Karl Ove Hufthammer wrote:
On Mon, 01 Mar 2010 06:37:30 -0500 Duncan Murdoch <murd...@stats.uwo.ca>
wrote:
Some functions output matrices where you would expect them to output
data frames, and then this problem occurs. (Is there a reason why '$'
could/should not be made to 'work' on matrices too?)
The reason for the difference is that data.frames are lists organized
into columns (so the $ handling comes from the list, where it means
"extract the component") whereas a matrix is a single vector displayed
in columns.
Sure, I know that. But is there are reason why the '$' can't be
overloaded to handle the extraction, as a *convenience* to the user?
See the second paragraph of my response.
Duncan Murdoch
After all, it *is* possible to extract columns by name from matrices
too (e.g., using d[,"Sepal.Width"]).
A similar type of overloading is used in the 'sp' class functions,
where you can basically treat a 'SpatialPointsDataFrame', a
'SpatialLinesDataFrame' or a 'SpatialPolygonsDataFrame' as a data frame,
with '$colname' indexing and '[' subsetting, even though the *internals*
of the objects have a completely different (and very complex) structure.
But as a convenience to the user, you don't need to bother with the
internals, and can handle the object *as if* it were a data frame. It's
a very comfortable way of working.
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