[SOLVED] Accessing named members of a list in an array

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"].

On 01.07.2012 08:45, Patrick Burns wrote:

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

Hello,

thank you for all your answers!
I found out that I can circumvent the Problem "atomic vector" by
making the object which I extracted a list by

class(a[[1,1]]) <- "list"

After all, it seems a bit unlogical to me, because
1) c(a=2, b=3)$a works as expected

2) I've created a matrix containing lists and not atomic vectors by
  a<- array(list(NULL),dim=c(2,2))
  however, my added element seems to be converted to an atomic vector
  - without need because

3) I avoided vectorization (thanks for the link to R inferno! Very apt
  comparison;-)) by using the [[ operator for assignment
  (which seemed to work because else more than one matrix element
 would have been filled) so there should be no need for dropping names
  (which actually doesn't happen, because a[[1,1]]["a"] works

but if the operator [[<- works like this, there is no point in
argueing?

So - what works:

a<- array(list(NULL),dim=c(2,2))
a[[1,1]]<- c(a=2,b=3)
a[[1,1]]
## a b
## 2 3

a[[1,1]]$a
## Fehler in a[[1, 1]]$a : $ operator is invalid for atomic vectors

#Synonym??
a[[1,1]][["a",exact=FALSE]]
## 2

a[[1,1]]["a"]
## a
## 2

class(a[[1,1]])<- "list"
a[[1,1]]$a
## 2

Regards, Moritz

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