R help - Jan 2011 - get list element names within lapply / sapply call

Hi all,

I would like to iterate through a list with named elements and access the
names within an lapply / sapply call. One way to do this is iterate through
the names and index the list with the name.  Is there a way to iterate
through the list elements themselves and access the element names within in
the function? For example,

mylist <-
list("a"=c(1,2),"b"=c(3,4),"c"=c(5,6))
sapply(mylist,function(x){
  #get name of list elements ("a", "b", "c")
  #then do other stuff
})

Thanks for your suggestions.

Jonathan

	[[alternative HTML version deleted]]

I one tried to write a function to do that, but it wasn't worth it /
didn't
work

I found this to be a better solution:
mynames = names(sapply(mylist, names))
for(nm in mynames){
    print(mylist[nm])
    # or "do other stuff"
}

You can use "browser" to look inside sapply, and the objects available
don't
seem to have the current index information, but I don't know how to look at
the ...
> sapply(mylist, browser)
Called from: lapply(X, FUN, ...)
Browse[1]> ls(all.names=TRUE)
[1] "..." "FUN" "X"
Browse[1]> FUN
function (text = "", condition = NULL, expr = TRUE, skipCalls = 0L)
.Primitive("browser")
Browse[1]> X
$a
[1] 1 2

$b
[1] 3 4

$c
[1] 5 6

Browse[1]>


On Sat, Jan 15, 2011 at 3:06 PM, Jonathan Flowers <
jonathanmflowers@gmail.com> wrote:
> Hi all,
>
> I would like to iterate through a list with named elements and access the
> names within an lapply / sapply call. One way to do this is iterate through
> the names and index the list with the name.  Is there a way to iterate
> through the list elements themselves and access the element names within in
> the function? For example,
>
> mylist <-
list("a"=c(1,2),"b"=c(3,4),"c"=c(5,6))
> sapply(mylist,function(x){
>  #get name of list elements ("a", "b", "c")
>  #then do other stuff
> })
>
> Thanks for your suggestions.
>
> Jonathan
>
>        [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help@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.
>
	[[alternative HTML version deleted]]

On Jan 15, 2011, at 4:06 PM, Jonathan Flowers wrote:
> Hi all,
>
> I would like to iterate through a list with named elements and  
> access the
> names within an lapply / sapply call. One way to do this is iterate  
> through
> the names and index the list with the name.  Is there a way to iterate
> through the list elements themselves and access the element names  
> within in
> the function? For example,
>
> mylist <-
list("a"=c(1,2),"b"=c(3,4),"c"=c(5,6))
> sapply(mylist,function(x){
>  #get name of list elements ("a", "b", "c")
>  #then do other stuff
> })
You can pass this result to "[[":
 > names(sapply(mylist, names))
[1] "a" "b" "c"

 > sapply( names(sapply(mylist, names)) , function(x) mylist[[x]]^2)
      a  b  c
[1,] 1  9 25
[2,] 4 16 36

>
> Thanks for your suggestions.
>
> Jonathan
>
> 	[[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.
David Winsemius, MD
West Hartford, CT

On Jan 15, 2011, at 5:37 PM, David Winsemius wrote:
>
> On Jan 15, 2011, at 4:06 PM, Jonathan Flowers wrote:
>
>> Hi all,
>>
>> I would like to iterate through a list with named elements and  
>> access the
>> names within an lapply / sapply call. One way to do this is iterate  
>> through
>> the names and index the list with the name.  Is there a way to  
>> iterate
>> through the list elements themselves and access the element names  
>> within in
>> the function? For example,
>>
>> mylist <-
list("a"=c(1,2),"b"=c(3,4),"c"=c(5,6))
>> sapply(mylist,function(x){
>> #get name of list elements ("a", "b",
"c")
>> #then do other stuff
>> })
>
> You can pass this result to "[[":
> > names(sapply(mylist, names))
> [1] "a" "b" "c"
>
> > sapply( names(sapply(mylist, names)) , function(x) mylist[[x]]^2)
>     a  b  c
> [1,] 1  9 25
> [2,] 4 16 36
Does seem the long way around, though:
 > sapply( mylist, function(x) x^2)
      a  b  c
[1,] 1  9 25
[2,] 4 16 36
>
>
>>
>> Thanks for your suggestions.
>>
>> Jonathan
>>
>> 	[[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.
>
> David Winsemius, MD
> West Hartford, CT
>
> ______________________________________________
> 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