R help - Jun 2012 - Accessing named members of a list in an array

Dear List,

I've created a two-dimensional array which shall contain a value and its
error, respectively.
These two values are concatenated in al list and bear the names "sl"
and
"sl_err"

But I can't adress them using the $-notation.

a<- array(list(NULL),dim=c(2,2))
a[[1,1]]<- c(a=2,b=3)
a[[1,1]]$a
## Fehler in a[[1, 1]]$a : $ operator is invalid for atomic vectors
a[[1,1]]["a"]       # This works however.
## a
## 2

I always thought these two methods of indexing are equal? Is there any
way to use the $-Style indexing?

Thank you,
Moritz

-- 
GnuPG Key: 0x7340821E

On Jun 30, 2012, at 9:35 AM, mlell08 wrote:
> Dear List,
>
> I've created a two-dimensional array which shall contain a value and  
> its
> error, respectively.
> These two values are concatenated in al list and bear the names
"sl"
> and
> "sl_err"
>
> But I can't adress them using the $-notation.
>
> a<- array(list(NULL),dim=c(2,2))
> a[[1,1]]<- c(a=2,b=3)
> a[[1,1]]$a
> ## Fehler in a[[1, 1]]$a : $ operator is invalid for atomic vectors
> a[[1,1]]["a"]       # This works however.
> ## a
> ## 2
>
> I always thought these two methods of indexing are equal?
You thought wrong (on two accounts as it happens). The "$" methods  
translate to "[[" with a quoted argument and there is no matrix/array
equivalent since vectors loose their names (if they had any to begin  
with) when they are put into a matrix or array. The equivalent method  
to x$a is x[["a"]], not x["a"].
> Is there any
> way to use the $-Style indexing?
>
> Thank you,
> Moritz
>
> -- 
> GnuPG Key: 0x7340821E
>
> ______________________________________________
> 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.
David Winsemius, MD
West Hartford, CT

Hi,

You can use these as well to access named members.
> a[1]
[[1]]
a b 
2 3 


?>a[1][[1]][1]
a 
2 
> a[[1,1]][1]
a 
2 
 >a[[1,1]][2]
b 
3?

?>identical(a[[1,1]]["a"],a[[1,1]][1],a[1][[1]][1])
[1] TRUE

> a[[1,1]][["a"]]
[1] 2
> a[[1,1]][["b"]]
[1] 3

or,
> a[[c(1,2)]]
[1] 3
?>a[[c(1,1)]]
[1] 2

>a$ab<-a[[1]]
> a$ab
a b 
2 3 
?>a$ab[1]
a 
2 
>a$ab1<-a[[1]][1]
>a$ab1
a 
2 


Hope this will be of some use for you


A.K.



----- Original Message -----
From: mlell08 <mlell08 at gmail.com>
To: r-help at r-project.org
Cc: 
Sent: Saturday, June 30, 2012 9:35 AM
Subject: [R]  Accessing named members of a list in an array

Dear List,

I've created a two-dimensional array which shall contain a value and its
error, respectively.
These two values are concatenated in al list and bear the names "sl"
and
"sl_err"

But I can't adress them using the $-notation.

a<- array(list(NULL),dim=c(2,2))
a[[1,1]]<- c(a=2,b=3)
a[[1,1]]$a
## Fehler in a[[1, 1]]$a : $ operator is invalid for atomic vectors
a[[1,1]]["a"]? ? ?  # This works however.
## a
## 2

I always thought these two methods of indexing are equal? Is there any
way to use the $-Style indexing?

Thank you,
Moritz

-- 
GnuPG Key: 0x7340821E

______________________________________________
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.

Your problem is that you are trying to
use `$` on an atomic vector rather than
a list:

 > a<- array(list(NULL),dim=c(2,2))
 > a[[1,1]] <- c(a=2,b=3)
 > a[[1,1]]$a
Error in a[[1, 1]]$a : $ operator is invalid for atomic vectors
 > a[[1,1]]
a b
2 3
 > a[[1,1]] <- list(a=2,b=3)
 > a[[1,1]]$a
[1] 2
 > a[[1,1]]
$a
[1] 2

$b
[1] 3


 From the description of the problem, perhaps
it would be easier to just have a 3-dimensional
array.

Pat


On 30/06/2012 14:35, mlell08 wrote:
> Dear List,
>
> I've created a two-dimensional array which shall contain a value and
its
> error, respectively.
> These two values are concatenated in al list and bear the names
"sl" and
> "sl_err"
>
> But I can't adress them using the $-notation.
>
> a<- array(list(NULL),dim=c(2,2))
> a[[1,1]]<- c(a=2,b=3)
> a[[1,1]]$a
> ## Fehler in a[[1, 1]]$a : $ operator is invalid for atomic vectors
> a[[1,1]]["a"]       # This works however.
> ## a
> ## 2
>
> I always thought these two methods of indexing are equal? Is there any
> way to use the $-Style indexing?
>
> Thank you,
> Moritz
>
-- 
Patrick Burns
pburns at pburns.seanet.com
twitter: @portfolioprobe
http://www.portfolioprobe.com/blog
http://www.burns-stat.com
(home of 'Some hints for the R beginner'
and 'The R Inferno')