 !








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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, toby_marks at americancentury.com 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 !
>
>
>
>
>
>
>
>
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> CONFIDENTIALITY NOTICE: This electronic mail transmission (i...{{dropped}}
>
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
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