R help - Feb 2013 - different behavior of $ with string literal vs string variable as argument

Hi everyone,

I ran into the issue below while trying to execute a command of the form

apply(list.names,1, function(x)  F(favorite.list$x) )

where list.names is a character vector containing the names of the elements
of favorite.list and F is some function defined on a list element.

Namely,  the $ operator doesn't treat the string variable 'x' as the
string
it represents, so that, e.g.
> ll <- list(ss="abc")
> ll$ss
[1] "abc"
> ll$"ss"
[1] "abc"

but
> name <- "ss"
> ll$name
NULL

I can get around this by using integers and the [[ and [ operators, but I'd
like to be able to use names directly, too -- how would I go about doing
this?

Thanks for your help in clarifying what might be going on here.

David

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On Sun, Feb 10, 2013 at 9:06 PM, David Romano <dromano at stanford.edu>
wrote:
> Hi everyone,
>
> I ran into the issue below while trying to execute a command of the form
>
> apply(list.names,1, function(x)  F(favorite.list$x) )
>
> where list.names is a character vector containing the names of the elements
> of favorite.list and F is some function defined on a list element.
>
> Namely,  the $ operator doesn't treat the string variable 'x'
as the string
> it represents, so that, e.g.
>
>> ll <- list(ss="abc")
>> ll$ss
> [1] "abc"
>> ll$"ss"
> [1] "abc"
>
> but
>
>> name <- "ss"
>> ll$name
> NULL
>
> I can get around this by using integers and the [[ and [ operators, but
I'd
> like to be able to use names directly, too -- how would I go about doing
> this?
>
You've already found it: the bracket operators....

MW
> Thanks for your help in clarifying what might be going on here.
>
> David
>
>         [[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.

On 13-02-10 4:06 PM, David Romano wrote:
> Hi everyone,
>
> I ran into the issue below while trying to execute a command of the form
>
> apply(list.names,1, function(x)  F(favorite.list$x) )
>
> where list.names is a character vector containing the names of the elements
> of favorite.list and F is some function defined on a list element.
>
> Namely,  the $ operator doesn't treat the string variable 'x'
as the string
> it represents, so that, e.g.
>
>> ll <- list(ss="abc")
>> ll$ss
> [1] "abc"
>> ll$"ss"
> [1] "abc"
>
> but
>
>> name <- "ss"
>> ll$name
> NULL
>
> I can get around this by using integers and the [[ and [ operators, but
I'd
> like to be able to use names directly, too -- how would I go about doing
> this?
>
> Thanks for your help in clarifying what might be going on here.
You can use names with [[, e.g.

ll[[name]]

will work in your example.  You can see more details in the help topic 
help("$"), in the section "Recursive (list-like) objects".

Duncan Murdoch

Please read the Help before posting.

?"$" says:

"Both [[ and $ select a single element of the list. The main
difference is that $ **does not allow computed indices** , whereas [[
does. x$name is equivalent to x[["name", exact = FALSE]]. Also, the
partial matching behavior of [[ can be controlled using the exact
argument. " [emphasis added]

In other words, $ does not evaluate its argument.

This also appeared just a couple of days ago on this list, so please
also search Help archives before posting.

-- Bert

On Sun, Feb 10, 2013 at 1:06 PM, David Romano <dromano at stanford.edu>
wrote:
> Hi everyone,
>
> I ran into the issue below while trying to execute a command of the form
>
> apply(list.names,1, function(x)  F(favorite.list$x) )
>
> where list.names is a character vector containing the names of the elements
> of favorite.list and F is some function defined on a list element.
>
> Namely,  the $ operator doesn't treat the string variable 'x'
as the string
> it represents, so that, e.g.
>
>> ll <- list(ss="abc")
>> ll$ss
> [1] "abc"
>> ll$"ss"
> [1] "abc"
>
> but
>
>> name <- "ss"
>> ll$name
> NULL
>
> I can get around this by using integers and the [[ and [ operators, but
I'd
> like to be able to use names directly, too -- how would I go about doing
> this?
>
> Thanks for your help in clarifying what might be going on here.
>
> David
>
>         [[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.


-- 

Bert Gunter
Genentech Nonclinical Biostatistics

Internal Contact Info:
Phone: 467-7374
Website:
http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm

Sorry, this was meant to go to the full list.  -David

On Sun, Feb 10, 2013 at 2:15 PM, David Romano <dromano@stanford.edu>
wrote:
>
>
> On Sun, Feb 10, 2013 at 1:59 PM, Bert Gunter
<gunter.berton@gene.com>wrote:
>
>> Please read the Help before posting.
>>
>> ?"$" says:
>>
>
> It helps to know that $ must be quoted, so thanks again goes to Duncan for
> pointing this out.
>
>
>>  "Both [[ and $ select a single element of the list. The main
>> difference is that $ **does not allow computed indices** , whereas [[
>> does. x$name is equivalent to x[["name", exact = FALSE]].
Also, the
>> partial matching behavior of [[ can be controlled using the exact
>> argument. " [emphasis added]
>>
>> In other words, $ does not evaluate its argument.
>>
>> This also appeared just a couple of days ago on this list, so please
>> also search Help archives before posting.
>>
>
> I did search, but as Ben points out in the next message in the thread,
> it's tricky to formulate the search to get hits, and, for example, I
> wouldn't have realized the post he refers to there involves the same
issue
> unless I already knew the answer.
>
> David
>
> -- Bert
>>
>> On Sun, Feb 10, 2013 at 1:06 PM, David Romano
<dromano@stanford.edu>
>> wrote:
>> > Hi everyone,
>> >
>> > I ran into the issue below while trying to execute a command of
the form
>> >
>> > apply(list.names,1, function(x)  F(favorite.list$x) )
>> >
>> > where list.names is a character vector containing the names of the
>> elements
>> > of favorite.list and F is some function defined on a list element.
>> >
>> > Namely,  the $ operator doesn't treat the string variable
'x' as the
>> string
>> > it represents, so that, e.g.
>> >
>> >> ll <- list(ss="abc")
>> >> ll$ss
>> > [1] "abc"
>> >> ll$"ss"
>> > [1] "abc"
>> >
>> > but
>> >
>> >> name <- "ss"
>> >> ll$name
>> > NULL
>> >
>> > I can get around this by using integers and the [[ and [
operators, but
>> I'd
>> > like to be able to use names directly, too -- how would I go about
doing
>> > this?
>> >
>> > Thanks for your help in clarifying what might be going on here.
>> >
>> > David
>> >
>> >         [[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.
>>
>>
>>
>> --
>>
>> Bert Gunter
>> Genentech Nonclinical Biostatistics
>>
>> Internal Contact Info:
>> Phone: 467-7374
>> Website:
>>
>>
http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm
>>
>
>
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