R help - Mar 2006 - lapply and list attributes

Hi

I have a list that has attributes:

attributes(lis[2])
$names
[1] "150096_at"

I want to use those attributes in a function and then use lapply to
apply that function to every element of the list, eg for simplicity's
sake:

my.fun <- function(x) {
        attributes(x)
}

Then

l2 <- lapply(lis, my.fun)

It seems that "attributes(x)" within the function is not the same as
eg.
"attributes(lis[2])" used outside of the function, ie the attributes
have changed.

Help for someone who's trying to understand lists?

Many thanks
Mick 


	[[alternative HTML version deleted]]

Hi,

"michael watson (IAH-C)" <michael.watson at bbsrc.ac.uk>
writes:
> I have a list that has attributes:
>
> attributes(lis[2])
> $names
> [1] "150096_at"
>
> I want to use those attributes in a function and then use lapply to
> apply that function to every element of the list, eg for simplicity's
> sake:
>
> my.fun <- function(x) {
>         attributes(x)
> }
This gets tricky.  You said the right thing: you have a _list_ with
attributes.  This does not mean the elements of the list have
attributes (unless they happen to). 
> It seems that "attributes(x)" within the function is not the same
as eg.
> "attributes(lis[2])" used outside of the function, ie the
attributes
> have changed.
"[" on a list returns a sublist (with names copied over).  The lapply
function is equivalent to accessing elements via "[[" which pulls out
a single element (no names).

For the specific case of processing a list with names, you may want:

for (name in names(x)) {
   el <- x[[name]]
   ...
}

You could also try mapply.

+ seth

michael watson (IAH-C) wrote:
> Hi
> 
> I have a list that has attributes:
> 
> attributes(lis[2])
> $names
> [1] "150096_at"

Here you get the attributes of "lis", while you will get the
attributes
of the *elements* of "lis" when applying the functions below, the
lattes
is comparable with
  attributes(lis[[2]])

Uwe Ligges


> I want to use those attributes in a function and then use lapply to
> apply that function to every element of the list, eg for simplicity's
> sake:
> 
> my.fun <- function(x) {
>         attributes(x)
> }
> 
> Then
> 
> l2 <- lapply(lis, my.fun)
> 
> It seems that "attributes(x)" within the function is not the same
as eg.
> "attributes(lis[2])" used outside of the function, ie the
attributes
> have changed.
> 
> Help for someone who's trying to understand lists?
> 
> Many thanks
> Mick 
> 
> 
> 	[[alternative HTML version deleted]]
> 
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