[R] subsetting with column name as string

[Prof Brian Ripley]

On Wed, 6 Aug 2008, Faheem Mitha wrote:

>
> Hi,
>
> Consider the following
>
>> x = c(1,2)
>> y = c(3,4)
>> d = data.frame(cbind(x,y))
>> d$x
> [1] 1 2
>> d$"x"
> [1] 1 2
>> 
>> foo = function(val)
> + {
> +   return(d$val)
> + }
>> 
>> bar = function()
> + {
> +   return(d$"x")
> + }
>> 
>> foo("x")
> NULL
>> bar()
> [1] 1 2
>
> I'm a little surprised that R accepts both the form d$x and d$"x", but I'm 
> mostly wondering why foo("x") doesn't work.

It does work: it returns d$val as instructed.  From the help

Usage:

      x$name

     name: A literal character string or a name (possibly backtick
           quoted).

or see the version in ?`$.data.frame`.

So you are 'a little surprised' that the function works as described on 
the help page!  Please explain to us how you can to that conclusion.

I think you are looking for [[ (on the same help page as $).

>                                                           Thanks, Faheem.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595