R help - Aug 2009 - how to index a list with a string?

Hi, everybody. I have a list obj L, sth like

$`aaa`
[1] "5753"

if the string 'aaa' is a returned value of a function foo(). what is
the right syntax form of L$foo()

I'm new to R, thanks in advance.

Tena koe

Try either
L[foo()]
or
L[[foo()]]

These return subtly (or not so subtly depending on your point of view)
different results.

HTH ....

Peter Alspach
> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Ning Ma
> Sent: Thursday, 27 August 2009 3:35 p.m.
> To: r-help at r-project.org
> Subject: [R] how to index a list with a string?
> 
> Hi, everybody. I have a list obj L, sth like
> 
> $`aaa`
> [1] "5753"
> 
> if the string 'aaa' is a returned value of a function foo(). 
> what is the right syntax form of L$foo()
> 
> I'm new to R, thanks in advance.
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide 
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

On Aug 26, 2009, at 11:35 PM, Ning Ma wrote:
> Hi, everybody. I have a list obj L, sth like
>
> $`aaa`
> [1] "5753"
>
> if the string 'aaa' is a returned value of a function foo(). what
is
> the right syntax form of L$foo()
What do you really want to achieve or to learn? Pretty much anything  
ending in () will be seen as a function and it does not appear that  
you have a function inside L.

 > L <-list()
 > L$aaa <- 5753
 > L$aaa <- "5753"
 > L
$aaa
[1] "5753"

Note that aaa, which is a valid object name, does not have surrounding  
back-quotes as in your question. And even if you created an aaa  
portion of L using backquotes they would not be displayed that way:

 > L <-list()
 > L$`aaa` <- "5753"
 > L
$aaa
[1] "5753"

If you constructed an invalid object name within back-quotes you might  
see them later:

 > L <-list()
 > L$`5753` <- "5753"
 > L
$`5753`
[1] "5753"

So what is your real question and what does str(L) report?

-- 

David Winsemius, MD
Heritage Laboratories
West Hartford, CT