[R] different behavior of $ with string literal vs string variable as argument

[Bert Gunter]

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]]
>
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-- 

Bert Gunter
Genentech Nonclinical Biostatistics

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