no warning with NULL component in data.frame

I sent a message a few days ago about NULL components in list or
data.frame. To reiterate, if I have a list or a data.frame, and try to
manipulate some elements using the $ symbol, no warning is given if the
elements do not exist.

"It's not a bug, it's a feature"

You can do is.null(list$component) to test very efficiently whether the
component is there, which would be hard if it gave an error. Also, it
turns out to be useful to be able to handle a list which may have missing
components without a lot of special cases and without errors.

Anyway, it's a fairly fundamental part of the language.

There might be a case for warning if you access a non-existent element of
a data frame. The problem here is that a lot of correct and functional 
code probably does access non-existent elements and the warnings would
get annoying.

Accessing non-existent dataframe columns with "[" does give an error, so
you could do that instead.

data(trees)
trees$aardvark

NULL

trees[,"aardvark"]

Error in [.data.frame(trees, , "aardvark") : 
        subscript out of bounds


	-thomas

Thomas Lumley
Assistant Professor, Biostatistics
University of Washington, Seattle

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