R help - Mar 2014 - From Strings to Variable name. When get does not work

Hi all,
I would like to turn some long strings like MyString$Myfield$MySubfield into
variables but it looks like that the get does not like lists



so for example:

 test<-list(a=2) 

test 
>$a 
[1] 2 


get("test") 
>$a 
[1] 2 


get("test$a") 
>Fehler in get("test$a") : Objekt 'test$a' nicht gefunden 
lapply(test,function(x) return (x$a)) 
>Fehler in x$a : $ operator is invalid for atomic vectors 
What should I do to read those lists I have as strings?

I would like to thank you in advance for your reply

regards
A

	[[alternative HTML version deleted]]

On 12/03/2014 07:42, Alaios wrote:
> Hi all,
> I would like to turn some long strings like MyString$Myfield$MySubfield
into variables but it looks like that the get does not like lists
Those are expressions not names, so you need to parse them.

 > eval(parse(text = "test$a"))
[1] 2
>
>
> so for example:
>
>   test<-list(a=2)
>
> test
>> $a
> [1] 2
>
>
> get("test")
>> $a
> [1] 2
>
>
> get("test$a")
>> Fehler in get("test$a") : Objekt 'test$a' nicht
gefunden
>
> lapply(test,function(x) return (x$a))
>> Fehler in x$a : $ operator is invalid for atomic vectors
>
> What should I do to read those lists I have as strings?
>
> I would like to thank you in advance for your reply
>
> regards
> A
>
> 	[[alternative HTML version deleted]]
>
>
>
> ______________________________________________
> 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.
>

-- 
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

You can use get to grab the object, then subset it:
> test <- list(a=2)
> get('test')[['a']]
[1] 2

This way you can even have the variable names in other variables:
> whichvar <- 'a'
> whichlist <- 'test'
> get(whichlist)[[whichvar]]
[1] 2

Even better would be to have any lists that you want to "get" in an
array or environment rather than just sitting in the global
environment so that you can access them using [[]] rather than needing
the get function.

If you have nested arrays (your syntax with multiple $) then you can
just create a vector with the path you want to follow:
> test2 <- list( a=list(b=list(c=5)) )
> whichvars <- c('a','b','c')
> test2[[whichvars]]
[1] 5


The reason you received an error is that "get" takes it argument
literally (does not parse), so it was looking for a variable named
`test$a` rather than the component named "a" in the variable named
"test" which is how it is parsed from the command line.

On Wed, Mar 12, 2014 at 1:42 AM, Alaios <alaios at yahoo.com>
wrote:
> Hi all,
> I would like to turn some long strings like MyString$Myfield$MySubfield
into variables but it looks like that the get does not like lists
>
>
>
> so for example:
>
>  test<-list(a=2)
>
> test
>>$a
> [1] 2
>
>
> get("test")
>>$a
> [1] 2
>
>
> get("test$a")
>>Fehler in get("test$a") : Objekt 'test$a' nicht
gefunden
>
> lapply(test,function(x) return (x$a))
>>Fehler in x$a : $ operator is invalid for atomic vectors
>
> What should I do to read those lists I have as strings?
>
> I would like to thank you in advance for your reply
>
> regards
> A
>
>         [[alternative HTML version deleted]]
>
>
> ______________________________________________
> 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.
>


-- 
Gregory (Greg) L. Snow Ph.D.
538280 at gmail.com