Bug in subsetting data frame (PR#13515)
Stavros Macrakis wrote:
On Tue, Feb 10, 2009 at 8:31 AM, Duncan Murdoch <murdoch at stats.uwo.ca>wrote:
Stavros Macrakis wrote:
It is a bad idea to set data.frame <- xxx since R has a single
namespace for functions and variables.
That's not quite accurate. R mixes functions and variables in namespaces,
but it has lots of namespaces, and it usually doesn't cause much trouble to
have a variable named data.frame in one, and a function named data.frame in
another. The evaluator recognizes the context of usage and will get the
function for a function call. If you retrieve data.frame without doing a
function call, you'll get whichever one it finds first, which is typically
the one in the global environment, as below.
Interesting. I was not aware of this rule. Can you point me to chapter and verse in the language definition ( http://cran.r-project.org/doc/manuals/R-lang.html)? I couldn't find it, but I probably looked in the wrong place.
No, I'm also unable to find it.
| Sometimes this causes trouble (e.g. if you passed data.frame to apply or do.call), but usually not, ... Indeed! It is quite surprising that functions are defined using ordinary assignment, but function lookup is different from normal variable evaluation, e.g. that after c<-4, c(10) is different from (c)(10). Was this inspired by some other language? I don't think it's done this way in any other language I can think of....
I think originally there was no difference, and it caused the obvious trouble when people used variable names like t and c other short function names, so this was added. I don't remember whether the different lookup rules showed up first in R or S. Duncan Murdoch